Air handler blower doesn't work in "Auto" mode
Hi everyone. New here and hoping to get some help. The problem I am having is that sometimes in "Auto" mode, the blower fan doesn't run. Thermostat calls for heat, the furnace starts and the pipes are hot. Only thing is that the blower fan doesnt start by itself. I suspected that the problem may be with thermostat but replacing the thermostat did not help. If I turn it to the "FAN ON" position it works great but fan will not kick in automatically the "FAN AUTO" position. What should I do? The problem is intermittent and seems to happen when the attic is very cold (not fully sure if this is the cause). Please help.
The air handler is made by FirstCo and is about 10 years old.