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Hot Air Furnace - Fan and Limit Controller - can't adjust high limit

Hello, I have a hot air furnace, and the fan limit controller was replaced with a new Honeywell device by a technician a few months back, but ever since then the burner stops working too early, and only luke warm air flows through the ducts. Which might heat up a normal house, but this 200+ year old house is full of holes, so unlike before, it can now take hours and hours to raise the temp in the house 10 degrees.

So I took off the cover of the fan limit controller, and saw the three levers. The were set at 110 degrees, 140 degrees, and 180 degrees. I could not adjust the burner set point lever higher than 180 because the adjustable limit stop was also set at 180.

Two questions:

1) How can I adjust the limit stop? I looked at various Honeywell instruction manuals, and they all say to stick a wire in the hole right above the word "caution" on the scaleplate, and that doing so is supposed to have the effect of releasing the lock on the limit stop. But there is no hole -- there is kind of a stamped squarish impression that looks like, if I forced it open, could be made into a hole. But I didn't want to break anything, so I did not try to force it. Any suggestions?

2) For a leaky old house, what should the temperature settings be for the three levers? If I set the third lever to 200 or maybe even 205, then what should the other two be to compliment that?




  • billtwocase
    billtwocase Member Posts: 2,385

    The high limit has a stop to prevent it from any higher than 180 which should be warm enough. I would check the length of the new control compared to the old one. It sounds like either the control is no good, been rotated manually, or it is longer and closer to the heat exchanger. I would also check your cold air returns and filter. That will also get it to hit high limit, and move very little air.
  • 180 is enough?

    Thanks for the advice. The technician had taken away the old controller when he replaced it. So nothing to compare it to.

    So I went to check the state of the filters -- SURPRISE, there were none! The technician had taken them out altogether, probably in hopes of correcting my complaint that the burner kept going off too early. So now I need new filters too.

    Anyway, could I set the high limit to be maybe 200 or 210 without problems, instead of 180 degrees? And if so, them my original question, how to how can I adjust the limit stop beyond the factory pre-set of 180 (see question #1 above)?

    If not, is there a way to measure the temp inside where the controller is, so I can verify if it's really cutting out at 180 (since it might be faulty and cutting out earlier).
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    I suspect another problem...

    Is there a humidity bypass loop in the ducting? If yes, close it off and see what happens.

    Does the duct work go through a substantial amount of unheated space? Does it leak? Is it insulated?

    As for sensing, you'd need a small meat cooking thermometer. Drill a small hole on the supply duct (watch out for AC coils!). Also check the return air temperature.

    As for losing filters, that's not a normal service protocal. If the appliance has been run for an extended period of time without filters, the curved blades on the blower may be severely impacted with dust/dirt/lint and may be severely limiting the air moving capacity of the blower. Generally speaking, you should expect to see around a 100 degree F rise in air temperature across the furnaces heat exchanger..

    If you know for a fact that the tech yanked your filters out, I'd call him and demand that he bring new filters back out and put them in place. Furnace shouldn't be run without them.

    As for adjustment of set point, you have to hold the round portion of the dial in place, and press in on the stops, and while it is in, can move it around, then release and allow it to pop into position (there are notches). I wouldn't adjust it to higher than 200 degrees, and if the furnace is in fact hitting that high of a temperature, then something is seriously wrong.

    You should also have them perform a CO test on the flue gasses, and an inspection of the heat exchanger to look for cracks or leaks.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • heatboy
    heatboy Member Posts: 1,468
    Why on earth......

    ....would you want to raise the house 10 degrees?  Setbacks of more than 2-3 degrees in an old drafty house is certainly not going to save any money at all.  Set it and forget it.  A system that has been designed correctly, recovery should take forever, if it all, when it gets to design temperatures outside.

    The Radiant Whisperer

    "The laws of physics will outweigh the laws of ecomomics every time."
  • billtwocase
    billtwocase Member Posts: 2,385
    another thought

    After reading Mark's post, the blower came to mind. Has the motor been replaced recently? Perhaps the wrong rotation? Sounds like the air filter was left out due to a known problem. We have a few that deliver adequate air only with a basic filter installed, but install a pleated filter, and it does not. If you can raise it to 200, ok, but no higher. If all is correct, it shouldn't hit that mark. It is only a safety, and not an operating temp.Can you supply us with some furnace info? Brand, burner, nozzle size, belt or direct drive blower, pics of unit, return air vents upstairs, etc.
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