Variable Air Volume (VAV) is the most used HVAC system in commercial buildings. In this article we’ll discuss the Variable Air Volume system and single duct VAV boxes with reheat coils. The Air Handler varies the amount of air flow (CFM) at the overall system level based on the demand required by the zone level VAV boxes, which vary air flow based on their local demand.
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The VAV box regulates the flow (CFM) to a zone in relationship to the demand of the temperature sensor in the space.
Variable air volume is more energy efficient than constant volume flow because of the reduction in fan motor energy due to reducing fan speed (RPM) at partial load. As the cooling or heating demand is reduced because of a mild temperature day, the VAV Air Handler system can reduce the amount of air flow (CFM) by reducing the fan speed.
Air Handler Unit with VFD
The air handler will deliver a constant temperature of 55ºF (13 ºC) supply air to the VAV boxes. While the supply air temperature stays constant the volume (CFM) of air will vary based on the total demand of all the zones on the system. There are several control strategies to adjust the speed of the fan which we’ll discuss below.
As the VAV boxes open or close due to demand called for by the temperature sensor in the space, the pressure in the main supply air duct will either increase or decrease. This pressure change is picked up by a static pressure sensor in the main supply air duct.
As the pressure increases in the main supply duct because the VAV boxes are closing their dampers and are adjusting their dampers towards the minimum open setting, the air handler supply fan VFD slows down the fan. The opposite will happen due to the VAV boxes opening because of increased demand and the dampers are opening, in this case the VFD will cause the supply fan to speed up when the pressure in the main supply air duct drops.
The VFD will try to maintain the speed (RPM) of the fan so that the static pressure in the duct at the location of the static pressure sensor maintains some minimum set-point, such as 1.25” sp. The static pressure sensor sends a signal to the VFD and the speed of the fan is adjusted according to the set-point required.
Basic Functions of a VAV Box
The VAV box at the zone level will operate in one of three modes: Cooling Mode that varies the flow rate (CFM) to meet a temperature setpoint; a Dead-Band Mode where the temperature setpoint is satisfied and the box is at minimum flow (CFM); and a Reheat Mode for when the space requires heat.
As you can see in the diagram above the VAV Damper goes from a minimum of 30% open, whatever the minimum required to meet ASHRAE 62, all the way to the damper being 100% open.
There are basically three modes in this control strategy. Mode #1 Is the Cooling Mode where the heating hot water control valve is closed and the VAV damper modulates from 30% to 100% open in order to satisfy the temperature sensor. Next is Mode #2 Dead Band Mode is when there is no need for cooling or heating, so the damper stays in its minimum position to meet the ventilation requirements of ASHRAE 62. And Mode #3 is the Heating Mode where the VAV box damper remains in the minimum position and the heating hot water valves modulates open to satisfy the heating requirements of the space.
Function and Components of a VAV Box
The VAV box has a damper at its inlet moved by an actuator that is controlled by the controller that takes its command from a temperature sensor. The process is very simple. When the temperature sensor in the space calls for cooling it sends a command to the VAV box controller which then adjust the supply air flow rate (CFM). The adjustment is done by an actuator rotating the VAV box inlet damper either open or closed in increments.
- Airflow Sensor – is used to adjust the damper position by measuring the air flow at the inlet of the box. The airflow sensor measures total pressure and static pressure to determine the Velocity Pressure which helps the controller determine the CFM through the inlet of the VAV box. Velocity Pressure = Total Pressure – Static Pressure.
- Actuator – Based on the airflow the actuator will power the rotation of the damper to meet the space demand.
- Damper – adjust airflow (CFM) based on the temperature sensor and airflow sensor input.
- Reheat Coil – Depending on the zone, there may be a reheat coil that provides heating from heating hot water, steam or electric. The use of electric is limited in some jurisdiction due to energy codes.
- VAV Box Controller – Taking input from the temperature sensor and the airflow sensor the controller will send and output signal to the damper or heating hot water valve to modulate open or closed. Controls can be pneumatic, electronic, or direct digital control (DDC). Pneumatic is an older form of control and is being replaced by the more energy efficient DDC system.
