HVAC Distribution Systems

HVAC Distribution Systems
WEATHERIZATION ENERGY AUDITOR SINGLE FAMILY December 2012

By attending this session, participants will be able to:
Learning Objectives HVAC DISTRIBUTION SYSTEMS By attending this session, participants will be able to: Name functions of the components of forced warm air, hot water, and steam distribution systems. Demonstrate common diagnostic and assessment methods for ducted distribution systems. Describe common problems for each distribution system type. Explain solutions to these common problems. By attending this session, participants will be able to: Name the functions of the components of forced warm air, hot water, and steam distribution systems. Demonstrate diagnostic and assessment methods for ducted distribution systems. Describe common problems for each distribution system type. Explain solutions to these common problems. December 2012

Good Ducted System Design
HVAC DISTRIBUTION SYSTEMS A well-designed duct system: Provides conditioned air to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. Is properly sized so that static pressure is within manufacturer specifications. Is sealed to provide proper airflow and prevent air from entering the house or duct system from polluted zones. Has balanced supply and return airflows to maintain a neutral pressure in the house. Minimizes duct air temperature losses between the air handler and supply registers. A well-designed duct system: Provides conditioned air to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. Is properly sized so that static pressure is within manufacturer specifications. (Static pressure is discussed in more detail later.) Is sealed to provide proper airflow and to prevent air from entering the house or duct system from polluted zones. Has balanced supply and return airflows to maintain a neutral pressure in the house. Minimizes duct air temperature losses between the air handler and supply registers. This is a tall order for a typical “seat of the pants” design and installation. Even when residential ductwork is sized and installed properly, there can be problems with balancing due to interior doors being opened and closed by occupants and furniture or rugs blocking registers, for example. “Balancing” means getting the proper airflow from each register. Q: How is airflow measured? A: In cubic feet per minute (CFM). In a forced air distribution system, the occupants live within the ducted system. (Of course, people do not live in the ductwork; they live between the supply registers and return grilles.) This is not the case with hot water and steam distribution. December 2012

Forced Air Distribution System
HVAC DISTRIBUTION SYSTEMS The parts include: An air handler at the furnace. A heat exchanger where the heat from combustion is transferred to the distribution air. A supply air plenum to which the supply trunks are attached. Branches attached to the supply trunk. Supply registers through which conditioned air flows to the living space. Return grilles through which air flows back to the furnace. Return branches and trunks attached to the return plenum. A forced air distribution system includes: An air handler at the furnace. A heat exchanger where the heat from combustion is transferred to the distribution air. A supply air plenum to which the supply trunks are attached. Branches attached to the supply trunk. Supply registers through which conditioned air flows to the living space. Return grilles through which air flows back to the furnace. Return branches and trunks attached to the return plenum. Try to identify these parts in the illustration on the next slide. Not included in the list above are balancing dampers, which should be located in the supply branches to balance the air distribution system. Because room airflows are interdependent, as one balancing damper is opened to increase airflow to a room, the airflow to all other rooms decreases. A filter is another important part of the system. Filters for air cleaning are located either just behind a return grille or in the return plenum just before it attaches to the furnace cabinet. In some cases, the filter is in the furnace cabinet and accessible by removing the blower access door. Filters are always on the return air side of the distribution system. December 2012

Ducted Distribution System
HVAC DISTRIBUTION SYSTEMS This is not a typical system, but one that shows as many parts of a ducted distribution system as possible. Note the return and supply sides of the system. The supply side is much more extensive than the return side. This is typical of ducted systems. The return side in this illustration includes panning or panned floor joists. This is usually galvanized metal sheeting attached to the underside of a floor joist cavity, forming a duct with the floor above as one surface, the sides of two joists forming two more surfaces, and the metal panning forming the fourth surface. For the past 10 to 15 years, duct design manuals for new ducted systems have recommended against or forbidden the use of panning under floor joists because leaks can occur where the metal joins with the joists. Because these leaks are on the return side of the ductwork, they can cause hazardous negative pressure in the combustion appliance zone (CAZ), increasing the likelihood of combustion gas spillage and draft reversal. Additionally, panned joists can be connected to the outdoors at the rim joist, decreasing the efficiency of the heating/distribution system. When you find panned floor joists, use a smoke stick to check for leaks while the air handler is running. If leaks are adversely affecting the safety of the occupants, seal the joints between the metal pans and the joists. December 2012

Ducted Distribution System, Cont’d
HVAC DISTRIBUTION SYSTEMS Note there are balancing dampers in some of the branches (also called runouts). These hand-operated balancing dampers allow the technician to regulate the airflow to the room served by a particular branch. Opening a balancing damper to allow more air (and heat) to a room will decrease the airflow (heat) to all the other rooms. This interdependence among room airflows can make system balancing a challenge. A branch duct should not be attached within 24 inches of any trunk end cap (see right side of illustration). This helps equalize the airflow to the various branches attached to the trunk. If there is a branch very close to an end cap, there will be much more air moving through that branch and less air moving through branches that are attached to the trunk before it. This guideline should be reinforced for technicians who work in mobile homes and often block the trunks just after the last branch. The system pictured is a “reducing plenum” distribution system (highlighted by the middle red circle). This indicates that the main trunk becomes smaller as branches run off it. This reduction in size helps balance the amount of airflow in the branches and keeps the air speed high. December 2012

Draft hood (open at bottom)
Open Returns HVAC DISTRIBUTION SYSTEMS Open return This is a photo of a disconnected draft hood and open return register on the furnace return air plenum. This is a dangerous combination. Q: Why is this a problem? A: Flue gases spill into the CAZ and are drawn into the conditioned house air by the furnace blower through the open return air register. This creates negative pressure in the CAZ. Open returns in the CAZ should always be sealed with sheet metal and then with mastic. Draft hood (open at bottom) Photo Courtesy of PA Weatherization Training Center December 2012

Atmospheric Gas Furnace
Distribution System Components HVAC DISTRIBUTION SYSTEMS Atmospheric Gas Furnace Find the: 1. Circulating fan 2. Air filter 3. Cold air return 4. Heat exchanger 5. Warm air to house Find these components: Circulating fan. Air filter. Cold air return. Heat exchanger. Warm air to house. This is a Category I, natural draft appliance with an atmospheric burner. All of the items in the list to the right of the illustration are components of the distribution system. The return plenum (largest return duct) would be connected at the bottom left of the illustration at “Cold Air Return.” The supply plenum (largest supply duct) would be connected at the top of the illustration at “Warm Air to House.” Source: Heating with Gas, Natural Resources Canada, 1998. December 2012

