AHUs in HVAC system design configurations
In this third portion, review guidelines, standards, and basic concepts for designing HVAC projects using AHUs.
In terms of functional definitions, most HVAC air systems fall into two major types of systems for designers to consider. One system type is to provide a constant air flow and vary the temperature of the air delivered, typically referred to as a constant-volume, variable air temperature system.
The second type used will vary the amount of air delivered to the space and keep the temperature the same, referred as a variable-volume, constant air temperature system. However, with new technology and increased sophistication of BAS, modern HVAC systems can be a hybrid.
An AHU in a constant-volume system delivers constant cubic feet/minute (cfm), or airflow volumes, all the time regardless of the HVAC full or partial loads. As designers know, HVAC loads can change for several reasons including: building shell load gains and losses as the day progresses and based upon the time of year, building occupancy levels, and building internal heat gains or losses. As these loads change, one or more room thermostats monitor these changes and will signal an adjustment, usually through a type of BAS or HVAC zone level controller, to the AHU supply or leaving air temperature (SAT and LAT, respectively).
Thus, these systems have variable supply air temperatures, and vary as needed to maintain the building HVAC zones temperature setpoints whether in heating or cooling mode. Note, however, the overall design of the HVAC system may place some system components outside of the confines of the AHU itself.
A variable air volume (VAV) system AHU is designed to deliver variable volume cfm at all partial load conditions but at a constant supply air temperature. As the HVAC loads change, the HVAC zone monitoring thermostat(s) signal a required adjustment to the amount of air being delivered to the space. This variable volume then maintains the HVAC zone air temperature at a constant setpoint. The AHU’s air volume can be varied by different methods, but the preferred technology is using a variable frequency drive (VFD) connected to the AHU’s fan motor. A VFD’s frequency outputs are programmed to match the airflow volumes required and then adjust the fan speeds accordingly.
Either constant- or variable-volume AHUs may contain several different types of components to provide the heating or cooling needed to maintain or vary the supply air temperatures, respectively. These could be steam, heating-hot water, or even electric strip heat coils for heating or chilled-water or direct expansion (DX) cooling coils for cooling.
There also may be several other components used such as energy recovery devices, which could affect the AHU air temperatures. Whatever the means, there will be some type of control sequence that modulates a component, such as a hot water or chilled water control valve, or multiple stages of DX or electric heat capacities to vary the AHU supply air temperature to match the load requirements for the required mode of operation.
Modulating steam, hot or chilled water coil control valves for part loads is typically easier to provide acceptable load matches and thus temperature setpoints. Staging electric strip or DX coil capacities is more difficult but proper controls sequences and appropriate setpoints with dead bands associated with selected thermostats works well.
Variable refrigerant flow (VRF) systems that are now being employed in many applications will function well with the capacity changes and provide very good temperature control. Some of the systems being designed can use the refrigerant in the system to heat some zones while other zones are being cooled.
Dependent upon the application, there are several subsets of the two systems above that can be used by designer’s when working on projects with AHUs. Some of the systems will be briefly described even though they may no longer used in designs, but because they can still be found in older buildings being renovated.
Constant-volume, single–zone (CV-SZ): Probably the most commonly used system in HVAC is the CV-SZ, sometimes also called constant volume, single duct. This AHU system is very simple and, regardless of several forms they can take, they are applied and provide room comfort to only one zone. Thus, a common trait is that the AHU is controlled by one room or zone thermostat only and has no terminal equipment under control. Examples of this system subset would be smaller SZ AHU room fan-coils units (FCUs), packaged DX single-zone rooftop systems (RTUs), and the ubiquitous residential split DX system.
In Figure 4 the center figure from point (A) to (D), through to Zone 01 and as shown in solid lines, could be a CV-SZ unit. There would likely be a heating coil ahead of the cooling coil, shown between the fan location options, or could be placed in the OA ductwork to preheat the incoming ventilation air when needed. As shown, the unit could be a draw-through unit or alternately be a blow-through unit dependent upon the placement of the supply fan. Although a simple system, depending on the application and capacity required, the AHU could have some more advance components and operate under full airside economizer mode. A return/relief fan shown in dashed lines may or may not be required. The schematic also indicates the possible input/output (I/O) points that a BAS might monitor and/or control for proper operation.
Constant-volume, single–zone, with face and bypass dampers on cooling coil: This system is basically a CV-SZ AHU with an extra bypass section and damper assembly, along with a face damper assembly in front of the coils. It’s not as common as in the past, but designers might see them in older buildings set for renovation. There are some designers that have been placing similar sections in recent hospital AHU designs. The dampers are used to adjust the amount of mixed air that goes through the face of the coils as well as the amount that bypasses, thus the name face and bypass.
