It's not uncommon for people to notice that a VAV zone damper sitting at minimum position when the zone is cold and requiring more heat. There is a reason for this! 


According to Guideline 36, a VAV zone damper only increases if two conditions are met. 

1. The zone heating loop output > 50%

2. The discharge air temp setpoint has been met. 


Almost every time I've been asked this question, the problem has been with #2 that the reheat was not able to satisfy the DATsp and the discharge air temp was coming out too cold. 

The idea here is that if you can't maintain DAT with a low volume of air, increasing the volume of air over the same reheat coil will not deliver any more heat to the zone, and in fact would deliver more cold air to the zone. 


For more information on how 75F implemented guideline 36 logic. Check out this article. 

VAV Terminal Profile Logic & Tuners - Heating




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