FAQ: Do I need 2 airflow sensors for every VAV with SmartNodes?

Yes!

For heating, ASHRAE Guideline 36 specifies the following sequence:

- When the Heating PI Loop is between 0% and 50%, reset the VAV discharge air temperature setpoint from minimum to maximum.

- When the Heating PI Loop is between 50% and 100%, reset the active airflow setpoint from minimum heating CFM setpoint to maximum heating CFM setpoint (or minimum heating damper position to maximum damper position in non TrueCFM applications).

In 75F algorithms, we look at the differential between the entering air temperature and the leaving air temperature.  If the leaving air temperature - entering air temperature is less than a tuner setpoint, then the active airflow setpoint will remain at the minimum (or the damper position will remain at the minimum position in non TrueCFM applications).

The tuner is called "reheatZoneToDATMinDifferential", and the default value is 9°F.  This can be set as low as 0°F and as high as 20°F in 0.5°F increments.

You can find this under the tuners section of the VAV zone:

NOTE: If you set the tuner value as 0°F, the damper will open when the heating PI exceeds 50%.  If the heating source is not working, the system will be providing an increasing amount of cool air to a space that needs to be heated.