1. Technician’s Guide and Workbook for Zoning Section 8: Balancing and Managing Excess Air

2. Excess Air Management Overview
There must be an excess air management plan for constant volume zone systems and for many variable volume zone systems that use a ducted air distribution system. What do you do with the air when a zone shuts down, what if two zones shut down,…etc.? In Chapter II we covered the option of riding the blower curve. However, that decrease in airflow is not enough to allow us to safely operate the equipment when we have relatively small zones. Airflow management techniques, and how to calculate excess airflow covered in this chapter include the following:

3. Excess Air Options (page 1)
Bypass Duct Relief: Routes a portion of the supply air back to the return side of the duct system.
Dump Zone Relief: Excess air is dumped into a space where temperature control is not important.
Distributed Relief: Zone dampers have an integrated barometric design or a manual setting so they do not fully close, allowing some airflow into zones that are not calling for heating/cooling.

4. Excess Air Options (page 2)
Overblow Relief: Overblow allows more air to go into the open zone than the design airflow value. Note: riding the fan curve and allowing excess air into remaining open zones.
Selective Throttling: Routes the excess air to a zone where the temperature is not important (unlike dump zone the zone may vary by occupancy or the zone control designer’s discretion.

5. Excess Air Options (page 3)
Capacity Control: Some zone controllers can inhibit high airflow/BTU capacity until a sufficient number of zones (≥ 50%) are demanding conditioned air.
Consolidate Zones: Small zones require more relief than large zones when operating alone. So, increasing the zone size by combining zones may be required to reduce extreme relief requirements.

6. Bypass Relief

7. Distributed Relief (1)
HVAC systems designed with distributed relief allow air to flow into zones that are not calling for heating or cooling. The zone dampers in a distributed relief setup will have a mechanical stop or an electronically controlled minimum set point that allows a small portion of air to always enter into the zone. This type of design is a good way to make sure that outside air, is distributed to every zone weather or not the zone dampers are open and calling for airflow. Generally, the design minimum bypass is between 10% and 30% of the designed airflow.

8. Distributed Relief (2)
Due to the constant addition of treated (heated or cooled) airflow, even when there are not necessarily any actual load requirement, a distributed relief system design may cause some rooms to overheat and/or overcool. Make sure the implications of reducing the design amount are considered before changing the system’s minimum setting to operate differently than the specified design amounts.

9. Distributed Relief (3) Calculating the percentage of a design CFM
To calculate a percentage of the design CFM simply multiply the design CFM by the percentage:
30% of 100 = 0.30 × 100 = 30
10% of 200 = 0.10 × 200 = 20
Thus, 25% of 850 CFM = 0.25 × 850 = CFM

10. Balancing Distributed Relief (1)
The distributed relief system designer will specify a CFM that should bypass into each zone. Setting the dampers requires the measuring the minimum airflow for each zone at each operating possibility. The best way to proceed with balancing with the least number of changes is to start by setting the smallest zone first with the others wide open. Generally, the smallest zone will have the most resistance to airflow when the larger zones close to their minimum setting.

11. Balancing Distributed Relief (2)
So, it is a good practice to start by setting the smallest zone and then working up to the largest zone. By following this order, you will generally decrease the number of adjustments needed to make the system operate properly under all of the possible zone combinations. Always finish by double checking zones balanced previously before considering the minimum airflow settings completed. A final check is required because air can do weird things when it is being balanced, and will not always react proportionally or as expected.

12. Dump Zone Relief
Dump zone relief is accomplished by having an area that takes the largest excess airflow hit whenever a zone closes down. The dump zone can be a storage area or any room where accurate temperature control is not required. Bedrooms will not work well as a dump zone. However, a back hall or a laundry room might make a good dump zone. The designer should consider the air temperature for cooling applications to make sure the area will not reach dew point.

13. Balancing A Dump Zone
Dump zone balancing is basically a system check to make sure the individual zones are getting the air they need when the system is operating. The dump zone is open whenever another zone is closed, so it must be set for the maximum airflow required by the smallest zone that can operate independently. The balancing technician must make sure the final dump zone damper setting allows every zone to receive its designed airflow when operating alone.

14. Overblow Relief (1)
Overblow relief allows the excess air created when a zone closes down to go to the remaining open zones. As the zones close down the duct system’s internal area where the airflow can travel becomes smaller. The smaller duct means the external static pressure on the blower fan is increased. As covered in Chapter II, this will decrease the total amount of airflow the blower is providing.

15. Overblow Relief (2)
It is important to double check the airflow across the heat exchanger on systems utilizing an overblow design. The airflow across the heat exchanger must meet the original equipment manufacturers (OEM) minimum airflow requirements. OEM tables for the fan chart will have maximum and minimum airflow CFM values, the system must be operating within the OEM’s range under all operating conditions.

16. OEM Fan Chart

17. Balancing Overblow Relief Systems
The critical factor for balancing is making sure the airflow across the heat exchanger is maintained for all zone configurations. Once all of the Zones are set for their design airflow, they can be closed down one at a time and the system ESP can be measured to evaluate the airflow for each single zone configuration. Note: There is generally no need to measure ESP for the case where two or more zones are open because the overblow will decrease when more than one zone is open. The final check to be done is to verify that the system is not too noisy in the overblow mode, and that there are no drafts in occupied areas of over 75 FPM of airflow in the cooling wet coil mode or of 50 FPM in the heating dry cool mode.

18. Selective Throttling (1)
Selective throttling routes excess air to one or more zones and dumps it there. The difference between selective throttling and a dump zone is there are additional control parameters used to select where and how much excess air is dumped. For example, occupancy could be used to make the selection: the excess air may be sent to a great room area when the bedrooms are all occupied, and the great room area is not occupied.

