Step 1: Check for cooling runtime and discharge air temps

The first thing I check is if the system has been calling for cooling recently and if it has, whether or not the discharge air temps drop in correlation with the cooling calls. 

Here's an example of a zone that has been heating recently, and the discharge temp rising into the low 100F range as would be expected if the unit were actually heating. Heating is a bad example of cooling, but its the middle of winter, finding a cooling example was a little tougher. 


If this all checks out, your question is probably answered. But there are two cases where this doesn't check out. 

1. Not seeing any cooling commands recently when you feel you should have expected to. You'll learn more about how to handle this issue in the steps to follow. 

2. I am seeing cooling commands, but my Airflow Temp Sensor isn't reflecting any cool air being produced. This issue suggests a mechanical or wiring issue that should be serviced by your HVAC contractor and can not be resolved remotely.  


Step 2: Check what conditioning mode your system or zone is set to. 

It's not uncommon for modes to get changed and forgotten. So before you dig too deep, make sure you check your modes are set such that heating can be expected. 

For a DAB or VAV AHU, this is found under the system. You can check both the current setting and the trend data for the mode to ensure the mode hasn't recently been changed.



For a HyperStat zone, you check the Conditioning mode from the zone itself. You can check both the current setting and the trend data for the mode to ensure the mode hasn't recently been changed 



As long as your zone isn't set to Off or Heat Only and your system should require cooling. Then you should be good here. 

Step 3: Understand when your system should heat

In general, there are two types of control algorithms used by 75F.

DAB - weighted average of the zone load determines how much heating or cooling to call for. This same algorithm is used for HyperStat profiles, the only difference is the weighted average is of a single zone. With DAB zone priority can also be taken into account. 

VAV - If any zone requires cooling, then the system will provide cooling. How much cooling is determined by a voting mechanism, trim and respond. More zones requiring cooling, the more cooling the system will provide regardless if one or many zones need heating. 


If you find yourself in a situation where your system just doesn't seem to be operating correctly. Contact your 75F rep so they can help look more closely and find out what the culprit is. 




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