Our HVAC Fan only blows in 'fan on' mode while heating. In other words the fan will not blow in heat mode if switched to "fan auto". Both fan on and fan auto modes work when cooling. See image below, any ideas?
I also forgot that I had the blower replaced 1 year back and am now wondering if the wiring of that could be the culprit. See this link for information on the original and new blowers.
Normally, when the fan is set to "auto", if the unit enters "heating" mode the burner ignites then a heat sensitive device sends a signal to a relay which turns on the fan. The fan pushes heated air into the plenum and through the ducts but it also removes heat from the combustion chamber, so if the fan does not start or fails during operation, the burner shuts off to prevent overheating, damage, fire, etc.
If your fan is not coming on it could be a bad heat sensing device or a bad relay (the fan motor and power supply seem fine as you state that it works under other circumstances).
You did not state whether or not the burner continues to operate without the fan running, if this is the case you may have an additional problem if your system is designed (as most are) to shut down the burner upon fan failure.
I had exactly the same problem on HVAC. 1. The furnace blows hot air only in "FAN" mode. 2. When the mechanical thermometer is switched to "AUTO", I could hear the click sound from the unit, however the FAN does not start blowing the hot air at all.
According to the professional, it seems to be caused by the defective heat sensor/thermostat on the furnace unit. The part itself costs about $150.