1. Lecture 14a: Introduction to Secondary Systems
Notes: __________________________________________________________________
________________________________________________________________________
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

2. Importance of this Lecture to the Simulation of Buildings
Simulation is important, but understanding what one is simulating is equally critical
Lack of understanding (treating an HVAC system like a black box) can lead to errors in input

3. Purpose of this Lecture
Gain an understanding of:
Different types of space conditioning systems
General characteristics about how they operate and what some of their advantages/disadvantages might be
Prepare for upcoming lectures on EnergyPlus system input

4. All-Air Systems: Overview
Provides all sensible and latent cooling, preheating, and humidification required by the zone(s)
Additional cooling or humidification at the zone rare (industrial systems)
Heating is either provided by the main air stream by the central system or locally at a specific zone
Classified into single- and dual-duct categories as well as constant and variable volume categories
Conditioning depends on air mass flow rate and temperature (enthalpy) difference between supply and room air

5. Single-Duct Systems
Main heating and cooling coils in series arrangement
Ducts supply air to all terminals at a common temperature
Capacity varied by varying temperature or flow rate
Types of single-duct systems
Constant Volume
Single zone
Multiple-zone reheat
Bypass VAV
Variable Air Volume (VAV)
Throttling
Fan-powered
Reheat
Induction
Variable diffusers

6. Dual-Duct Systems Main heating and cooling coils in parallel
May use separate warm and cold air duct distribution systems, blending air at the terminal device
May blend air near the main unit and have separate duct for each zone
Most vary supply temperature, limited number (around 1% of all installed systems?) vary flow rate
Types of dual-duct systems
Single zone (“dual duct”)
Constant volume
Variable air volume
Dual conduit
Multizone
Three-deck multizone

7. Variable Volume vs. Variable Temperature
Throttling back flow when less heating/cooling required
Reduced flow results in reduced fan energy
Potential concerns about outdoor air quantities for IAQ and humidity control
Variable Temperature
Temperature of supply air changes as thermal loading conditions change
Variation in temperature may require additional energy (or use of more outside air)

8. Advantages vs. Other System Types
Flexible: high degree of flexibility on how to meet loads, distribute air, etc.
Low noise: most equipment kept away from occupied spaces
Control: probably the best control of both temperature and humidity can be achieved with air systems (precise control situations)
Most easily understood and popular
Indoor air quality: IAQ is part of the system (not an “afterthought”)

9. Disadvantages vs. Other System Types
Space: require additional space for air distribution ductwork (vertically and horizontally) adds to building size in all directions
Concerns about access to terminal devices
Requires air balancing which can be difficult on large systems
Perimeter heating not always available during construction

10. Single-Zone Constant Volume
Also known as single zone draw through system
Simplest all-air system, meets all conditioning needs of space
System can be in zone or at a remote location, with or without distribution ductwork
Little or no ductwork means low pressure drop and lower fan energy
Systems can be turned off without affecting adjacent systems

11. Multiple-Zone Reheat (Constant Volume)
Also known as terminal reheat in some circles
Single air stream at a fixed temperature delivered to various spaces
Local supply temperature is varied by the use of a terminal reheat coil
Main duct temperature typically cooled to cold deck temperature (all year)large amount of energy consumption
When main duct air temperature varies, could have humidity problems

12. Constant Volume Bypass
Bypass replaces reheat coil
Total flow of system remains constant but flow of air to space is varied
Excess flow is bypassed around zone and directly into return duct
Saves the energy that would be used by the reheat coil
Requires a return fan to avoid air short circuiting back into the room from the bypass; still have contact supply fan energy
Usually only used in smaller applications where humidity control is not as important

13. Variable Air Volume Systems
Objective: reduce flow rate when loads are not as high to save fan energy (fan energy proportional to flow rate cubed)
Especially effective for perimeter zones that may receive solar heating
Temperature is maintain same for all zones, flow rate varied to each
Flow rate bounded at lower end by a minimum air fraction
Concern 1: indoor air quality—outside air may limit lower bound of VAV flow rate
Concern 2: humidity—it may also limit lower bound of VAV flow rate
Terminal devices used to further reduce cooling or to provide heating
From maximum cooling point, VAV first throttles back flow and then adds reheat (or uses terminal device)

14. Dual-Duct Systems: Overview
Two air streams are conditioned at a central location: one warm, one cold
Air distributed to spaces through:
Two ducts (one warm, one cold) and mixed locally, disadvantage of two ducts (both must be sized to handle their maximum flow rate though this may be different for heating and cooling, cost, space)
A single duct per zone after mixed centrally, disadvantage(?) of multiple ducts
Tends to require more energy than a VAV system
Dual-duct can serve one or more thermal zones
May have more than one fan (dual fan—one for each duct)

