Blower always blows (slowly) on gas furnace

I have a Furnasman HG 112-36 gas furnace that I believe was the original furnace in the house I just bought. The house was built in 1968.

I replaced the thermostat to a Ritetemp programable immediately after purchasing the house (4-wire setup R, W, Y, G) The problem I'm having is that the blower always blows, albeit at a slow speed. Everything else is functioning properly. The blower will kick in to a high speed when I switch from auto to on, and it maintains the proper temperature.

I switched back to the original mechanical thermostat, and I get the same behaviour.

I am assuming this is not normal behaviour, as it blows cold air when the furnace is not burning.

Do you think this a problem with the limit switch?

 

If you disconnect all the wires from the thermostat (don't let any of the bare ends touch each other), does the fan still run?


Perhaps there is another switch in the house (I once found one under a built-in book case in the living room) or even right on the furnace to turn on the fan for circulation.

Tracing out the wiring will uncover the source of the power.

 

Even when I disconnect the thermostat, the fan still runs slowly. There is a three-way switch on the furnace itself that will make the fan kick up to the high speed, but it will not shut the fan off. The only way I can turn the fan off is to throw the breaker for the furnace.

When I go back to the house, I'll try tracing the wires to see if there is a third switch.

 

This rules out the tstat as the problem.




Yes, your fan control could be temperature activated and the cut out temperature is lower than the ambient temperature.


The three position switch could be bad among other possible problems.


Bottom line: you have to figure out how electric is getting to the fan.

 

It depends on how it's wired up. Could be a relay put in to run the fan on low till a/c heat comes on, and kicks in a higher speed.

Lot of people are running the fan on slow to keep the air movement going to help keep the house temp balanced out. I run our fan slow 24/7 to help keep the family room in the basement warm.