Step 1: Check for heating runtime and discharge air temps
The first thing I check is if the system has been calling for heating recently and if it has, whether or not the discharge air temps rise in correlation with the heating 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.
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 heating 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 heating commands, but my Airflow Temp Sensor isn't reflecting any hot 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 Cool Only and your system should require heating. 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 - ALL zones must require heating before system heating is engaged at the AHU. According to Guideline 36 the bulk of the heating is taken up by the reheats.
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.