Heil unit doesn't work in auto position

[Johnboy2978]

Hello all. I have a 25 year old Heil gas forced air furnace. Last year the I was having problems with the blower motor and the service guy replaced the squirrel cage and that seemed to do it. He recommended replacing the unit as it was 25 years, at the end of its life expectancy and only about 70% efficient. It is now cold enough to need a little heat in the mornings and today I attempted to turn it back on. My thermostat has both an "auto" and "on" position for the fan. If I leave it in auto, the furnace is no longer turning on the blower motor. If I turn it to "on" it turns on the blower motor which then never turns off but I have heat at least. Suggestions as to the problem here? Would this be the limit control switch maybe?

[HayZee518]

I would check the wiring diagram of the thermostat for what exactly is turned on when in auto mode.
Most fans [circulation blowers] operate off a plenum temperature control. It is essentially a flat bi-metallic element that twists as it expands thereby turning on a set of contacts. The twisted part is called a HELIX.
In the ON setting all limits are bypassed and the motor is always running.

[jimmybob]

I have the same problem,but my blower will come on sometimes and shut off when the burners come on.