Re: OT furnace fan limit switch question??

Posted by Bus Driver on October 24, 2006 at 12:16:06 from (66.44.250.218):

In Reply to: OT furnace fan limit switch question?? posted by PhilcaseinWPa on October 24, 2006 at 11:41:02:

Sometimes there are two switches and sometimes one with two functions. The fan control starts the fan when the heat exchanger temp gets up to the preset ON temp. This is so that the fan does not blow cold air for the time that the heat exchanger is warming up. If the system is designed and installed correctly, the fan will carry away the heat as fast as the furnace can create it. Clogged filters, undersized ducts, slipping fan belt, things like that can mean that the heat continues to build. The second switch and/or function is the high limit which will shut off the burner if the heat exchanger temp reaches that value. It is possible that some furnaces regularly cycle on the high limit without the occupants being aware of it. The second function of the fan control switch is to stop the fan when the heat exchange temp drops to the lower/off setting. This lets the fan get a bit more stored heat from the unit after the burner stops, increasing efficiency.

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