The auto fan works fine when set to air conditioner, however, it does not work when put to heat (furnace) side. The furnace works because when you put it to fan only it will blow out heat. Just won't blow out heat when put to auto and using the furnace. Works fine with air conditioner Can you possibly tell me how to solve this problem.

What kind of T-stat is it
Digitals have a switch or setting to turn the fan ON with heat
Gas-no fan
Elect-fan with heat

Some non-digitals have a wire (I think it is yellow screwed to an A terminal) that needs to be moved to the Y terminal.

The Honeywell round T-stats have different bases. The base is the part that has the switches for heat cool and fan. They sell different bases for them. I think that A-base is no fan on heat. J-base is with fan

Gas heat normally turns on the fan itself.
Most electric does also except for American Standard or their other names like Trane.

Hope this helps