15. Dual-Duct Constant Volume
Total air flow rate remains fixed, but volume through each duct varies with heating/cooling load
Single fan with reheat
Similar to the terminal reheat system
Reheat applied at a central location rather than at each individual zone
Air is not cooled and then reheated as in terminal reheat
Uses less energy than terminal reheat because some air is heated and other air is cooled
Constant volume system—air flow is constant and thus fan energy is always same (high)

16. Dual-Duct Constant Volume (cont’d)
Single fan, no reheat
Similar to a single-duct system
Simply has a cooling coil bypass and is very similar to a single duct system
Does not expend energy to reheat air, simply uses a mixture of return and outdoor air
Less air is sent through cooling coil, may result in less dehumidification and thus moisture problems during parts of the year (spring/fall)

17. Dual-Duct Variable Air Volume (DDVAV)
Blends warm and cold air in variable volume combinations
Flow reduced below maximum load and flows mixed at minimum flow rate (which might be limited by outside air or humidity concerns)
Can be combined with single duct VAV systems for zones that are cooling only (interior spaces)
Dual-duct terminal units can also serve as separate VAV systems (one warm, one cold) which can reduce fan energy and heating and cooling energy

18. Single Fan DDVAV
Fan sized for the “coincident peak” of the hot and cold deck volumes
Control via two static pressure controllers (one in each deck)
Cold deck typically kept at same temperature though it may be varied
Hot deck sometimes adjusted up when temperature outside is low or when humidity is high (to force more air through the cold deck)
Some systems may use a precooling coil for the mixed or outside air

19. Dual Fan DDVAV Volume of each fan independently controlled
Each fan sized for anticipated maximum coincident hot or cold volume (not sum of instantaneous peaks)
Cold deck maintained with either outside air (free cooling) or mechanical refrigeration
Hot deck can use return air, heating coil, or outside air (rare, humidity concerns) to provide heating

20. Dual Conduit Really a “dual system” configuration
Primary air (constant flow) system:
Used to meet exterior transmission (perimeter) loads
Run only during peak periods
Runs without outside air (can be fairly local)
Must be sized to account for action of secondary system at minimum air fraction
Secondary air (variable volume) system:
Runs year-round
Serves both interior and perimeter spaces
Used to meet the loads from people, lights, equipment, and solar heating
Used to bring in outside (fresh) air
Variation on this system mixes the two air streams using a dual duct terminal box—in this case the primary system is heating only and the secondary system must meet the entire cooling load

21. Multizone Systems: Overview
Similar to a dual-duct system (though only for more than one zone)
Multizone systems share same advantages and disadvantages as dual-duct systems
Multizone systems can be obtained as “packaged” units (lower first costs) of up to approximately 12 zones

22. Three-Deck Multizone Has an additional duct for “unconditioned” air
This duct is in addition to the warm and cold air ducts (hence the name)
Only two of the ducts unconditioned and either warm or cold used at any given time (seasonal switchover)
Generally not used due to the extra initial expenses
Can be mimicked by a standard dual-duct by seasonally scheduling coils

23. Summary Single-duct systems: Dual-duct systems
Main heating and cooling coils in series arrangement
Ducts supply air to all terminals at a common temperature
Capacity varied by varying temperature or flow rate
Dual-duct systems
Main heating and cooling coils in parallel
May use separate warm and cold air duct distribution systems, blending air at the terminal device
May blend air near the main unit; separate duct for each zone
Most vary supply temperature

24. Lecture 14b: Introduction to Secondary Systems
Notes: __________________________________________________________________
________________________________________________________________________
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

25. Special Systems: Dual Air Stream System
Two interconnected all air systems (diagram)
Primary air system used for outside air
Secondary air system used to condition space
Used when there are high space gains or high air flow rates required
Both systems might have heating and cooling coils

26. Special Systems: Under-floor Air Distribution (UFAD)
Also called “up air system”
Air distributed from floor rather than overhead
Air enters space through floor registers or through furniture
Individual control and high degree of flexibility
Floor surface is also “conditioned” (pseudo-radiant system)

27. Special Systems: Evaporative Cooling
Water vapor added to air stream to reduce the dry bulb temperature
Raises the humidity level so this tends to only have application in drier climates
Cooling can either be direct (of supply or outside air stream) or indirect (return air with heat exchanger)
Can offset cooling needs, requires water as a resource

28. Special Systems: Low-Temperature Systems
Concept: lower the supply air temperature for cooling so that we can reduce the flow rate (less flow means less fan energy)
Typically only used in conjunction with ice storage systems
Ice can be used to produce lower supply air temperatures
Cannot produce colder air directly using chiller because lower evaporator temperatures will make chiller less efficient
Terminal units may be required to maintain a sufficient amount of air movement within the space