- Other components used on various other versions of the VAV box, such as fan powered boxes would include fans and filters.
Zoning and Building Loads
Before we get any deeper into this subject we need to cover the basics of zoning. Zoning is how the Engineering divides up the building into separate VAV zones, with each zone getting its own VAV box. To keep cost down its best to limit the amount of VAV boxes used, as each box adds additional cost for material, labor, controls and electrical.
After a heating and cooling load is completed on a building, the spaces will be divided up into zones. Each individual zone will have similar load profiles and be served by the same VAV box. A typical individual zone maybe offices that share a southern glass exposure or interior spaces. Look for a Zone drawing in any set of mechanical plans that has a large area broken down into zones. (See example of a Zone Map Drawing below)
The idea of zoning is to breakdown large areas of a building into smaller zones with similar load profiles. When a zone on the south facing portion of a building is calling for maximum cooling, the north facing zones may be in minimum cooling or heating mode. Zoning allows different spaces the ability to provide cooling or heating and vary the flow (CFM) depending on the demand of that zone’s temperature sensor.
All the zones on a floor of a high-rise maybe fed from the same air handler, but each zone can adjust its CFM according to their specific needs. Depending on the size of the floor plate, there maybe two Air Handlers per floor, or for smaller floors the Air Handler may feed more than one floor. The Air Handler can be located on the floor within a mechanical room or located on the roof.
The supply air main is considered the high side of the system. The high side being the main supply duct from the air handler to the inlet of each VAV box. The main is considered upstream of the VAV box, while downstream of the box is considered the low-side supply.
The air handler will provide 55 F degree (13 Celsius) supply air to the VAV box. The Variable Air Volume VAV box will then determine how much air (CFM) to pass through to the space based on the demand of the space. The air handler is sized to meet the maximum block load of the area it serves. The block load is basically the peak heating or cooling load of all the zones combined. It is not the total CFM of all the peaks of each zone, but the total based on the worst month, day and time of year where the total block is at its maximum load.
Each zone above is reacting differently to the early morning sun. Some zones are in cooling mode with their dampers at different percentages of being open, while other zones are in heating and one zone is off and receiving minimum air for ventilation. This is a very basic diagram of how zones may differ and why it’s important to consider how spaces are grouped together, as each space may have a different solar exposure and cooling load profile. As the sun travels across the sky the zone dampers will open or close depending on their need for heating or cooling.
Corner spaces are often difficult to include with other spaces because they have two exposures. It’s like living on the corner in your block, you have two streets. Looking at the image we can see that there are two cooling zones between corner spaces that are on the south exposure that could be grouped into one zone, Zone #5 below. The same is true for the two zones between corner spaces on the North exposures, Zone #2. If you had interior zones they would be separated from any exterior zone because interior zones are often exclusively in cooling mode due to internal heat gains and the lack of heat loss from any exterior surfaces.
During cooling mode, the Variable Air Volume VAV box will modulate between a minimum CFM setpoint and the calculated design maximum cooling CFM setpoint based on the zones peak cooling demand. When the hot summer arrives and the sun shines through windows and conducts heat through the walls and roofs, the need for cooling will be sensed by the temperature sensors in the space which will call for the VAV box to open its damper and let more cold air into the room. Or, if you’re in a room located within the interior of the building, like a conference room, then the heat from the people, lights and plug loads will cause the temperature sensor to initiate an opening of the VAV box damper for more cold air.
For exterior zones and in certain cases interior zones there will be a reheat coil or an electric heater attached to the VAV box The reheat coil can be served by heating hot water, steam or electric. When in heating mode, the flow (CFM) through the box will be at a minimum setpoint to avoid wasting energy. Remember that the air handler is sending the VAV box 55 F degree (13 Celsius) supply air which was most likely cooled by chilled water from a chiller.