Function of Heat Exchanger
HVAC DISTRIBUTION SYSTEMS The furnace heat exchanger is where the heat from combustion gases—usually between 70% and 95%— is transferred to the distribution air in the ductwork. The heat exchanger is an extremely important part of any furnace because it can have a significant impact on efficiency and health and safety. Combustion byproducts must not mix with distribution air. The furnace heat exchanger is where 70%-95% of the heat from combustion gases is transferred to the distribution air in the ductwork. The heat exchanger is an extremely important part of any furnace because it can have a significant impact on efficiency and health and safety (H&S). Combustion byproducts must never mix with distribution air Q: Why must combustion byproducts never mix with distribution air in a furnace? A: Because these byproducts may enter the living space and hurt the occupants. This can occur when a heat exchanger leaks due to corrosion, rust, or a split seam. If a heat exchanger is cracked, carbon monoxide might leak from that crack and taint the indoor air. However, it is more likely that when the furnace blower runs, air from the distribution side of the defective heat exchanger will enter the combustion chamber. The furnace distribution fan is MUCH stronger than the natural draft up the chimney, so distribution air from the fan will blow from the distribution side to the combustion side through the defect in the heat exchanger. This blowing action might cause flame roll-out and/or carbon monoxide production. If a heat exchanger is defective, it must be replaced. Repairing a defective heat exchanger is not acceptable. In many furnaces, the heat exchanger can’t be replaced, so the entire furnace must be replaced. December 2012

Heat Exchanger Leakage Testing
HVAC DISTRIBUTION SYSTEMS Test methods: Visual inspection Inspection with small torch CO reading Wavering flame Tracer gas Damaged area of heat exchanger There are a few ways to test for defective heat exchangers. Test methods: Visual inspection – Visual inspection is considered the best method, but this is sometimes difficult or impossible to do. A good inspection mirror and strong light are required. Some technicians will remove the heat exchanger for inspection if they suspect there is a problem. This can be time consuming, but in some cases it is worth it. Inspection with small torch – A method similar to visual inspection uses a small torch called a gas match. With a gas match, a good inspection mirror, and a strong flashlight, a technician can usually identify a problem heat exchanger. A gas match has a 3-foot flexible probe with a butane torch at one end. The torch is ignited by a continuous sparking igniter, so that if air from a defective heat exchanger blows the flame out, the spark igniter will light it again. The air blowing out of the distribution side of the heat exchanger when the blower comes on will cause the flame on the gas match to waver or go out, indicating a defective heat exchanger. Carbon monoxide (CO) reading – Another method uses an instrument that detects carbon monoxide. With the CO instrument probe inserted in the vent connector BEFORE the furnace blower activates, the technician notes the CO concentration, measured in parts per million (ppm). The technician should carefully watch the CO instrument when the furnace blower turns on. If blower activation significantly alters the CO ppm level (usually it increases), the heat exchanger is probably defective. In this case, a thorough visual inspection is in order. Wavering flame – Watch the burner flames at the time the furnace blower kicks in. If any of the flames waver, the heat exchanger is probably defective. If wavering is seen, further visual inspection is warranted. Tracer gas – Probably the most rigorous heat exchanger test is the one developed by the American Gas Association (AGA, December 1986), included as a resource to this section. This method uses a tracer gas (methane) and a combustible gas detector. Photo courtesy of New River Center for Energy Research and Training (NRCERT) December 2012

Ductwork Efficiency HVAC DISTRIBUTION SYSTEMS Specify duct sealing where ducts are located in unconditioned spaces. Ducts in unconditioned spaces should be insulated to recommended levels. Seal all returns in spaces where atmospheric fossil fuel appliances are located. Specify duct sealing when ducts are located in unconditioned spaces. Insulate ducts in unconditioned spaces to recommended levels, typically at least R-6. Insulation levels are based on cost-effectiveness. They are determined through a DOE-approved audit or priority list. Seal all returns in spaces where atmospheric fossil fuel appliances are located. Ducts located in unconditioned spaces may leak to the outside and are therefore included in building shell measures. Similarly, a distribution system that uses building framing for supplies or returns must be sealed. It is often necessary to remove duct insulation to properly seal ducts. Seal with mastic, then re-insulate. Photos courtesy of NRCERT December 2012

Ductwork This sheet metal ductwork
HVAC DISTRIBUTION SYSTEMS These photos show a well-constructed ducted system with sealed joints. This sheet metal ductwork is located within the building envelope, so it does not need to be insulated. Joints should never be sealed with duct (duck) tape. Heat causes the adhesive on this tape to break down. Mastic is the best choice for duct sealing. Duct leaks on the supply and return sides closest to the air handler will leak the most. This is because static pressure is greater closer to the air handler. Q: Should all duct joints within the thermal envelope of a dwelling be sealed? A: It depends. If a duct leak is adversely affecting the pressure in the CAZ (making it excessively negative), it should be sealed. However, if a duct leak is within the thermal envelope of the dwelling, it should not be sealed unless testing shows it is leaking so excessively that it can’t deliver adequate air to other rooms, and/or it is creating a hazardous pressure in the CAZ. This sheet metal ductwork is located within the building envelope, so it does not need to be insulated. Photos courtesy of R. Karg December 2012

Ducted System Controls
HVAC DISTRIBUTION SYSTEMS The primary controls are: Thermostat. Fan and limit control. Balancing dampers. Motorized dampers (these are not common). These are the primary controls for a ducted distribution system. Each will be described in more detail on the following slides. Thermostat Fan control and limit control Balancing dampers Motorized dampers (uncommon) December 2012

Furnace Thermostat The thermostat activates the burner on a furnace.
HVAC DISTRIBUTION SYSTEMS The thermostat activates the burner on a furnace. The fan and limit switch turns the air handler blower on and off. The programmable thermostat pictured here is quite simple to operate. When the thermostat for a furnace calls for heat, it activates the burner directly. If it activated the air handler blower instead, cool air would be distributed until the burner heated the distribution air. This would cause discomfort. The fan and limit control switch turns the air handler blower on and off. Photo courtesy of R. Karg December 2012

Fan and Limit Control This control turns the air handler blower on and
HVAC DISTRIBUTION SYSTEMS This control turns the air handler blower on and off at set temperatures. This is the fan control. It also shuts down the blower if the heat exchanger area gets too hot (usually at about 200). This is the limit control. Recent versions are electronic and cannot be adjusted in the field. There is a bi-metallic coil protruding from the opposite face of this fan and limit control, shown in the inset photo. This bi-metallic coil is in the heat exchanger area of the furnace; it rotates with temperature changes, turning the air handler blower on and off. This is the fan control. The coil also shuts down the blower if the heat exchanger area gets too hot (usually at about 200F). This is the limit control. Modern versions are electronic and cannot be adjusted in the field. Excessive temperatures in the heat exchanger area can be caused by a broken fan belt, a faulty motor, or a very dirty filter, among other things. Graphic source: Heating with Gas, Natural Resources Canada, 1998. Photo courtesy of Honeywell. December 2012

Dampers help control airflow to rooms.
Balancing Dampers HVAC DISTRIBUTION SYSTEMS Dampers help control airflow to rooms. Dampers help control airflow to rooms. Manual balancing dampers are not as common as they should be. Sometimes they need to be added. Hand-operated balancing dampers allow the technician to regulate the airflow to the room served by a particular branch. Opening a balancing damper to allow more air (more heat) to a room will decrease the airflow (heat) to all the other rooms. This interdependence among room airflow volumes can make proper balancing a challenge. Balancing damper Graphic source: Heating with Gas, Natural Resources Canada, 1998. Manual balancing dampers are not as common as they should be. Sometimes they need to be added. December 2012

