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Blue Springs, MO

Gas Furnace Acting Crazy


I have a gas, forced air furnace system in my home that has started acting up over the last few (coldest) days.

I might have the themostat set to 74 degrees. It may get down to 68 or 69 degrees and doesn't kick on. Sometimes it will, sometimes it won't. It's just been sporadic. I can hear furnace ignite downstairs, and the fan down there blows, but the blower doesn't kick on for the house.

Here's the thing. I played with the thermostat settings when it stopped working and had a few weird results.

Thermostat set to Heat setting, and fan setting set to AUTO....the air doesn't blow in the house. All I hear is the furnace ignite downstairs.

Thermostat set to Heat setting, and fan setting set to ALWAYS ON...same thing as above.

Thermostat set to OFF, and fan setting set to ALWAYS ON....the fan starts blowing...albeit cold air.

Any ideas on this? Does it sound like a bad thermostat, or something do do with the furnace? The thermostat is a decent digital one, and haven't had any problems until now.


Computer Help Forum

Heavy Artillery For The Little Guy
Tulsa, OK
Hmmm, could be the blower motor thermostat. Blower motors are thermostatically controlled to turn on after the furnace has reached operating temperature and to remain on after the burners quit till the temperature drops below the threshold. If yours isn't working it will not kick on when trying to heat and then the safety will kick it off.

Do you have the manual for your furnace?
"Fascism should more properly be called corporatism because it is the merger of state and corporate power." -- Benito Mussolini

Fairfield, CA

2 edits
reply to shane
Check the furnace filter for blockage, the furnace should turn off the burner if the temperature in the plenum gets too high due to a blower blockage, so the fan will kick on then turn off soon after and the house will get cold.

Leander, TX

1 recommendation

reply to shane
You can't troubleshoot a furnace in the basement by playing with the thermostat on another floor.


Gillett, WI
reply to KrK
Newer furnaces actually time when the blower kicks on and off. This function is now controlled by the circuit board usually located in the blower compartment.


Gillett, WI
reply to shane
I suggest you actually verify by sight that the burners actually firing when the thermostat calls for heat.

Old and Cranky
Cortland, OH
reply to shane
Sounds like a dirty "flame sensor"

Novelty, OH
reply to shane
You need to confirm if the burners are actually igniting.

You say that a "fan down there blows" but the main blower is not kicking in. I assume you are describing the inducer fan. If that is not working properly, or the air flow sensor is not working, then it won't try to ignite the burners.

Or, perhaps the gas valve is stuck, so not actually allowing gas flow to the burners.

If they are indeed igniting, then you need to look at why the main blower is not starting. Potential problem with plenum heat sensor, or bad motor/starter.

stay warm
reply to shane
It is either 2 things as mentioned in earlier post,
either the blower motor is shot (I favor this answer) or the furnance is not lighting up.
If it is blockage like PrntRhd suggest the blockage is most likely carbon build-up which you better pray it is not as most repair people will not service it.
Call in a repair person as soon as you can before something bad really happens.

·NorthWest Tel
reply to shane
Is this a newer furnace with an inducer fan and electronic control board?

If so, the control board should indicate an error code through a blinking light pattern (refer to instruction manual for decoding).

The error code will give you a better idea of what's going on. Is it failing to sense flame or is the blower failing to come, causing the furnace to trip on high limit? That's what you want to know before any further troubleshooting can be done.

Also, you can diagnose this manually by simply watching the furnace while it turns on with a call for heat. If the flame burns for only a few seconds and turns off, you've got a flame sensor problem. If the flame runs for around a minute or two then turns off, then it's tripping on high limit since the blower is not coming on.
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