what would cause the home furnace fan to contantly run?

boring

boring

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The furnace fan would not shut off. no fan switch on the thermostat. temperature above thermostat setting. shut off power to the furnace, turned power back on after 10 seconds..no difference. Shut off power to the furnace for about a half hour, seems to be working ok now. I am concerned that the furnace may be on its way out, any thoughts?
 
The first thing I would do is check to see if you have a On/Auto fan switch. It's a flimsy white button. Auto means it comes on when the furnace is ready for a heat cycle. On means it will run constantly.

If you do not have that button, then the solution gets a little trickier.
 
It is the thermostat mine did the same thing replaced with new one from Lowes 20.00$ good luck and oil the blower motor and replace the filters.
 
Check for a stuck fan switch. It's that box on the side of your furnace, there maybe a button on it and pull or push on that, because it maybe set to fan mode and then it will run continuously. If that doesn't work replace the fan swich. These are typically 110V, turn off the power remove the power leads and install a new one. When you pull it out it will have a long sensor containing a spring. It operates by sensing heat from the furnace and turns your fan on and off.
 
There are hundreds of furnace models in service. Depending on the model, the problem could be mechanical, electrical or electronic. Most likely candidates are a bad fan switch, bad fan relay or bad temperature sensor, depending on the model. Probably won't be expensive to repair, but it could get that way if you don't know what you're doing.
 
thanks for the responses everyone. There is no fan switch on the thermostat itself. Would the pilot light in the furnace being higher than normal cause the fan to run? It was very windy here last night. The furnace runs more when its windy, but the strange part is the thermometer shows higher than what the thermostat is set to when its windy and the furnace runs more.
 
I'm not sure if I follow. Is the furnace putting out heat and won't shut off or is it just the blower?
 
In order of likelihood:

Thermostat (cheap and easy to replace)
Relay (A little more expensive and DIY if you're handy)
Control board (expensive and best done by a trained tech)

To rule out the thermostat, just turn off power to the system at the circuit breaker, disconnect all of the wires going to the thermostat (after taking notes as to which goes where), and turn the breaker back on.
 
just the blower that was running nonstop. The burner seemed to be working as normal, accept it seemed higher than normal yesterday. Thanks for the continued help.
 
If you've got a metal box like this on the outside of your furnace, try killing giving it a rap with a screwdriver handle:
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If that kills the fan, you know it's the fan relay. Don't open the box unless you've killed power to the furnace via the breaker panel because there's 110v or more running through that circuit. If that is your problem, the relay is pretty easy to replace but if you doubt your ability to work on electrical stuff, leave it to a pro.
 
I keep expecting it to do it again. It has not done it since friday. any thoughts on what caused it? I shut off power to the furnace for about 50 minutes for the problem to go away. Thanks again everyone.
 
It could have been residual magnetic field on a relay combined with a weak spring, assuming it has a fan relay. That's a shot in the dark since I have no idea what brand, model, age or even fuel type the furnace is.
 
I had a similar problem a few years ago except my blower fan would turn on and off on its own with all settings turned off.

Turns out that my control board had fallen off of its mount and was shorting out against the furnace housing.
 
a bunch of stuff fried on it overnight. Time for a new thread. Thanks just the same everyone for the help.
 
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