The dampers are controlled by a room thermostat to maintain a constant room air setpoint. As room temperature drops, the bypass dampers open at the same time the face dampers close thus raising the AHU’s discharge supply air temperature. As the room temperature rises, the dampers are modulated to provide more air through the coil section and less to the bypass. This system design was developed as it generally did a better job of maintaining the zone’s relative humidity at part load over the previous CV-SZ design.
Constant-volume, single–zone, with reheat: Because the CV-SZ system controls to constant LAT, and the supply air is dehumidified regardless of the room part load conditions, there could be comfort issues in the zoned space. This system could be provided with terminal reheat for the zone so the supply air is reheated to satisfy the room thermostat (Figure 4 with a reheat coil in the Zone 01 ductwork). This is a little more expensive to operate as its costs to both cool and dehumidify the supply air, then warm up or reheat the airstream. For energy reasons, standards like ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings, and several codes, limit this system type. Some designers using this system type, use recovered energy to assist in providing the reheat in the form of condenser heat from the cooling system, heat recovery chillers or condenser reheat plus sensible heat recovery from exhaust or return airstreams using energy recovery ventilators (ERVs), etc.
Constant volume–multiple zone (CV-MZ) system: The additional of terminal reheat with the constant volume AHU system above could provide fixed volumes of cooler supply air to more than one zone (Figure 4, with the two dashed zones Zone 02 and 03 added), and could be considered a multi-zone (MZ) system. Each zone then reheats this air as required by zone thermostat. Control becomes slightly more important as the supply air temperature must meet the cooling needs for the worse zone. The reheat terminal equipment can be any of those mentioned above, and generally provides good control for a combination of zones with similar temperature and/or humidity requirements. However, the system also works well in zones with different HVAC loads.
Early MZ systems were modified to enhance the control and delivery of conditioned air. The modified units became very popular and were truly used to provide a way to get good zone control when serving multiple zones in buildings. They are not commonly designed anymore, but may be seen in some older existing buildings today. The design began to be replaced when VAV systems were implemented and when energy costs became less affordable. The early systems were like the single zone AHUs as discussed above, but instead of a single duct, there were numerous runs of ductwork to each zone or space from the AHU. Figure 5 will be used to indicate a few versions of this system type.
The systems were typically blow-through and usually consisted of both a cooling and heating coil, but placed in parallel and separated internally in the AHU. In Figure 5, there are two zones coming off the end of the AHU and would be considered a small AHU. Smaller AHUs could have up to four or so duct runs routed from the unit. Larger MZ AHUs could have as many zone duct runs as possible, all routed out of the mechanical room where the AHU was placed, with only space constraints as the main restriction. Case in point: a maximum of 14 zones have been seen on one AHU, and those units were in older facilities on some military installations.
These constant volume, MZ system AHUs were designed to provide comfort to multiple zones by mixing cold air and hot air so the mixed air (MA) in each duct run would satisfy the zone thermostat. The AHU had both a cold deck (CD) and a hot deck (HD), where both maintained a constant leaving air temperature (maybe 55°F and 95°F, respectively), with mixing dampers mounted on the front or top of the AHU. Refer to Photo 3, which illustrates damper actuators on a smaller MZ system.
Each set of zone dampers were on a common shaft and offset by 90 deg, so if cold deck was full open, the hot deck was closed and vice versa. A damper actuator was controlled by a room thermostat, and it positioned the dampers to meet the room temperature setpoint. Too cold, the actuator opened more to the hot deck to warm the mixed air and increase the supply air temperature, thus warming the room. If the zone was too warm, the actuator modulated to open more of the cold deck damper, and close the hot deck damper, to cool the mixed air and lower the supply air temperature. Figure 5 illustrates these dampers schematically.
A twist on this system was called a Texas MZ AHU. This AHU was designed for hot and humid climates, and replaces the hot deck with a bypass deck, or neutral air deck, for the return air (RA) coming from the zoned spaces. Typically, the cold deck is overcooled for dehumidification and space humidity control in these hot/humid climates. To save reheat energy, the heating coil would be removed and RA used to reheat the mixed air stream. A reheat coil would be placed in the zone ductwork (Figure 5) to provide any additional heating required when the neutral deck dampers were fully open, and the space continued to drop in temperature. Photo 3 is a Texas MZ with hot water reheat coil in the ductwork above in the upper right.