Motorized dampers are uncommon.
HVAC DISTRIBUTION SYSTEMS Motorized dampers are uncommon. Motorized dampers are used for zoning a ducted distribution system, rather than for balancing. When a zone requires heat, the thermostat of that zone opens the zone damper and activates the furnace burner. Motorized dampers are essentially zone control devices for ducted systems. They are not really for balancing the system as manual dampers are. Motorized dampers are connected to thermostats. When a thermostat in a zone calls for heat, the thermostat opens that damper and fires the gas or oil burner to create and deliver heat to that zone. Photo courtesy of R. Karg December 2012

Common Ductwork Problems
HVAC DISTRIBUTION SYSTEMS Common problems include: Duct leakage to/from the outdoors. Restricted return side. System not balanced. Temperature too high or too low at heat exchanger. Static pressure out of range. Airflow of air handler fan not matched to system. People live within the distribution system. Closing a bedroom door or covering a register or grille can significantly alter airflow. Common ductwork problems include: Duct leakage to/from the outdoors. Return side restrictions. System imbalance. Temperatures that are too high or too low at the heat exchanger. Static pressure out of range. Airflow of the air handler fan not matched to the system. A closed door or furniture covering a register or grille. These can significantly alter airflow. Duct leakage, especially return side leakage, is probably most serious if it is in the CAZ. This can cause hazardous negative pressure, resulting in backdrafting of natural draft combustion appliances. It is very common for the return side of a ducted system to be more restrictive than the supply side. This is usually caused by too few returns and/or the return part of the ducted system being too small. As mentioned, balancing refers to supplying the proper amount of air to each room. This can be a daunting task because opening one balancing damper causes less air to flow to all the other registers. If the system is balanced properly with all the interior doors open but an occupant closes the door to a room with a supply register but no return grille, the ducted system will be out of balance. If the temperature at the heat exchanger is too high or too low, discomfort and/or damage to the heat exchanger might result. Static pressure is like blood pressure. If there is too much pressure because of restricted ductwork or obstructions, the static pressure will be high and the airflow will go down. On the other hand, leaky ductwork can cause low static pressure. Each air handing unit should have an airflow (in CFM) that matches its British thermal unit per hour (BTUH) output. If the airflow is too high or low for the firing rate of the burner, problems can result. December 2012

Diagnostics for Ductwork
HVAC DISTRIBUTION SYSTEMS Diagnostics include: Pressure pan (duct leakage). Duct blower (duct leakage). Static pressure. Temperature rise. Room-to-room pressure differences (door restrictions). Air handler blower CFM flow. Diagnostics include: Pressure pan (duct leakage). Duct blower (duct leakage). Static pressure. Temperature rise. Room-to-room pressure differences (door restrictions). Air handler blower flow. December 2012

Pressure pan testing of the duct system
HVAC DISTRIBUTION SYSTEMS Test the duct system with a pressure pan and blower door to identify: Leakage to the outdoors when ducts are located outside the thermal boundary. Leakage pathways from duct-containing building cavities to the outdoors (e.g., floor-joist cavities adjacent to porch roofs, cantilevers). Pressure pan testing of the duct system Test the duct system with a pressure pan and blower door to identify: Leakage to the outdoors when ducts are located outside the thermal boundary. Leakage pathways from duct-containing building cavities to the outdoors (e.g., floor joist cavities adjacent to porch roofs or cantilevers). With the blower door depressurizing the house to 50 pascals (Pa), use a pressure pan to block a duct register or grille. A manometer with a hose attached from the input port to a pressure tap on the pressure pan measures the pressure difference. The degree of pressure difference indicates the leakage to the outdoors. A reading of 1 Pa or less is the goal. Leaking ductwork in mobile homes is fairly common. Leaking ductwork in a CAZ is usually a health and safety problem rather than an energy problem. Leaking ductwork outside the thermal envelope should be sealed and insulated, usually to at least R-6. Photo courtesy of NRCERT December 2012

The handle allows for easy testing of hard-to-reach ducts.
Pressure Pan Procedure Summary HVAC DISTRIBUTION SYSTEMS Depressurize house to Pa with blower door. Test each register and grille. Document readings. Registers too large or oddly shaped may be covered with plastic and taped on edges. Seal duct leaks and retest. Goal = readings lower than 1 Pa Photo courtesy of NRCERT The handle allows for easy testing of hard-to-reach ducts. Test procedure: Depressurize the house to 50 Pa with the blower door. Test each register and grille. Document the readings. Registers that are too large or oddly shaped may be covered with plastic and taped on edges. Seal any duct leaks and retest. The goal is readings lower than 1 Pa. A blower door must be operating at 50 Pa for any pressure pan testing. It is a good idea to start with the registers or grilles closest to the blower door and work clockwise around the house, first floor first and then second floor. It is very important to record the pressure pan readings before and after duct sealing. Q: What kind of housing stock tends to benefit most from pressure pan testing? A:Many weatherization programs use a pressure pan test to check for duct leakage in mobile homes because leakage can be much more significant than in site-built homes. Q: Why? A: Because mobile home ducts are commonly located outside of the thermal boundary. The goal stated in the slide is a suggestion only. Some weatherization programs use more stringent standards, for example, a total of 3 Pa for ALL the grilles and registers in a mobile home. Some programs use a less stringent standard. The pressure measures leakage from the register or the grille to the outdoors. This means that even if there is a significant leak close to a register in a site-built home but the basement walls and windows where the duct is located are very tight to the outdoors, the pressure pan reading will be small. A digital manometer is connected to the other end of the orange hose that is connected to the pressure pan in the photo. The manometer is measuring the pressure difference between the house and the pressure pan. For more detailed instructions, refer to the Pressure Pan User Manual provided with this curriculum. December 2012

Pressure Pan Use Sample mobile home duct pattern Bedroom 0.8 Pa
HVAC DISTRIBUTION SYSTEMS Sample mobile home duct pattern Bedroom Pa Bedroom Pa Bath Pa Furnace Closet (living room) 2.6 Pa Living Room Pa Kitchen Pa Kitchen Pa Total: Pa 2.4 Pa at the bath register and 2.6 Pa at the living room register indicate a large leak between them, probably at the furnace plenum. Here are some sample readings from pressure pan testing conducted on a mobile home. Many readings were greater than 1 Pa, indicating it will be cost-effective to seal the ducts. Photo courtesy of NRCERT December 2012

Duct Blower for Duct Leakage
HVAC DISTRIBUTION SYSTEMS Manometer Use duct pressurization testing to quantify: Total duct leakage (to indoors and outdoors). Duct leakage to outdoors. By pressurizing ducts of forced air systems with a duct blower, you can measure total duct leakage (to the indoors and outdoors), expressed in CFM of airflow at a standard pressure difference of 25 Pa. When used in conjunction with a blower door, you can measure duct leakage to the outdoors. Assessing duct leakage to the outdoors is much more important than total duct leakage because of the energy penalty associated with the loss of conditioned air to the outdoors. The pressure pan is really a “poor man’s duct blower.” The pressure pan does not give you the CFM of airflow or square inches of leakage as a duct blower does, but it does give a relative degree of leakiness and is much easier and faster to use than a duct blower. Duct blaster Photo courtesy of The Energy Conservatory December 2012

