Originally Posted by tdmaymac
I think I discovered why the overhead fan unit comes on when the furnace setting is selected. There are four fan positions that are available under the FURN mode and these relate not to the furnace blower but the overhead AC unit. I noticed that mine was set to LOW and the overhead fan was running, but when I changed it to AUTO, the overhead unit shut off and didn't come back on. I think this may be the trick for avoiding cold air blowing from the overhead unit when the furnace is selected. If someone has better insight into this problem, please chime in. Thanks.
First, ALL furnaces have an initial purge cycle and after running on heat cycle, the thermostat is satisfied, the gas valve closes to extinguish the LP flame and the blower continues to run to purge any LP from the burner area and to cool the unit after operation. All modern LP furnaces follow the same operational routine.
When you toggle your CCC to furnace, is the fan setting still on? When you exit the cooling mode, your A/C fan should automatically go off and should not have any operational relevance to the furnace.
If you have a heat pump unit, the furnace will automatically commence operation in low ambient temperatures (around 40*F) when the heat pump can no longer produce interior heat. The furnace should automatically take over.
If you are getting the A/C blower when your CCC is set to furnace, you might have your dip switches on the relay board of the upper A/C unit set improperly, the unit might be mis-wired or you simply need to do a re-set of your CCC.
Check it out and get back with what you find.