furnace fan

I have an oil furnace in which the fan continously runs. I have checked the digital thermastat and the fan setting is on auto, I have tried to switch fan setting from auto to on and back again hoping it would correct itself but as of now it still just runs. The fan has run for 2 days straight now, what could this be? Any help would be greatly appreciated!! Thanks so much in advance!
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l snipped-for-privacy@hotmail.com wrote:

There are a number of possible causes, some depending upon the model. FWIW, this is not something that you should be tinkering with on your own. Do yourself a favor and call a respectable service company.
hvacrmedic
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RP wrote:

Call me ignorant, but wouldn't there be a fan limit control on the furnace itself?
On our furnace if the fan limit is set too high then the fan will run continous. Recommended settings are on the control box.
-Felder
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you:.
BTW...you said that you made sure that the fan was set to AUTO and not Manual...so if that is the case and you are still having problems...then here goes.....:
The typical way a furnace runs is that the thermostat is set for a desired temperature. The thermostat tells the furnace that heat is needed so the burners kick in and heat up the heat exchanger. When the desired heat is reached in the exchanger of the furnace, the fan kicks in and blows warm air all over the place until the thermostat reaches it's temperature goal. Then the thermostat tells the furnace it has enough heat and then tells the burners to stop....BUT....the fan will keep running until the heat exchanger has cooled down enough to safetly kick the fan off. . The reason for the fan to cool the heat exchanger after the heat cycle is so the heat exchanger does not heat warp or becomes heat fatigued. The heat exchanger will "eventually" break down after many years of use...but the fan running for about 3-5 minutes after a heat cycle prolongs that from happening. Knowing this,.......... there is a temperature sensor in the heat exchanger that sends a signal to shut the fan off after it has reached its "cool" point and THAT I beleive is your problem... the sensor is not telling your fan to shut down. Print this out and call a furnace repair dude.....give him the make and model of your furnace and you'll save time and money as he'll know what to bring. It's about a 50 dollar fix plus his costs. Don't do it yourself as if you set it up wrong, you could do serious damage to your exchanger.. hope that was of some help....Jim
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Jimi wrote:

I believe that the reason for the delayed off on the blower is to extract the remaining heat in the hx, thus providing increased efficiency. Another reason is to prevent limit trips on shut down. I've honestly never heard anything like warping of the hx as a reason. If anything, I suspect that the rapid cool-down will increase metal fatigue.

Not necessarily. Most newer systems have timed blower on/off cycles, and most of the boards that control this delay will also cause continuous blower operation on a limit trip, which is one of the best reasons for this homeowner not to tinker with it; limits don't usually trip unless there is a potentially serious problem with the system.

HTH.
hvacrmedic
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If you can, disconnect and seperate the wires at the T-stat( mark them so you know where they go). If the fan shuts off, you have a problem with the T-stat. However if the fan still runs, go disconnect the T-stat wire at your furnace. If the fan shuts off, you have a shorted T-stat wire somewhere between the furnace and the T-stat. If however the fan still runs, you have a problem with the furnace. Perhaps it could be a stuck relay.
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