Duct Blower Procedure Summary #1
HVAC DISTRIBUTION SYSTEMS To test for total duct leakage: Open a window or door to the outdoors. Install duct blower to the air handler compartment. Temporarily seal all supply registers and return grilles. Insert manometer hose into a supply duct. Open up rooms containing ducts. Pressurize the ducts to 25 Pa and record the airflow. This is the standardand briefprocedure for setting up a duct blower to measure the total duct leakage: Open a window or door to the outdoors. Connect the duct blower to the air handler compartment. Temporarily seal all supply registers and return grilles. Insert the manometer hose into a supply duct. Open doors to rooms containing ducts. Pressurize the ducts to 25 Pa and record the airflow. December 2012

Duct Blower Procedure Summary #2
HVAC DISTRIBUTION SYSTEMS To test for duct leakage to outdoors: Close all exterior windows and doors. Set up blower door to pressurize the house. Connect duct blower to air handler compartment and manometer hose to air handler compartment. Temporarily seal all supply registers and return grilles. Pressurize the ducts to 25 Pa. Pressurize the house until the pressure difference of the house and the ductwork is 0 Pa. Record the airflow at the duct blower. This is the standardand briefprocedure for setting up a duct blower to measure leakage to the outdoors: Close all exterior windows and doors. Set up the blower door to pressurize the house. Connect the duct blower to the air handler compartment and the manometer hose to the air handler compartment. Temporarily seal all supply registers and return grilles. Pressurize the ducts to 25 Pa. Pressurize the house until the pressure difference between the house and the ductwork is 0 Pa. Record the airflow at the duct blower. For more detailed instructions, refer to the Duct Blaster User Manual provided with this curriculum. December 2012

Room-to-Room Imbalances #1
HVAC DISTRIBUTION SYSTEMS Closed doors that prevent supply air from getting back to a return cause positive pressures in those rooms with supply vents. . . …which starves the return for air, causing negative pressure in the zone where the return is located. Return Supply Closed doors that prevent supply air from getting back to a return cause positive pressures in those rooms with supply vents. This starves the return for air, causing negative pressure in the zone where the return is located. In this case, which is quite common, the closed door acts as a closed damper in the distribution path of the forced air system. If the closed door serves a bedroom, for example, the air in the room will become stuffy, adequate heated air from the furnace is unlikely to be delivered to the bedroom, and exfiltration from the bedroom to the outdoors will increase. Q: Why is this a problem? A: Increased exfiltration can cause moisture problems in walls in cold climates, and it can significantly increase the air changes per hour (ACH). Of course, this problem increases heating costs. Closed door December 2012

HVAC DISTRIBUTION SYSTEMS Master Bedroom Utility Room Kitchen Whole-house return in hallway Living Room This illustration shows which rooms have positive pressures and which have negative pressures when the doors are closed. Click to reveal the resulting pressure (positive or negative) in each area. Bedroom Bath December 2012

HVAC DISTRIBUTION SYSTEMS Measure room-to-room pressure imbalances Room pressure imbalances over 3 Pa should be remedied by adding supply or return air. Then retest. Measure room pressure imbalances. Remedy imbalances over 3 Pa by adding supply or return air. Then retest. Use a manometer for testing the pressure difference across a closed interior door with the forced air system running. Do not use the blower door for this test. The mathematical sign of the number on the manometer is not significant for this test, only the absolute value of the number. Photo courtesy of PA Weatherization Training Center December 2012

HVAC DISTRIBUTION SYSTEMS Solutions Undercut door. Add jumper duct. Add door grille. Add wall grille. Add transfer grille. Install return in affected room. Solutions for room-to-room imbalances include: Undercut the door. Add a jumper duct. Add a door grille. Add a wall grille. Add a transfer grille. Install a return in the affected room. Q: Does the positive or negative sign on the manometer matter? A: No, only the absolute value of the number matters. Open the door with the air handler running until the absolute value of the number on the manometer falls to 3 Pa or less. This is the opening size needed to relieve the room-to-room pressure imbalance caused by the closed door. Measure the size of this opening (usually a long thin rectangle and a small triangle at the top of the door) to find the area (in square inches) required for the free vent area of your selected solution. Graphic source: Air Distribution System Design, DOE, 2003. Find the size of the free vent area of your solution by opening the door while the air handler is running. When the manometer reading falls below 3 Pa, measure the in2 of door opening. This is the in2 of free opening for your solution. December 2012

Measuring Static Pressure #1
HVAC DISTRIBUTION SYSTEMS Magnet There are two types of pressure in a closed system such as ductwork: Static pressure pushes the duct walls out. This is the pressure that expands a balloon when you blow it up. Velocity pressure causes the air to move in one direction. This is the pressure that causes the balloon you blew up to fly around when you let go of the valve. These two pressure types are created by the air handler blower. Static pressure plus velocity pressure equals total pressure. Measuring the static pressure in a ducted system can help us diagnose problems in the system. Photo courtesy of Rob deKieffer Photo courtesy of R. Karg Point the tip into the air stream Static pressure tip December 2012

Measuring External Static Pressure
Measuring Static Pressure #2 HVAC DISTRIBUTION SYSTEMS Measuring External Static Pressure Check nameplate for External Static Pressure (ESP). Measure both return and supply sides of the air handler as the unit was shipped. Measure at inlet and outlet of blower. Have a clean filter in place (suggested). Don’t measure beyond the A/C coil unless it shipped with unit. Add return and supply pressures together, IGNORING the negative sign. Steps for measuring static pressure include: Check the nameplate on the furnace for external static pressure (ESP). The ESP is the static pressure the manufacturer assumes the air handler blower will create to move air through the ductwork. If an air conditioning coil was shipped with the air handler, the static pressure it imposes will be included in the manufacturer’s ESP rating on the label. If the air conditioner coil was added as a retrofit, the static pressure imposed by this coil will not be included in the manufacturer’s ESP rating. Most manufacturers include in their ESP label rating a typical static pressure imposed by a furnace filter and a register and grille, but don’t worry about these when you do this test. With a static pressure tip at one end of a test hose and your digital manometer connected to the other end, test the static pressure on the supply and return sides of the operating air handler. You don’t need the blower door for this test. Measure at the inlet and outlet of the blower. It’s best to have a clean filter in place. Don’t measure beyond the air conditioner coil unless it was shipped with the unit. Add the supply side and return side static pressures together, ignoring the negative sign on the return side. For example, if the supply side static pressure is +31 Pa and the return side static pressure is -34 Pa, the total static pressure for the ducted system is = 65 Pa. December 2012

Measuring Static Pressure #3
HVAC DISTRIBUTION SYSTEMS General External Static Pressure and Fan Relationship External Static Pressure IWC (Pa) Air Handler Fan Flow Cubic Feet per Minute (173) 1,350 (155) 1,400 (138) 1,450 (118) 1,500 (98) 1,550 (78) 1,600 If the static pressure is too high, the fan flow will drop. This table shows the general relationship between static pressure and air handler blower airflow. For any given fan speed, the air handler airflow decreases as the static pressure in the ducted system increases. If the airflow gets too low, there can be detrimental effects on the distribution system, such as increased temperature rise. There are approximately 250 Pa per inch of water column (IWC). Thus, 1 Pa equals about IWC (1/250 =.004). If the static pressure is too high, the fan flow will drop. December 2012

