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High limit switch

I've got a fan limit switch on a 275000 btu oil fired Thermo-pride furnace that wont shut the fan off. I've changed the switch and even changed the location of the switch but the fan just runs forever until you tap it. The settings are 75 degree's off and 150 degrees on and 200 degrees for high limit.Ive changed the settings a couple of times to higher turn off point but it still seems like it sticks . The system will cool down to room temp (around 62 degrees) and hold. Does anybody know of another kind of fan limit switch to use other than honeywells dial type? Any help would be great .Thanks John


  • Firedragon_4
    Firedragon_4 Member Posts: 1,436

    use to make one, try contacting Metromatic in Medford, MA



  • Glen
    Glen Member Posts: 855
    fan off is too high -

    move fan off position to above 100 deg F. This way the forced air won't feel drafty to your client (body temp 100 Deg F) on a cool down cycle and the fan should shut off.
  • Empire_2
    Empire_2 Member Posts: 2,343
    I agree

    85* air will feel cold to most people, especially to the elderly. Raise your cut off. Also, is the control heater Assist? 24vac)???
    You could use a "Cam Stat" control with a timing circuit, but I personally dont like them.
  • Big Ed
    Big Ed Member Posts: 1,117

    I try to look at it from a different angle if you do a repair and it does the same thing. As the guys mentioned 85*cut out is too low for a helix f&l. And you mentioned you already tryed a higher off setting.

    Maybe it's doing it's job and it's not getting enough air to cool the heat exchanger ? Could be a simple solution like a dirty air filter..

  • C-TEC
    C-TEC Member Posts: 14
    Fan limit

    I also agree that the "fan off" setting is too low. Try raising it above 100F. Also, if has a heater built in, it could be wired improperly and keeping the element hot. Check the wiring and make sure that the heater, if it has one, is wired to energize only on a call for heat.
  • clammy
    clammy Member Posts: 3,094
    Printed circuit

    Perhaps you sould check in the main control box and see if there is a fan time delay board had a unit not so long ago doing the same thing ended up being the fan time delay board was intermittly bring the fan on and some times it would work correctly .Replaced the board and all is fine i believe it was a honeywell part and was on a ducane oil fired unit peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • clammy
    clammy Member Posts: 3,094
    chk clean outs

    Did you check the clean out sometimes if there ia alot of soot and debris in the flue passes it will give you residual heat and bumthe fan limt back on 'check to see if chimmey and all furnace clean out make sure she's good andf clean peace clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
    EBEBRATT-Ed Member Posts: 15,322

    Make sure the fan switch on the control is in the auto position and not calling for continous fan.
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