furnace not responding to thermostat

[devilonmyback]

I have a MGHA 077ABFC-01
77k BTU

I came home one day in June and the furnace was running full blast even though the thermostatwas in the off position. Didn't matter what I did with thermo it would not shut down. I had to shut furnace off manually on unit itself. This happened once before about 18 months ago. My contractor fixed it and it was fine all this time. I called him and he told me he could not recall what he did and would have to revisit. Trust me he's not scamming me for another house call he's a personal friend. He honestly can't remember what he did. In meantime I tried new thermostat thinking maybe this went bad. No luck, problem persisted. In meantime since this first occurred 18 months ago, no work has been done on wiring or electrical, its just a spontaneous thing. One of my neighbors had same exact thing happen in her unit.

I'm thinking its a short in system and some kind of bad relay that tells thermostatto shut down furnace. Can anyone offer advise where to look?

[DaleM]

This happened in June, how hot was it in the place that day?

[devilonmyback]

Are you asking how hot it was before or after the furnace was running? The day it happened was warm/hot to begin with. When it happenedthe temp went sky high over 100 degrees.

[DaleM]

The reason I asked is that there are times when a pilot model furnace will reach a high enough temperature to turn the fan switch on. The little bit of heat from a burning pilot combined with the heat from the sun beating on the roof of the home and the furnace roof jack causes a high internal temperature in the furnace. Normally doesn't happen if the air conditioner is running. NORMALLY the burner is not operating, just the fan is blowing. Most just turn the furnace switch off or turn the control valve to OFF so there is no pilot burning.

[devilonmyback]

OK that's fine for when it was summer and temps were high. Problem still persists now that its cooler. My unit does not have AC regardless. Where else should I be looking?

[DaleM]

You can try cycling the furnace without the thermostat connected to eliminate the wiring to the thermostat and the thermostat as the problem. Pull all the t-stat wires off at the furnace. Wire just red and white at the furnace together then turn it on. After it runs for about 4 minutes disconnect them. Furnace should cycle as normal. Do this a few times, if blower is eratic at all something wrong in furnace. If not, problem is most likely in wiring or thermostat.