Measuring External Static Pressure
Measuring Static Pressure #4 HVAC DISTRIBUTION SYSTEMS Measuring External Static Pressure If ESP is too high, the airflow might be blocked or the ductwork might be too small or restricted. If ESP is too low, the ductwork might be very leaky or the fan might be dirty or damaged. Typical ESP totals are around 0.5 IWC or 125 Pa with an air conditioning coil and filter. Typical ESP totals are around 0.25 IWC or 62 Pa without an air conditioning coil and filter. It is preferred to have the return and supply sides of similar magnitudes, for example, a return of -34 Pa and supply of +31 Pa. If the ESP is too high, the airflow might be blocked, or the ductwork might be too small or restricted. A number of things can increase the static pressure in a ducted system, including a dirty furnace filter or clogged air conditioning coil. This reduces the airflow across the heat exchanger. If the ESP is too low, the ductwork might be very leaky, or the fan might be dirty or damaged. Typical ESP totals are around 0.5 IWC or 125 Pa with an air conditioning coil and filter. Typical ESP totals are around 0.25 IWC or 62 Pa without an air conditioning coil and filter. It is preferable to have the return and supply sides of similar magnitudes, for example, a return of -34 Pa and a supply of +31 Pa. December 2012

Temperature Rise Test #1
HVAC DISTRIBUTION SYSTEMS Place thermometer in supply side as close to furnace as possible but out of “sight” of the heat exchanger. Use manufacturer’s recommended measurement method, or Use the four-corner method (measure at each corner or supply plenum and average readings). Place thermometer in return side. Fire furnace. When the supply-side temperature reaches steady state, subtract return-side from supply-side temperature to get temperature rise. Check specified temperature rise on furnace name plate. Actual should be in the middle of the nameplate range. To determine the temperature rise: Place the thermometer in the supply side as close to the furnace as possible but “out of sight” of the heat exchanger so you are measuring true air temperature and not radiant heat gain. Use the manufacturer’s recommended measurement method, or use the four-corner method (measure at each corner of the supply plenum and average the readings). Place the thermometer in the return side. Fire the furnace. When the supply-side temperature reaches steady state, subtract the return-side temperature from the supply-side temperature to get the temperature rise. Refer to the temperature rise noted on the furnace nameplate. The measured temperature should be in the middle of the nameplate range. December 2012

Measuring Temperature Rise & Calculating CFM
Temperature Rise Test #2 HVAC DISTRIBUTION SYSTEMS Measuring Temperature Rise & Calculating CFM The temperature rise for this example is 70: 145 supply side -75 return side 70 temperature rise The temperature rise for this example is 70F. 145F supply side - 75F return side = 70F temperature rise Q: What determines an acceptable temperature rise? A: Temperature rise should be maintained per manufacturer recommendations. Manufacturers’ recommended temperature rises usually range from 40F to 70F. December 2012

HVAC DISTRIBUTION SYSTEMS Excessive temperature rise can result from: Low fan output. Wrong fan speed, bad motor bearings, low voltage to motor, dirty blower wheel, wrong motor rotation, slipping blower belt. Low airflow from restrictions in system. Undersized or restricted ducts, dirty filter, dirty cooling coil. Overfired burner (gas pressure or oil nozzle). Too little airflow relative to the firing rate results in excessive temperature rise. This can be due to: Low fan output. Wrong fan speed, bad motor bearings, low voltage to motor, dirty blower wheel, wrong motor rotation, slipping blower belt. Low airflow from restrictions in the system. Undersized or restricted ducts, dirty filter, dirty cooling coil. Overfired burner (gas pressure or oil nozzle). December 2012

HVAC DISTRIBUTION SYSTEMS Low temperature rise can result from: Excessive fan speed. Excessive duct leakage. Underfired burner. Low gas pressure. Oil nozzle not matched with airflow rate. Too much airflow relative to the firing rate results in low temperature rise. This can be due to: Excessive fan speed. Excessive duct leakage. Underfired burner. Low gas pressure Oil nozzle not matched with airflow rate December 2012

HVAC DISTRIBUTION SYSTEMS Temperature rise that is too high can: Damage the heat exchanger. Cause rocking on the high limit. Temperature rise that is too low can: Lead to condensation. Cause excessive soot buildup. Lead to occupant discomfort. Temperature rise that is too high can damage the heat exchanger or cause rocking on the high limit (also known as cycling on the high limit). Rocking on the high limit can be caused by a loose or broken fan belt, very dirty fan, clogged filter, or very slow fan (i.e., not enough airflow across the heat exchanger). Here’s how it works: The thermostat calls for heat and causes the burner to fire. When the heat exchanger reaches the fan-on temperature, the fan is activated. However, because the fan is not moving enough air across the heat exchanger to cool the heat exchanger area by sending the heated air to the conditioned spaces in the dwelling, the temperature in the area of the heat exchanger continues to rise. When it reaches about 200F, the high limit on the fan and limit control shuts the burner down. During this cycle, little, if any, heated air is distributed to the conditioned spaces. The thermostat will call for heat again, restarting the futile cycle. This can go on for a long time before the occupants realize the furnace is malfunctioning. Low temperature rise can lead to condensation if temperatures in the system don’t get high enough to evaporate any remaining moisture in the system. There is also the possibility of soot buildup from low temperatures. Low temperature rise can also lead to occupant discomfort because the air distributed by the air handler and ductwork might not be warm enough as it exits the registers. December 2012

TrueFlow® Air Handler Flow Meter
Measuring Air Flow at Air Handler #1 HVAC DISTRIBUTION SYSTEMS TrueFlow® Air Handler Flow Meter Measures airflow in residential air handlers The TrueFlow Air Handler Flow Meter is a fast and accurate method for measuring air handler airflow. It is more accurate than the temperature rise method, the static pressure and fan curve method, and the duct blower isolated return method. One of two sizes of metering plate provided with the kit is inserted into the filter slot (or filter grille) with the help of spacers for a tight fit. Full instructions for measuring air handler airflow come with the equipment. #20 #14 Photos courtesy of The Energy Conservatory December 2012

Measuring Air Flow at Air Handler #2
HVAC DISTRIBUTION SYSTEMS General Minimum Airflow Values Airflow in CFM Furnace BTUH Input 500 Less than 60,000 700 60,000 to 79,999 900 80,000 to 99,999 1,200 More than 100,000 The values in the table are generalized, giving the relationship between furnace input (in BTUH) and airflow (in CFM) across the heat exchanger. Out-of-range values can signal a number of problems, including high or low static pressure in the ductwork or the wrong fan speed. December 2012

Analysis of Existing Ductwork - 1
Assessing Ductwork #1 HVAC DISTRIBUTION SYSTEMS Analysis of Existing Ductwork - 1 Interview occupants about the thermal comfort of the existing system. Ask about: Uncomfortable rooms. Excessive noise. Frequent cycling of furnace. It is always important to ask occupants about comfort or discomfort associated with the heating system. Uncomfortable rooms may indicate a distribution system that is not balanced properly, significant duct leakage, or a duct run needing insulation. Q: What causes excessive noise in a ducted system? A: Excessive noise from the ducted system usually means that the air speed within the ductwork is too fast. Generally, the speed should be below 500 to 700 feet per minute (FPM); 88 feet per minute is the equivalent of one mile per hour. Frequent cycling of the furnace might indicate a number of problems, including a thermostat anticipator that is set improperly, an oversized furnace, or malfunctioning fan and limit controls. Plumbing through return duct! Photo courtesy of R. Karg December 2012