29. Terminal Units: Overview
Terminal unit is the last (hence terminal) device on the air distribution system (between the duct and the conditioned space)
Two types:
Supply outlet—register or diffuser: goal is to supply air to the space without causing drafts or excessive noise
Terminal unit: controls the quantity and temperature of the supply air (by further conditioning the air)

30. Constant Volume Reheat Terminal Units
Used mainly in terminal reheat systems
Reheat coil adds heat to avoid overcooling (supply air temperature generally cooled down to around 13C first)
Can lead to excessive energy consumption (frowned upon by ASHRAE Standard 90.1)

31. Variable Air Volume Terminal Units
Terminal unit reduces the flow rate in an attempt to provide only enough cooling to match the actual cooling load
Care must be taken to avoid too much flow in areas close to the air handler

32. Throttling Units Throttling Unit Without Reheat
Air flow rate can be throttled down to meet the cooling load
Sometimes flow can be completely shutoff (air quality concerns)
Concerns about noise from throttling (typically have some sort of sound attenuation)
Throttling Unit With Reheat
Air flow rate can be throttled down to some minimum
Below the minimum, reheat energy would be required
Can also be used in cases where there are actual heating loads
Flow rate reduced first, then reheat added
Some systems will ramp up flow rate again once reheat turned on to avoid excessive reheat energy
Reheat is sometimes replace with a baseboard unit

33. Induction Unit
Flow from room or ceiling induced (Bernoulli effect) and mixed with primary air stream
Primary air flow rate reduced while keeping actual flow relatively constant (avoids stagnant air concerns)
Requires higher static pressures to achieve induction effectthis leads to higher fan energy (though overall it should be reduced since this is VAV)

34. Fan-Powered Terminal Unit
Also referred to as “fan-powered VAV box”
Basic idea is to reuse heat from space, lights, etc. to provide the “reheating” (similar to induction except we are now actively moving air with a fan)
Better circulation due to increased air movement (over throttling unit)
Parallel arrangement—local fan is outside primary air supply stream
Series arrangement—local fan is inside primary air stream and runs when space is occupied
Various heater options available to provide perimeter heating (coil, baseboard, radiant system)
During unoccupied hours, main system can be shut off and local fans can be run to meet loads as needed
Increase in fan energy over throttling units possibly offset by reuse of heat from space
Noise a potential problem since fan is so close to occupied space

35. In-Room Terminal Systems: Main Characteristics
Combined air and water system components:
Central air-conditioning equipment
Duct and water distribution systems
Room terminal units

36. In-Room Terminal Systems: Main Characteristics
“Primary” or ventilation air provided either centrally or locally
Central air: handles indoor air quality requirements and latent loads (humidity)
Centralized maintenance and lack of moisture condensation in the terminal units
Requires ductwork but typically smaller than all-air systems
Air can be supplied through the terminal unit or separately from it
Moisture addition/removal can be accomplished centrally
Local air: handles all conditioning requirements locally through building apertures
Eliminates requirements for ducts since outside air is introduced through terminal unit
Increase in maintenance costs due to outside air handling “distribution”
Generally lower first costs

37. In-Room Terminal Systems: Main Characteristics
Water supplied to local coils (“secondary water”) used to provide additional conditioning beyond that which is done by the central air
Usually sensible only—latent cooling at the terminal unit would significantly decrease the life of the unit and could lead to odors/bacterial growth
Any condensate is typically left in space to get reabsorbed by room air at some later time (drain usually recommended)
Can be either heating or cooling
Applications
Primarily exterior spaces with mainly sensible loads and no strict humidity control requirements
Spaces where individual control is important or preferable
Common installations: hospitals, hotels, schools, apartment buildings, office buildings, research laboratories

38. In-Room Terminal Systems: Main Characteristics
Potential for heating without the circulation of air (unoccupied periods, similar to baseboard heating)
Main categories/arrangements
Induction units
Fan-coil units
Two-, Three-, and Four-Pipe arrangements
Packaged units

39. Induction Units: Characteristics
(Requires) centrally conditioned “primary” air supplied to unit
Uses Bernoulli effect to draws secondary air through secondary coil
Requires medium to high pressure to achieve secondary air flow
Secondary air flow is simply room air drawn into unit (through filter/screen and coil)
Units typically installed around perimeter of building near windows

