Variable Air Volume Systems - Variable Air Volume (VAV) systems are the best way to zone especially in large buildings. A VAV system with the proper controls and set-up will help the building owner realize large savings in energy usage. There are many VAV systems out there set up in different ways. The most common set-up (for DDC) is, of course, usually the cheapest to install.
They have an air handler (DX or Chilled Water) which supplies the VAV Boxes with a fixed pressure and temperature of the conditioned air. The VAV Boxes have electric reheat which will heat the air if that particular zone calls for heat (it is not uncommon to also find VAV boxes that have hot water reheat although this usually cost more to install but can save in energy costs). More on how VAV boxes work.
Variable Air Volume Systems - VAV System Diagram and Design
Fan Powered VAV Boxes | VAV Box Advantages
The three main types of VAV boxes available are:
- Parallel boxes (both these boxes (series and parallel) are fan powered)
- and just a plain old VAV without a fan
Either type of box can offer zone comfort and energy savings if the proper controls are installed. Secondly, if they have the appropriate programming for the sequence of operation. Additionally, the system needs to be properly balanced and calibrated. All that being assumed, let’s look at a sequence of operation for a DDC controlled VAV box. We start at the thermostat which is reading the zone temperature.
If the zone temperature is too cold the DDC backs down on the damper. The damper allows air from the primary source (VAV AHU) to feed the zone. Depending if the box is parallel or series then the blower fan kicks on. This allows air from the plenum (or above the ceiling) to be redistributed into the zone (this air is usually warmer than the primary air supply). If the zone temp continues to drop then DDC will close the damper to minimum position (not closed but an engineered rate of CFM minimum flow) maximizing the use of warmer plenum air and activate electric or hydronic heat.
Fan Powered VAV Boxes | Set Points
Additionally, in the background, the zone controller is sending a heat request back through the network to the AHU equipment controller. As long as the equipment controllers (which is receiving input back from all the zone controllers) does not have any requests for cooling then it should adjust the Supply Air Temperature set point up (with the proper programming). This prevents the compressors (or chilled water valves from opening) from cycling using unnecessary energy. Additionally, as more zones call for heat and their particular dampers begin closing, the static pressure will rise.
Through DDC, the variable frequency drives should slow the speed of the AHU blower allowing the static pressure to settle at a predetermined pressure. This application uses only the amount of energy necessary to keep the zones satisfied and is far better than systems which used to run flat out with little control. Note that some systems do not have variable frequency drives to control static pressure but use vortex dampers on the blowers.
Types of VAV Boxes
Additionally, there are VAV boxes that are fan powered and there are VAV boxes that are not fan powered. The main purpose of fan powered VAV boxes is to make use of the plenum air. The air above the ceiling where the fan powered VAV box is located. This air is usually warmer than the air supplied by the VAV air handler. So in the sequence of operation for a fan powered VAV box, and a non-fan powered VAV box, would include closing the damper to allow minimal air from the air handler and at the same time energize the fan contactor so that the fan in the VAV box comes on and pulls warmer air from the plenum.
Plenum air in most VAV box applications is also, usually, the return air for VAV air handling unit. When the fan in the fan-powered VAV kicks on it pulls this plenum air into the VAV box. Furthermore, it mixes with the minimal airflow coming from the VAV air handler. Then it either hits a reheat hot water coil or electric heat strips. The air is warmed further to a desired supply air temperature (SAT) set point. This happens in the DDC program if the VAV system is so equipped. Finally, there are also other set points which the DDC program will monitor including CFM’s.
VAV Box Control and Schematic
The analog signal coming from the DDC controller will modulate the damper open and close (and everywhere in between to maintain the programmed set point) to maintain the desired CFM’s in either the fan powered VAV boxes or the non-fan powered VAV boxes. Fan-powered VAV boxes are used to help increase efficiency by making use of the warmer plenum air and mixing it with the cooler air handler air. It takes less BTU’s to heat 70-degree air than it does to heat 60-degree air.
DDC/Building Automation Specialty Link Resources:
VFD Cost Savings – see an example of how much you can realize in energy savings from using Variable Frequency Drives compared to the older systems. An example shows a one-year payback for installation and equipment costs. Department of Energy Resource – a link to building energy software tools. CtrlSpecBuilder.com – CtrlSpecBuilder is an excellent resource especially for engineers.
Reliable Controls® Corporation manufactures simple, flexible, and economical BACnet® software and BACnet® controller products that control the Heating, Ventilation, and Air-Conditioning (HVAC) equipment found in commercial, institutional, and agricultural buildings. Automated Buildings.com – an online magazine and web resource that provides news and connections to the building automation industry.
EPA School Design Tools – design specs for schools. HPAC – heating, piping, and air conditioning engineering magazine. BACnet.org – a website is dedicated to providing the latest information on BACnet. Portland Energy Conservation, Inc. – good energy conservation site concerning HVAC. Variable Air Volume Systems…….
Variable Air Volume Systems - VAV Boxes