Assessing Ductwork #2 HVAC DISTRIBUTION SYSTEMS Analysis of Existing Ductwork - 2 Inspect air handler and ductwork for such things as: Disconnected ducts. Duct leakage. Restricted returns. Panned floor joists. Ducts in unconditioned spaces. Balancing dampers. Disconnected duct! Inspect the air handler and ductwork for: Disconnected ducts. Duct leakage. Restricted returns. Panned floor joists. Ducts in unconditioned spaces. Balancing dampers. If these are not properly calibrated, they could be restricting airflow or allowing too much airflow. These things – disconnected ducts, restricted returns, etc. – reduce the effectiveness of the system and may lead to building degradation and moisture issues in some climates. Photo courtesy of R. Karg Photo courtesy of R. Karg December 2012

Very dirty blower vanes!
Assessing Ductwork #3 HVAC DISTRIBUTION SYSTEMS Do technical testing and appraisal of the duct system and equipment. Duct leakage Pressure pan testing Duct blower testing Room-to-room pressure imbalances Static pressure Temperature rise Blower CFM Test the duct system and equipment, appraising: Duct leakage using pressure pan or duct blower testing. Room-to-room pressure imbalances. Static pressure. Temperature rise. Duct blower CFM. Is the system operating within manufacturer specifications? Are there areas for cost-effective improvements? These are the questions you as an auditor will have to answer when you model the system in an approved energy audit. Very dirty blower vanes! Photo courtesy of NRCERT December 2012

Assessing Ductwork #4 HVAC DISTRIBUTION SYSTEMS Analysis of Existing Ductwork - 5 Determine strategies for duct repair: Write down possible problems. Determine required alterations to furnace and ductwork. Decide on consumer education strategies. Determine strategies for duct repair: Identify the problem(s). Determine necessary alterations to the furnace and ductwork. Necessary alterations could include something as simple as changing the furnace filter or something as complex as adding a return duct run. Decide on consumer education strategies. Client education includes teaching clients to maintain the furnace filter, having the system regularly cleaned and tuned, and keeping supply registers and return grilles uncovered. December 2012

Good Hot Water Distribution Design
HVAC DISTRIBUTION SYSTEMS Good design: Provides conditioning to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. Heats the dwelling quietly. A good hot water distribution design: Provides conditioning to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. Heats the dwelling quietly. Hydronics refers to a system of heating (or cooling) that involves transfer of heat by circulating water or water vapor. So this segment on hot water distribution and the following segment on steam distribution both refer to hydronic systems. Hot water or steam distribution is often referred to as “hydronic.” December 2012

Hot Water Distribution System
HVAC DISTRIBUTION SYSTEMS The parts include: Thermostat(s) that activate the circulator pump. Circulator pump(s). Might include zone valves rather than two or more circulator pumps. Aquastat control. A heat exchanger where the heat from combustion is transferred to the distribution water. Supply and return piping at boiler. The expansion tank. Hot water baseboard units (convector) where the thermal energy is transferred to the conditioned rooms. A hot water distribution system includes: A thermostat that activates the circulator pump, a water pump turned on and off by the room thermostat. A circulator pump, which might include zone valves rather than circulator pumps. Some hot water systems have one circulator pump; others have more. An aquastat control, which keeps track of the boiler water temperature. A heat exchanger where the heat from combustion is transferred to the distribution water. As with furnace heat exchangers, boiler heat exchangers can become defective. However, furnace heat exchangers can be defective without the occupants’ knowledge. A defective boiler heat exchanger will leak water that will puddle on the floor under the boiler. Supply and return piping at the boiler. The expansion tank. Hot water baseboard units (convectors) or radiators (convectors) where the thermal energy is transferred to the conditioned rooms. December 2012

Series Loop Hot Water System
HVAC DISTRIBUTION SYSTEMS A series loop hot water distribution system is probably the most common system layout because it is the least expensive. A series loop hot water distribution system is probably the most common system layout because it is the least expensive. Notice that the water is supplied at one end and the return is at the other end. The heated water flows through each baseboard distribution convector. Note the labeled part of the circulator, expansion tank, hot water supply pipe, convector, and cold-water return pipe. Based on graphic from the International Association of Certified Home Inspectors, Inc. December 2012

Series Loop Hot Water Baseboard
HVAC DISTRIBUTION SYSTEMS Typical hot water baseboard distribution Damper fin The damper fin at the top of the baseboard unit can be opened all the way (pointing to the ceiling) or closed. This damper can be used to balance the hot water baseboard distribution system. For a typical series loop hot water distribution system, opening this damper on one baseboard unit means less heat is delivered to the downstream units. The smaller photos to the left show the typical construction of the inside of a baseboard convector. The thin aluminum fins increase the efficiency of heat transfer between the hot water in the copper tube and the room air flowing through the baseboard unit. Rugs and carpeting must not obstruct the open space at the bottom of the baseboard unit faceplate. Photos courtesy of Slant/Fin December 2012

Expansion Tanks HVAC DISTRIBUTION SYSTEMS Old-style tanks (above) and newer tanks (right) allow for expansion of heated water and contraction of cool distribution water. Q: Why is an expansion tank an important component of any hot water distribution system? A: Expansion tanks allow for expansion of heated water and contraction of cool distribution water. An older tank is pictured at left and a newer tank is pictured at right. If an older expansion tank is not drained and refilled annually, the air cushion in the tank will be absorbed into the water. Newer expansion tanks have a bladder that separates the air from the water to prevent the air from being absorbed into the water. Photos courtesy of R. Karg December 2012

Hot Water Distribution Controls
HVAC DISTRIBUTION SYSTEMS Basic controls include: Thermostat Circulator pump(s) Zone valves Aquastat A hot water distribution system has three basic controls. Thermostat Circulator pump or zone valves Aquastat December 2012

Boiler Thermostat The thermostat activates the boiler circulator or
HVAC DISTRIBUTION SYSTEMS The thermostat activates the boiler circulator or zone valve and circulator. The aquastat controls the burner. There are many types of thermostats. The unit pictured is electronic and programmable, and is quite simple to operate. Q: What device in a hot air furnace controls the burner? A: For a furnace, the thermostat turns the burner on and off. However, for a boiler, the thermostat turns the circulator pump on and off. The burner is directly controlled by the aquastat. If a boiler has zone valves, the thermostat opens a valve and turns on the circulator so that hot water is distributed to the appropriate zone. Photo courtesy of R. Karg December 2012

Boiler Aquastat An aquastat: Maintains boiler water temperature.
HVAC DISTRIBUTION SYSTEMS An aquastat: Maintains boiler water temperature. Provides high-limit temperature protection. Will not allow circulator to operate if boiler water temperature is too low. Also assists with DHW temperature control if the water heater is tankless or indirect-fired. This is a photo of an aquastat with its cover removed. The aquastat is similar to the fan and limit control on a furnace. A boiler aquastat: Maintains boiler water temperature. Provides high-limit temperature protection. Will not allow the circulator to operate if the boiler water temperature is too low. The aquastat also assists with domestic hot water (DHW) temperature control if the water heater is tankless or indirect-fired. The aquastat includes relays (line voltage to low voltage), thermostat controls for the boiler water, differential temperature controls (if domestic hot water is generated by the boiler), and a high-limit switch. The aquastat is the control hub of the distribution system. Just behind the aquastat control on the pictured system is a tankless coil for domestic hot water. Not all boilers have tankless coils. To the left of the aquastat control is the temperature/pressure gauge. Photo courtesy of R. Karg Normally, the aquastat control is covered. December 2012