40. Induction Units: Advantage
Individual temperature control on separate thermostats at fairly low cost (biggest advantage)
Ducts and air handling units can be reduced in size since much of conditioning can be done locally
Moisture addition/removal, filtering, and outside air can be done centrally
Space heating during unoccupied hours does not require fan operation
Components last relatively long (15-25 years) with limited maintenance (cleaning filters and nozzles)

41. Induction Units: Disadvantages
Usually limited to perimeter spaces, requiring another system for interior spaces
Primary air flow is constant and flow to units cannot be shutoff individually (local coils can be shutoff)
Secondary air flow dependent on condition of lint screen and nozzle
Potentially lower chilled water temperature needed due to reduce air flow to zones and desire to avoid local cooling coil condensation
Not appropriate for spaces with high ventilation requirements since primary flow is reduced
Local moisture sources (open windows, etc.) can cause unexpected condensation
Higher initial costs and higher operating costs (due to high pressure requirements) than most air systems

42. Fan-Coil Units: Characteristics
Components: coils, fan/fan motor, filter, insulated condensate pan/drain, controls/valves, return and supply air openings
Heating and cooling (though maybe only one at a time)
Forced convection from coil to air using local fan
Outside air locally or (usually) separate from the unit via a central source (example: high-rise hotels)
Local outside air eliminates balancing problem if windows are opened
Local outside air not allowed in commercial buildings because wind pressure changes outside reduce control over outside air
Local outside air systems might require coil freeze protection
Water (hot and/or cold) supplied from central source
Electric heater might be needed for a two-pipe system for shoulder seasons

43. Fan-Coil Units: Types/Locations
Vertical units
Floor mounted, can also be low profile style
Usually installed at the perimeter under window sill
Low profile units can present maintenance problems
Horizontal units
Ceiling mounted, saving floor space
May use ductwork to distribute air (greater pressure required at fan)\
Can also combine air with central (outside) air
Maintenance and condensate handling more difficult though initial costs lower
Chase-enclosed units
Unit is typically floor to ceiling configuration
More likely to see these in hotels and residential buildings
Back-to-back placement with sound treatment

44. Fan-Coil Units: Selection and Capacity/Control Issues
Common technique is to select a unit that can meet the cooling needs of a space at the medium speed setting of a three-speed unit (safety factor and less noise during most operation conditions)
Sizes can be reduced if outside air handled separately and introduced at a temperature close to room air conditions (~21C)/reasonable humidity
Automated control on water flow rate, manual or auto control on air flow rate
On-off control not recommended (noise/circulation issues)

45. Fan-Coil Units: Water Distribution Schemes
General considerations
Pipes refer to fluid pipes entering and leaving the unit
More pipes increases initial costs
Less pipes requires “changeover” strategies and may result in lack of heating or cooling when needed
All can be used with central ventilation

46. Fan-Coil Units: Water Distribution Schemes
Two-Pipe
One inlet pipe, one outlet pipe (single coil)
Heating and cooling happen seasonally with changeover from one to the other (problems during intermediate seasons)
Changeover typically determined by outdoor air temperature, all zones changeover at same time
Can result in frequent changeovers
Two-pipe changeover with partial or full electric strip heat can help avoid frequent changeovers by meeting smaller heating loads
Changeovers not necessary if building is dominated by either heating or cooling

47. Fan-Coil Units: Water Distribution Schemes
Three-Pipe
Two inlet pipes, common return pipe (two coils)
Generally not recommended since mixing the hot and cold water return is a waste of energy
Four-Pipe
Two inlet pipes, two outlet pipes (two coils)
Highest initial cost but best performance of fan coil units
May include a deadband between heating and cooling to avoid simultaneous heating/cooling

48. Fan-Coil Units: Advantages
Individual control of spaces/central water production
Pipes require less space than ducts (easier for retrofits?)
Potential lack of central AHU may also save space

49. Fan-Coil Units: Disadvantages
Greater maintenance costs, maintenance in occupied areas
Multiple condensate drains problematic
Ventilation may not be uniform or guaranteed if outside air is provided locally

50. Packaged Terminal Systems
Similar in concept to fan-coil units except that cooling provided locally by “window AC” type unit
Heating can either be through central water/steam source or local (electric coil/heat pump)
Advantages/disadvantages similar to other air/water systems with additional concerns:
Units have relatively short (appliance) life
Concerns about outside air and water leaks into building through unit
Noise from compressor can be excessive and variable

51. Summary Induction Units: Fan-Coil Units:
Centrally conditioned “primary” air
Bernoulli effect to draws secondary air through secondary coil
Fan-Coil Units:
Forced convection from coil to air using local fan
Outside air locally or separate from the unit via a central source
Water (hot and/or cold) supplied from central source
Many other system types possible: packaged, low temperature, evaporative, etc.