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My furnace is running OK and heating the house. The problem is that I have the thermostat set to 66°F and it does not turn off when it reaches that temperature. I can turn it off manually at the thermostat but the blower continues to run for a very long time before it shuts down.

I have a very old dial thermostat. Could it have gone bad?

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What type of furnace (brand, model, etc.), and how old is it? –  Tester101 Nov 29 '11 at 20:16
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When you say it "runs for a very long time" how much time are you talking about exactly? Furnances normally run the blower for a while after the actual heating shuts off -- basically, until the core is actually cold (I think they are both timer-based and temperature-based controllers, depends on the furnace). –  gregmac Nov 29 '11 at 21:44
    
good point that @gregmac brings up .... you could check if the furnace is still running or if it's just the blower circulating the air around. I know the blower on my furnace stays on for a good 5 minutes after the furnace has turned off. –  Scott Vercuski Nov 30 '11 at 2:23

2 Answers 2

In my experience the old mercury trip thermostats with the bi-metallic strip are wildly inaccurate. My experience was similar to yours, I had to crank the thermostat down as low as it could go to get the furnace to turn off and crank it high to get it to turn on.

I'd recommend going to the local hardware store and picking out an inexpensive digital thermostat. Here is a link to a good example. It doesn't have a ton of features but it will get you the accuracy you're looking for. They're pretty easy to install, only a few wires to hookup (2 for your furnace and 3 for central AC if I remember correctly).

I've installed a similar thermostat in every place I've lived in if a mercury trip thermometer was in place ... even rented apartments.

There may be a problem with your furnace but this is a good first step to take and even if it doesn't solve the problem it's a good investment for your place. Easy to read, easy to change and very convenient.

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If the blower is running constantly after the heat has turned off, it might be that the limit switch needs replaced. They will keep blowing the smaller blower trying to cool off the furnace because the limit switch isn't letting it know to turn off.

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