Zone Valves Zone valves are controlled by thermostats in each zone.
HVAC DISTRIBUTION SYSTEMS Zone valves are controlled by thermostats in each zone. This house has 3 zones with one thermostat for each. The 4th zone valve is for domestic hot water from the boiler. Zone valves are controlled by thermostats in each zone. This house has three zones with one thermostat each. All boilers with hot water distribution have at least one circulator pump. The unit in the photo has one circulator pump and four zone valves. Three of these valves are for distribution of hot water to separate heating zones, and each one is controlled by a separate thermostat. When a thermostat calls for heat, its companion zone valve opens and the circulator pump turns on, sending hot water to that zone. Multiple zones can be active at the same time. The fourth zone valve in the photo (obscured by a copper pipe) is for controlling the temperature of the indirect-fired DHW heater. If this hot water distribution system had four circulators rather than one circulator and four zone valves, most technicians would consider it to be a higher quality system. This is because circulators are more durable than zone valves. However, zone valves are less expensive to install than circulators. Photo courtesy of R. Karg Zone valves take the place of circulators. December 2012

Potential Problems with Hot Water #1
HVAC DISTRIBUTION SYSTEMS Photo courtesy of R. Karg Poor maintenance: If a hot water distribution system is maintained properly, there is little that can go wrong. Oil-fired boilers should be cleaned and tuned every year. Gas-fired boilers should be cleaned and tuned once every three years. If a hot water distribution system is not maintained properly, it will develop problems. Oil-fired boilers should be cleaned and tuned every year. Gas-fired boilers should be cleaned and tuned every three years. If the system is maintained properly, little can go wrong. December 2012

HVAC DISTRIBUTION SYSTEMS Poor expansion tank maintenance Older expansion tanks (at left) should be drained and refilled annually. If not, the air cushion in the tank can be absorbed into the water, so the tank loses its ability to absorb the expansion of the water. Then it gets hot. As a result, the distribution system might become noisy and the pressure relief valve will start releasing small quantities of hot water. Newer expansion tanks (at right) require very little maintenance. These tanks have a bladder that separates the air from the water, so the air cannot be absorbed into the water over time. Older tanks (above) should be drained and refilled annually. Newer expansion tanks (right) require very little maintenance. Photos courtesy of R. Karg December 2012

HVAC DISTRIBUTION SYSTEMS Air bleeder vent Expansion tank Air in the distribution system If the air bleeder valve malfunctions, air will not be purged from the distribution system. This air (oxygen) will create sludge and make the system noisy as the air is pumped with the water. Air in the distribution system: The expansion tank is always on the supply piping of the hot water distribution system. The air bleeder vent is usually on top of the expansion tank in newer systems. If the air bleeder valve malfunctions, air will not be purged from the distribution system. This can cause the system to become noisy as water pumps through. It can also create sludge in the system because of oxygen in the distribution water. December 2012

Good Steam Distribution Design
HVAC DISTRIBUTION SYSTEMS A good design: Provides conditioning to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. A good steam distribution design: Provides conditioning to meet all room heating loads. Provides thermal comfort evenly in all conditioned rooms. December 2012

Steam Distribution System
HVAC DISTRIBUTION SYSTEMS The parts include: A thermostat(s) that activates the circulator pump. Pressure control (Pressuretrol). A heat exchanger where the heat from combustion is transferred to the distribution water/vapor. Supply and return piping at boiler. For one-pipe distribution, the supply and return pipes are the same. For two-pipe distribution, there are separate supply and return pipes. Steam radiators that transfer thermal energy to the conditioned rooms. A steam distribution system includes: A thermostat. A pressure control (Pressuretrol). A heat exchanger where the heat from combustion is transferred to the distribution water/vapor. Supply and return piping at the boiler. For one-pipe distribution, the supply and return pipes are the same. For two-pipe distribution, there are separate supply and return pipes. Steam radiators that transfer heat to the conditioned rooms. Identify these parts in the illustrations and photos on the upcoming slides. Steam distribution systems do not have pumps. The steam is moved around by pressure. December 2012

Steam Distribution Controls and Gauges
HVAC DISTRIBUTION SYSTEMS Basic controls include: Thermostat Pressure control (Pressuretrol) Sight or gauge glass Low-water cutoff The basic controls for a hot water distribution system include: Thermostat. Pressure control. Sight glass or gauge glass. Low-water cutoff. December 2012

Steam Boiler Thermostat
HVAC DISTRIBUTION SYSTEMS The thermostat activates the steam boiler burner. The Pressuretrol turns the burner off when the set pressure is reached. There are many types of thermostats. The unit pictured is electronic and programmable, and is quite simple to operate. For a furnace, the thermostat turns the burner on and off. For a steam boiler, the thermostat turns the burner on. The Pressuretrol turns the burner off when the set pressure is reached. The Pressuretrol is similar to a fan and limit control on a furnace and an aquastat on a boiler. It controls when the burner fires and shuts down in response to a call for heat from the thermostat. Photo courtesy of R. Karg December 2012

Steam Boiler Pressuretrol (pressure control) Sight glass
HVAC DISTRIBUTION SYSTEMS Pressuretrol (pressure control) Sight glass Low-water cutoff Steam boilers are rated as high or low pressure. Most residential steam boilers are low pressure, meaning they operate at pressures less than 15 pounds per square inch (psi). The pressure control allows the operating pressure to be set. This turns the burner on and off to maintain pressure when heat is called for. One of the most important rules for the safe operation of steam boilers is to maintain a constant, proper water level at all times and as conditions permit. If water is not visible in the sight glass, shut the boiler off immediately until a safe water level has been determined. The low-water cutoff is the most important electrical/mechanical device for maintaining a safe water level on a steam boiler. It is designed to shut down the boiler if a low water condition is detected. A low-water cutoff is required by code. Steam distribution systems do not have expansion tanks like hot water systems. The entire steam distribution system provides space for the steam to expand in the radiators. When the steam (water vapor) condenses in the radiators, it releases 970 British thermal units (BTU) of thermal energy per pound of condensate. This condensed water runs back to the boiler to be heated and vaporized again. Oil burner Photo courtesy of R. Karg December 2012

Steam Distribution Controls
HVAC DISTRIBUTION SYSTEMS Sight (gauge) glass Low-water cutoff Photo courtesy of R. Karg The low-water cutoff will shut off the burner if the water falls to an unsafe level. This is required by code. The sight or gauge glass provides an easy way to determine the water level in a steam boiler. The low-water cutoff is the most important electrical/mechanical device on a steam boiler for maintaining a safe water level. It is designed to shut down the boiler if a low water condition is detected. If a low water condition develops, it could very well result in overheating and explosion of the boiler. The low-water cutoff should be tested at least weekly. The sight glass provides an easy way to determine the water level in a steam boiler. December 2012

