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wowbw7

garage fuel oil furnace fan problem

wowbw7
9 years ago

I've installed a 10 year old upflow fuel oil furnace in my garage ( northern minnesota). everything works fine except the fan. it short cycles on and off during and after the time the burner is on. the limit switches for the fan are 2 'snap' switches, one for overheat protection to shut off the burner should the firebox get too hot, and the other one for the fan control itself. I believe it turns the fan on at 130 degrees and off at 110 degrees. I've figured out that the problem occurs due to the cold air (40 degrees or less) being sucked off the floor, up the plenum, hitting the snap switch and cooling off. hence the fan stops. 20-30 seconds later it is heated to the 130 degrees and the fan turns on again. and the cycle repeats until the thermostat is satisfied. the question is... how do I trick the snap switch or replace it with something to solve this. thanks in advance for any help or suggestions you may offer.

Comments (2)

  • jackfre
    9 years ago

    I guess you could put in a different limit switch or put in a time delay on the limit circuit. Rather than going there why not raise the return air grill so it is seeing a more blended air temp. You could also drop a supply branch down to the floor to blend that return air temp.

  • wowbw7
    Original Author
    9 years ago

    thanks for the reply Jackfre. I thought about your suggestions about rerouting the warm air down to the return air and it got me thinking. instead of doing that I pulled the snap limit switch from its location low in the plenum and relocated it higher in the plenum and closer to the heat exchanger. now it starts the fan sooner and runs continuously until a minute after the burner shuts down. exactly what I wanted. problem solved. thanks again, Jackfer

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