Commercial building heating, ventilation and air conditioning (HVAC) systems commonly use variable air volume (VAV) systems for their heating and cooling needs. VAV systems can be more energy efficient when properly controlled and operated. We frequently find these systems performing less than optimally and recommend a variety of energy saving solutions depending on current operations. One way to increase energy efficiency and yield other benefits, such as improved occupant comfort, is an approach called time-averaged ventilation (TAV).
What is time-averaged ventilation?
ASHRAE Standard 62.1 and California Title 24 allow for ventilation to be provided based on average conditions over a specific period. This approach allows a VAV damper to be closed for a short period of time, before being opened again, during occupied periods. We call this time-averaged ventilation (TAV), aka intermittent ventilation. By using this strategy, zone airflows can be effectively lowered to values below the VAV box controllable minimum value, while still maintaining enough fresh air for occupants.
How does TAV save energy?
The VAV box and the damper controller determine the controllable minimum airflow. Ventilation code applied to the area the VAV box serves, and the expected occupancy of the area, will determine the required ventilation minimums. When the required minimum ventilation is lower than the controllable minimum of the VAV box, then TAV can be applied to reduce the airflow. Lower airflow can save energy by reducing fan energy and reducing mechanical cooling loads due to tempering ventilation air and providing additional tempered air to cooling-only zones.
Addition benefit of TAV
Time-averaged ventilation can also increase building occupant comfort through reducing the risk of overcooling. In interior zones that do not have reheat coils (cooling-only boxes), there is no way to warm the air above the temperature that the air handler provides. If critical zones (think perimeter or high load areas) require cold air, then that same air will be delivered to those cooling-only zones. If those zones are in deadband, this may be colder air (more cooling) than those zones need, possibly causing overcooling.
An example of this is a cold interior office during a hot summer day. TAV can allow for less of that cold air to be delivered, which can provide a more comfortable space. The minimum air delivered to those spaces is the code-minimum allowed amount over an average time.
TAV is typically applied to a deadband condition when VAV box airflow is at its lowest. When more cooling is needed, then the airflow is increased, and the controllable minimum is typically not an issue.
How do I Implement TAV?
The old rule of thumb for VAV boxes was that the controllable minimum is 30% of the max cooling airflow of the box. More recently, this has moved to be about 20% of max cooling airflow. Research has shown that most boxes and modern controllers can reliably control to even lower minimums. TAV control strategies make the controlled minimum less relevant, because the airflow will be intermittent, cycling the box damper between open and closed. So, no matter what occupants have enough fresh air to exhaust contaminants from zone air.
Note: if your VAV box controllable minimum is greater than 30%, we would recommend that you do a functional test to determine if it can be reduced to 30% or lower. ASHRAE Guideline 36 has a procedure for determining the controllable minimum.
TAV is now included in ASHRAE Guideline 36, 2018 version (High-Performance Sequences of Operation for HVAC Systems). Guideline 36 has a host of great sequences and TAV is just another excellent component. The sequences look like this:
The following diagram illustrates TAV as detailed in Guideline 36.
TAV Diagram (ASHRAE Guideline 36)
Note that the sequence calls to start the closed period after a random number (fractional) multiplied by the open period. This randomized start helps ensure that multiple zones do not enter TAV mode at the same time and to avoid the synchronized opening and closing of VAV dampers. This approach also aims to ensure that the total airflow is as constant as possible over the total cycling time even if all of the VAV terminal units enter TAV mode at the same time (e.g. when a building-wide temperature setback occurs).
Start benefiting from TAV
The TAV approach can help improve your building’s comfort, save energy, and save money. If you need help in evaluating the minimum ventilation requirements in your building, or with implementing optimal sequences of operation, let us know. We’re here to help.