Pressure Control for Steam
HVAC DISTRIBUTION SYSTEMS This device determines the operating range of the boiler during the heating cycle. When the thermostat calls for heat, the burner will cycle up to the cut-out pressure setting of the Pressuretrol. The burner will then shut off. A Pressuretrol determines the operating range of the boiler during the heating cycle. When the thermostat calls for heat, the burner will cycle up to the cutout pressure setting of the Pressuretrol. The burner will then shut off. Photo courtesy of Honeywell Controls December 2012

One-Pipe Steam Distribution
HVAC DISTRIBUTION SYSTEMS One-Pipe Steam Distribution Both steam and condensate use the same pipe. Steam travels to each radiator, condenses (giving off heat), and flows back to the boiler through the same pipe as condensed water. A one-pipe steam distribution system uses the same pipe for the steam vapor and the condensate. Because of this, the piping is larger than piping for a two-pipe system. Steam travels to each radiator, condenses (giving off heat), and flows back to the boiler through the same pipe. One-pipe systems are difficult to convert to hot water distribution systems. The amount of heat distributed to each room is controlled by the vent valve. Graphic based on Basic Steam Heating Systems, Hoffman Specialty, ITT Industries, 1999, p. 2, December 2012

Two-Pipe Steam Distribution
HVAC DISTRIBUTION SYSTEMS Steam moves to the radiators in one pipe and the condensate flows back to the boiler through the other pipe. These pipes are usually a smaller diameter than one-pipe systems. A two-pipe steam distribution system uses separate pipes for the steam and the condensate. Because of this, the piping is smaller than it is for one-pipe systems. Two-pipe systems are much easier to convert to hot water distribution systems than one-pipe systems. The amount of heat distributed to each room is controlled by the thermostatic trap. Graphic based on Basic Steam Heating Systems, Hoffman Specialty, ITT Industries, 1999, p. 2, December 2012

Potential Problems with Steam #1
HVAC DISTRIBUTION SYSTEMS Steam distribution pipes are sometimes covered with asbestos insulation. If this material is friable, be careful; it might be best to avoid blower door testing. Steam distribution pipes are sometimes covered with asbestos insulation. If this material is friable, take care not to disturb it. “Friable” means particles can be easily crumbled or pulverized, and can then become airborne. You want to prevent this, as you do not want anyone to inhale asbestos. Photo courtesy of R. Karg December 2012

HVAC DISTRIBUTION SYSTEMS If one- or two-pipe radiators don’t heat up, the supply valve may be closed or the air valve may be blocked. Supply valve Air valve If one- or two-pipe radiators don’t heat up, the supply valve may be closed or the air valve may be blocked. Two-pipe radiator Photo source: The Open Fire Centre Ltd., Yorkshire St., Oldham, Lancashire, UK. December 2012

HVAC DISTRIBUTION SYSTEMS Steam pressure is often set too high. This can cause distribution problems and wastes energy. For most residential low-pressure, one-pipe systems, 2 psi cut-out pressure or less will work fine and maximize efficiency. Steam pressure is often set too high. This can cause distribution problems and it wastes energy. The Pressuretrol has a cut-in pressure (pressure at which the burner fires) and a cut-out pressure (pressure at which the burner shuts down). The cut-out point should be set for about 1 psi higher than the cut-in pressure. For most residential low-pressure one-pipe systems, cut-out pressure of 2 psi or less will maximize efficiency. Too high a pressure can prematurely close the radiator air valves, preventing steam (the heat source) from entering the radiator. A lower steam pressure will not shut the air valve as quickly, allowing more steam to enter the radiator. If the pressure is set too high, the boiler must run longer to create the higher steam pressure, which wastes energy. Photo courtesy of Bill Van der Meer High pressure can cause distribution problems and waste energy. December 2012

HVAC DISTRIBUTION SYSTEMS When replacing a steam boiler, the new unit must be sized to match the installed radiation, rather than the heat load of the house. When we weatherize dwellings, we reduce their heating needs. When replacing any heating system except steam, consider de-rating (downsizing) the system so that it is not oversized for the reduced heating needs of the weatherized dwelling (design heat load or DHL) by more than 25%. When replacing a steam boiler, the new boiler must be sized for the existing steam radiators rather than the DHL of the house. Downsizing the system could lead to insufficient heating capacity in the radiators. Photo courtesy of R. Karg December 2012

Summary #1 HVAC DISTRIBUTION SYSTEMS The function of all distribution systems is to provide even thermal comfort in all rooms of the home. Major components of forced air distribution systems include an air handler, heat exchanger, supply air plenum, supply and return registers, grilles, branches, and ducts. Forced air system diagnostic procedures include duct leakage testing, measuring static pressure, temperature rise, room-to- room pressure imbalances, and airflow across the heat exchanger. Common problems associated with ducted systems include room pressure imbalances, improper temperature rise, and energy wasted through duct leakage to the outdoors. The function of all distribution systems is to provide even thermal comfort in all rooms of the home. Major components of forced air distribution systems include an air handler, heat exchanger, supply air plenum, supply and return registers, grilles, branches, and ducts. Ducted distribution is the most complex and potentially problematic of the three distribution types. Several diagnostic procedures can be used to assess ductwork, including duct leakage testing, measuring static pressure, temperature rise, room-to-room pressure imbalances, and airflow across the heat exchanger. Common problems associated with ducted systems include room pressure imbalances, improper temperature rise, and energy wasted through duct leakage to the outdoors. Present this slide as an interactive discussion, soliciting personal examples from the trainees. Add your own personal examples and knowledge to supplement. December 2012

Summary #2 HVAC DISTRIBUTION SYSTEMS Major components of hot water distribution systems include the thermostat, circulator pump, aquastat control, heat exchanger, supply and return piping, expansion tank, and hot water baseboard units. Major components of steam distribution systems include the thermostat, pressure control (Pressuretrol), heat exchanger, supply and return piping at the boiler, steam radiators, sight glass or gauge glass, and low-water cutoff. Some common problems associated with hot water or steam distribution include expansion tank degradation, low water levels (steam), or improper sizing once the home is weatherized. Compared with ductwork, hot water and steam distribution are relatively trouble-free, and diagnostic procedures are easy. We merely ask the occupants if there are problems with the thermal comfort of the home and conduct a simple inspection of the distribution components. The major components of hot water distribution systems include the thermostat, circulator pump, aquastat control, heat exchanger, supply and return piping, expansion tank, and hot water baseboard units. The major components of steam distribution systems include the thermostat, pressure control (Pressuretrol), heat exchanger, supply and return piping at the boiler, steam radiators, sight glass or gauge glass, and low-water cutoff. Some common problems associated with hot water or steam distribution include expansion tank degradation, low water levels (steam), or improper sizing once the home is weatherized. Compared with ductwork, hot water and steam distribution are relatively trouble-free and diagnostic procedures are easy. We merely ask the occupants if there are problems with the thermal comfort of the home and conduct a simple inspection of the distribution components. Present this slide as an interactive discussion, soliciting personal examples from the trainees. Add your own personal examples and knowledge to supplement. December 2012

HVAC Distribution Systems

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