Multiple zone, dual duct (MZ-DD) constant volume and variable volume system: This system type is like the CV-MZ, as it has both a hot deck and cold deck on the discharge side of the supply fan, but there are no mixing dampers at the AHU (Figure 6). Operation of the AHU is the same as the MZ, and the hot deck and cold deck leaving air temperatures are again maintained at a constant setpoint. The separate airstreams are delivered to the zones by two sets of supply ductwork routed around the building parallel to each other. Each zone is provided a dual-duct mixing box with two taps, one each connected to the cold deck and hot deck and controlled by the zone thermostat. The mixing box now contains the two dampers on a common shaft, offset by 90 deg, modulated from a single actuator and controlled by the room thermostat to provide the required zone comfort.
With a MZ-DD constant volume system, a constant volume of air is provided to each zone mixing box, and the mixture of hot and cold air is changed to vary space temperature as signaled by the thermostat. Control of the AHU becomes important as the ductwork static pressures in both hot deck and cold deck continually vary as the boxes try to maintain their constant flowrates. To save energy, the hot deck and cold deck setpoint temperatures can be reset higher or lower depending upon the season and the mixed air temperature requirements, but overall the operation of these constant volume MZ-DD systems was an improvement over constant volume reheat systems.
Dual duct variable volume (DD-VAV) systems are even more efficient than the constant volume MZ-DD because the AHU’s hot deck and cold deck airflows can be modulated and controlled more efficiently in each zone by the zone thermostat. Energy savings can be realized through reduced reheat and supply fan power. Typically, this is accomplished by using DD VAV boxes reducing the total volume of air to the space to a minimum amount prior to mixing either hot deck or cold deck air with the minimum airstream.
These MZ-DD systems are expensive to operate as the hot deck and cold deck are kept at a constant leaving air temperature. Energy efficiency is reduced because of the cooling and reheating process, and the fact that comfort is only achieved by creating a mixed airstream. Designers must be aware that not only does the cold deck supply air temperature need to be low enough to satisfy the worst zone relative humidity, the hot deck supply temperature must be warm enough to satisfy the worst zone heat loss during the heating system.
Use of these systems are again limited by most energy codes due to the high energy usage required for proper operation. Many of the systems have been replaced throughout the years, but some still exist as retrofitting them to new systems can be costly as well.
Variable air volume for MZs—VAV and VAV with reheat: Variable air volume (VAV) systems started to become popular when designers, determined to lower operational costs, started to replace the higher energy using constant volume systems. VAV systems typically use AHUs with supply fans on variable frequency drives (VFDs), and with a single supply duct which will provide a constant discharge supply air temperature to a building’s multiple zones.
Airflow is varied by VAV box terminal units which have a single-damper that modulates the airflow to a zone as signaled and controlled by zone thermostat. At the required full load condition in a zone, the VAV box damper should be wide open. At all part-load conditions, the box damper is always partially closed to some amount due to the lower demand for zone cooling.
Variable volume systems are more energy efficient than constant volume systems because overall total airflow requirements are reduced. Each VAV box will typically have a minimum airflow setpoint for ventilation, and can modulate down to that flow when the room/zone thermostat allows. Modern BAS should use fan pressure optimization type sequences to poll all the VAV boxes in a building, and reduce fan speeds and associated duct static pressure setpoints to maximum this potential for total building fan power savings.
A cooling-only VAV box is typically used for building interior zones and provides only cooling year-round. Because ventilation requirements fix a minimum VAV box airflow, some zones may begin to overcool at various times of low cooling loads. VAV boxes can then be provided with reheat coils. Once the lowest airflow is met, a reheat coil will be modulated and/or staged on to increase the supply air temperature to satisfy the room/zone thermostat. These VAV with reheat boxes will also be specified for perimeter zones where heating is needed during the year.
Another option for zones requiring reheat is to employ parallel fan power boxes (FPB) with reheat to be more energy efficient. A parallel FPB adds a small plenum on the side of a cooling-only box along with a small direct drive fan with a back-draft damper on the discharge side. When the box reaches minimum flow, and is fixed, a reheat sequence begins by starting the fan. This fan pulls RA from the plenum, typically a few degrees warmer than the supply air, into the box to mix into the discharge airstream. If additional heating is necessary, the reheat coil on the parallel section will modulate to increase the mixed air temperature further until the zone thermostat is satisfied. The sequence is reversed once the space starts to overheat.