This primary supply air will also bring a percentage of mandatory ventilation air (Outside Air). In some systems the supply air temperature could be increased to a temperature that is just cool enough to cool the most-demanding zone with its VAV box set to maximum flow, thereby saving additional energy.
The heating hot water valve will modulate open providing a range of heating hot water flow (GPM) to meet the heating load. The minimum CFM setpoint can be somewhere between 30% and 50% of the maximum cooling setpoint. Minimums are set by some code jurisdiction so that the minimum ventilation rate is always achieved. In California see Title-24 Sec 120.1 Requirements for Ventilation and Indoor Air Quality. See Ventilation section next.
Using electric heat is not approved in various jurisdictions. Check your local code for approved sources for the heating requirements.
Building Automation System
We’ll mention two control strategies for optimizing energy efficiency using a VAV system. These are the 1) Constant Static Pressure Control Method, and 2) Static Pressure Reset. (Required if there is a DDC system to the zone level)
When the VAV boxes are connected to a building automation system that monitors the function and status of the boxes there are various options for control. This is based on using a DDC system.
#1 Constant Static Pressure Control Method
Usually, a pressure sensor is installed 2/3 rds. of the way down the main supply air duct. When VAV boxes start closing their dampers because they need less cooling an increase in pressure will occur. When the static pressure in the supply duct increases due to the VAV boxes closing their inlet dampers the static pressure in the main supply air duct increases.
The pressure sensor in the duct will send a signal to the Variable Frequency Drive (VFD) causing the supply and return fans to slow down or reduce its RPM. If the pressure in the duct decreases because the VAV boxes are opening due to the need for additional cooling, the pressure sensor will send a signal to increase the fan speed (RPM).
The pressure sensor is set to maintain a constant pressure in the main supply duct which often causes excess static pressure to be provided when compared to option two below. The reduction in the fan speed provides energy savings.
#2 Static Pressure Reset
The use of this strategy is required by Title-24 (California) and ASHRAE 90.1 for system that have DDC to the zone level. The static pressure setting in the main supply duct is reduced to a point where one VAV box damper is nearly full open. This is the zone that requires the most pressure. This would require that the VAV box actuators can report their damper position, best performed with an analog output. Look for Trim and Respond logic for more information.
These options provide a good opportunity to save energy by reducing the fan speed and possibly increasing the supply air temperature in small increments with continuous polling. If the supply temperature can be reset above the economizer set point, then the compressors can stage off and the cooling can be provided by modulating the return air and outside air dampers to deliver the desired supply air temperature.
Using a DDC control system with VAV boxes that have a flow station and temperature sensor at the supply air discharge the system can determine the amount of reheat.
Q = CFM x 1.08 x Delta-T
Q = Btu/Hr
1.08 = A constant based on standard air conditions
Delta-T = (Discharge Air Temperature – Primary Supply Air Temperature)
The building automation system can track and trend over long periods of time the following: Damper position, static pressure, reheat valve position, airflow rate (CFM), supply air temperature, zone temperature and occupancy status.
There are other types of VAV boxes not discussed here such as: Fan Powered VAV Box, VAV Mixing Box (Dual Duct Systems), CAV (Constant Air Volume).
Ventilation ASHRAE 62
Ventilation air (Outside Air) is required for all occupied spaces according to ASHRAE standard 62.1. When using VAV boxes the minimum volume setting of the box needs to ensure the larger of the following:
1. 30 percent of the peak supply volume;
2. Either 0.4 cfm/sf or (0.002 m3/s per m2) of conditioned zone area; or
3. Minimum CFM (m3/s) to satisfy ASHRAE Standard 62 ventilation requirements. VAV terminal units must never be shut down to zero when the system is operating. Outside air requirements shall be maintained in accordance with the Multiple Spaces Method, Equation 6-1 of ASHRAE Standard 62 at all supply air flow conditions.
The use of Variable Air Volume (VAV) has been shown to save energy when combined with a supply fan VFD’s. As the demand in the spaces fluctuate the VAV box dampers open or close proportionately and the air handler fans respond through various control strategies. Variable air volume systems save more energy than a constant volume system.