A variable air volume (VAV) system is designed to meet the heating load of a building with the minimum supply airflow defined via the "Minimum Turndown Ration" value (typically 30% of the full airflow capacity). If the minimum airflow is not sufficient to meet the heating load in the zone(s), the system will first try to increase the zone reheat coil hot water supply temperature starting from the working setpoint value and increasing to the maximum temperature in the loop.
If raising the temperature in the hot water loop is not sufficient to meet the building loads, the air handing unit (AHU) will slowly try to raise the total supply airflow. Since most of the time buildings are not exposed to the maximum heating or cooling loads, an actual airflow through the AHU unit is almost always less than the maximum (sizing) airflow value. Unlike a VAV system, AHUs in constant air volume (CAV) systems are always supplying the same amount of airflow regardless of the actually heating or cooling load.
The following example highlights how changing the airflow affects the mixed air temperature in AHU unit:
- CAV and VAV AHUs are sized to total supply airflow of 10,000 ft3/min (cfm)
- Outside Airflow is 2,000 cfm.
- Minimum Turndown Ratio for the VAV unit is 3,000 cfm.
- Zone Air Temperature is 70º F (21.1º C)
- Outside Air Temperature is 20º F (- 6.7º C)
- Supply Air temperature is 55º F (12.8º C)
Let's assume the heating load in the building is as such that an air handing unit in the variable air volume system is supplying 4,000 cfm. When the total supply airflow is 4,000 cfm, 2,000 cfm of the return air at 70º F will be mixed with 2,000 cfm of the outside air at 20º F, resulting in the mixed air temperature of 45º F.
At the same time, the CAV system will mix 8,000 cfm of the return air at 70º F with 2,000 cfm of the outside air at 20º F resulting in the mixed air temperature of 60º F.
Since the Supply Air Temperature setpoint is set at 55º F, the AHU heating coil in the VAV system will have to add heat to the airflow to meet 55º F temperature setpoint. Meanwhile, the heating coil in the CAV system will not have to add any additional heat because the temperature of the mixed air will be 60º F, or 5º F higher than the actual setpoint.
Let's take a look at the CAV (top) and VAV (bottom) 24hr period airflow diagrams for a typical heating day where the blue line is the total supply airflow and the red line is the outside air airflow.
The graphs below show the VAV system will most likely increase the total supply airflow in the morning during the warmup period. As the day progresses, the AHU in the VAV system will start throttling down the total supply airflow since raising the temperature in the hot water loop will be sufficient to meet the zone heating loads in a heated building. Because of this, the amount of return airflow at 70º F will start decreasing and the temperature of the mixed air will become lower and lower, resulting in the AHU heating coil being used more frequently to maintain the 55º F supply air temperature.