19. Selective Throttling (2)
Any control scheme or combination of control schemes could be used for selective throttling. For example, a smart thermostat might know that a room becomes occupied at a certain time and ignore the setback settings. The related zone becomes a dump zone until the normal occupancy period approaches. The advantage selective throttling has over the simple dump zone is, the area where the excess air is temporarily dumped can operate normally and maintain the design temperature once it becomes occupied.

20. Balancing a Selective Throttling System
For selective throttling to work the zones need duct systems and control dampers sized to accommodate the CFM needed for relief. That amount may vary for the same room depending on which zone is open along with it. So, there may be more than one damper position for the selected zone, in order to provide the proper airflow to the zone calling for heating or cooling. Flow controls with more than one setting for the dampers may be required for a sophisticated throttling system design. It might even be controlled through a high tech home computer/monitoring system utilizing outside air temperatures, room occupancy, or programmed logic in order to decide where the excess air needs to be sent. Technician’s balancing the airflow for a selective throttling system must fully understand the control sequences and must make sure the proper airflow is going across the heat exchanger in all of the possible operating configurations.

21. Capacity Control
Capacity control requires a variable speed system that has the capability of decreasing the load output and airflow at the same time in parallel. The fan control operates to maintain the required CFM flow through the main supply trunk. Thus, as a zone closes the blower motor reduces the speed and the CFM decreases as needed to maintain the required airflow in the supply trunk. As with other control options the minimum airflow across the heat exchanger must be maintained. If humidity control for a humid climate is not a concern this design will work well. However, as the speed ramps down, the ability to remove latent heat also goes down. Since the infiltration and outside air does not decrease an alternative system is needed for moisture removal in humid climate zones.

22. Balancing Capacity Control Systems (1)
Balancing a capacity control system depends on the motor speed controller being properly set. For most residential systems the blower motor control is set up by selecting motor taps. The motor then maintains the CFM by the HVAC systems internal controls. However, when a SP sensor needs has been installed for motor control purposes, the most common mistake for sensor placement is to put the sensor too near to the blower in the air handling unit. Another common mistake is to place a SP sensor behind a zone damper, so it is in a zone.

23. Balancing Capacity Control Systems (2)
The first step for balancing this type of system is to verify the SP signal to the controlling device is an accurate reading.

24. Balancing Capacity Control Systems (3)
The second crucial step is to measure the SP at that point and make sure the sensor is sending a signal for that value back to the controller. The rest of the system’s operation depends on the accuracy of the SP measurement. After the SP sensor is set, the individual zones can be balanced.

25. Consolidate Zones
There is no way to relieve enough airflow to make a master closet a separate zone. Combining areas is required, when the rooms are too small. There will be a maximum amount of airflow that can be relieved for every system. Understanding the maximum amount of relief CFM that can be bypassed, throttled, and dumped, and explaining what areas will be controlled together up front, will provide the customers with reasonable expectations. Thus, avoiding callbacks from angry customers who expected complete temperature control in every nook and cranny of the home.

26. All of the Above
Utilizing two or more of the airflow management techniques listed above offers designers many options. For example, overblow and distributed relief works well in many locations and would ensure that some outside air was flowing through all the zones. Another combination that works well in most applications is overblow, distributed relief and bypass relief.

27. Balancing Combinations (1)
Combining the relief types also combines the balancing requirements. For example, when the system has overblow and distributed relief, the best practice would be to set the minimum airflow for the zones when the largest zone is in overblow to limit the excess airflow going into zones that are not calling for heating or cooling. This is followed by setting the minimum for the largest zone when the smallest zone is the only one calling for heating and cooling. After any change in settings is made the best practice is to check all the airflow combinations again and make sure the airflow across the heat exchanger is still correct.

28. Balancing Combinations (2)
When bypass relief is added to the combination listed above, one would generally have to adjust the bypass airflow settings, then the minimum settings for the smaller zones that were not calling for heating and cooling with the largest zone operating in overblow mode, followed by setting the largest zone’s minimum position with the smallest zone in overblow. As with any balancing, the airflow across the heat exchanger and the maximum and minimum to each room must be double checked in the end. It may take several passes through the system to get it balanced to work safely and correctly all operating combinations.

29. Balancing Combinations (3)
The final word on balancing all zone systems is check, double check, and document the final settings and measured airflows in the maximum and minimum positions. Additionally, one last check with all zones open should be made to make sure 80% of the required airflow is going into each zone or room.

30. VAV Systems
Variable air volume systems may be found in some large high end residences. VAVs come in different styles and types and provide a variable airflow to a commercial space. The airflow can be constantly cool with or without reheat as shown, hot and cool in a duel duct system as shown, or constantly cool with a mixing box that brings in return air to temper the cool air.

31. Balancing VAV Systems
VAV system balancing is covered in Manual B. Manual N applies to the zone load calculations for a VAV system. However, VAV duct designs are generally done by an engineer since Manual Q does not apply to them since the static pressure (SP) for the main supply trunk is usually 1.5” or above. The reason the SP is so high, is to decrease the duct size required for the long commercial runs. As the air enters the VAV the pressure drops, and the outlet side becomes a low pressure low velocity duct system. So, all the Manual Q calculations can be used on the outlet side of a VAV system.

32. Field Notes
The homeowner’s complaint is simple, The laundry room is always cold in the summer, and then guess what, it is always hot in the winter, what is causing this to happen? The Technician immediately knew the answer based on the system’s design, the laundry room was being used as a dump zone. The technician explained why a dump zone was needed and asked the homeowners if they would like to use another room as the dump zone. In the end they kept it the way it was.