c_shah wrote:
Furnace fan can be set to "auto" and "ON", I ususally keeps it on the
"auto"

I think when you keep the fan "ON" the fan will run contineously while
on "auto" it will start and stop automatically as per the need.

Does anyone know which weather condition you should keep the fan to
"ON"? Thanks in advance.

It is to be used by a service man.