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  1. #1
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    Electric furnace high limit?

    Hello folk's, I'm hoping someone with experience on electric furnaces can help me determine why mine is being shut off by the high limit switches. We have disconnected 2 elements to limit the output to 10kw. My problem is, no matter which elements we activate, after a few minutes running they will trip the high limit switches and cycle on and off.

    I have a 20kw Borg Warner electric furnace, model 2XM06502006A that I am putting back into service after sitting a few years unused. I am installing it with no duct work as it will be used to warm a cold building after being away and then not used while we are present.

    As mentioned above it doesn't matter which/how many elements we activate, they will all trip their perspective high limit switches. The blower is on high and we have tons of air flow, we have no restriction in the cold air return, in fact it does it with no return or filter installed.
    The output side is open with no duct work connected other than a short 90 degree duct which is 12"x12" (same as furnace output size).The elements glow very slightly at the bends, just enough glow to detect they are on.

    While trying to diagnose the problem I noticed the wiring schematic refers to the high limit switches as "switch limit, combination cycling & single operation". I'm not sure what combination cycling means?

    Although very unlikely, how probable is it that I have four over sensitive high limit switches?

    Would anyone have any suggestions on what may be causing this?

    Much appreciate any tips.

  2. #2
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    Quote Originally Posted by Canuck View Post
    Hello folk's, I'm hoping someone with experience on electric furnaces can help me determine why mine is being shut off by the high limit switches. We have disconnected 2 elements to limit the output to 10kw. My problem is, no matter which elements we activate, after a few minutes running they will trip the high limit switches and cycle on and off.

    I have a 20kw Borg Warner electric furnace, model 2XM06502006A that I am putting back into service after sitting a few years unused. I am installing it with no duct work as it will be used to warm a cold building after being away and then not used while we are present.

    As mentioned above it doesn't matter which/how many elements we activate, they will all trip their perspective high limit switches. The blower is on high and we have tons of air flow, we have no restriction in the cold air return, in fact it does it with no return or filter installed.
    The output side is open with no duct work connected other than a short 90 degree duct which is 12"x12" (same as furnace output size).The elements glow very slightly at the bends, just enough glow to detect they are on.

    While trying to diagnose the problem I noticed the wiring schematic refers to the high limit switches as "switch limit, combination cycling & single operation". I'm not sure what combination cycling means?

    Although very unlikely, how probable is it that I have four over sensitive high limit switches?

    Would anyone have any suggestions on what may be causing this?

    Much appreciate any tips.
    Can you check what the amp draw of your blower motor is suppose be ,and check it and see what amps it is drawing. With no duct work there might not be enough back pressure against the blower which will make it run fast, over amp, but not move enough air. It could be loading up and bypassing air.
    Also check your 240 voltage, make sure it not too low.
    Without having you wiring diagram its hard to trouble shoot. Do you have the wiring diagram? later Paul

  3. #3
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    Thanks for the response Paul.

    We are seeing approx a 3 amp draw from the blower motor. A friend mentioned that there is a spec on the furnace for this and we should be providing the proper input back pressure to match the rated /tagged rating. I'm not so far able to find this tag.

    I did install all the covers and the air filter and it runs much longer before the high limit trips out, so I'm assuming what you mentioned may be where we have to go to get it right.
    I did attempt to add more inflow air resistance by partially covering the air filter, which only made the elements glow real hot which I know is not good or proper.

    Our supply voltage varies from 248-253v.

    Between a buddy and myself, I believe we have very good understanding of the schematics and that we are connected and functioning as designed.

    That said, is there any other info I can provide that may lead to a way to diagnose this further?

    Am I correct in assuming it would be near impossible to have 4 bad high limits switches? I'm almost desperate enough order them in to try it.

    I much appreciate your assistance!
    Last edited by Canuck; 03-25-2010 at 09:02 AM.

  4. #4
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    A further update.....if I provide a considerable amount of restriction to the output air flow, it runs without tripping the high limits.
    It sure makes the elements glow red and the air is very hot, in fact too hot to have your face in front of.

    I have read that bright red glowing elements is not good or proper, so I'm back to thinking a couple of new H/L switches might be in order along with a slight amount of output back pressure????

  5. #5
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    Quote Originally Posted by Canuck View Post
    A further update.....if I provide a considerable amount of restriction to the output air flow, it runs without tripping the high limits.
    It sure makes the elements glow red and the air is very hot, in fact too hot to have your face in front of.

    I have read that bright red glowing elements is not good or proper, so I'm back to thinking a couple of new H/L switches might be in order along with a slight amount of output back pressure????
    Are you sure you are getting the right limit switches, Do you know what size
    the blower motor is? The amp draw of the motor is on a plate that is on the motor. Is your blower wheel good and clean? If the blower needs restriction,
    when you restrict it the blower will move more air, so if you go too far your elements can get hot. If you want you can email me a copy of your wiring
    diagram so i can look it over. paulm989@hotmail.com There is no restriction
    of air flow where the limit switch is at??
    What you are saying about it not turning off when you put a lot of restriction and coils red hot, does not make sense. You could take pic. and send them also. Later Paul

  6. #6
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    Sorry, I may not be describing what I'm doing very well...I'll try again.

    The limit switches are the original switches, I have not replaced them. Probably 20 yrs old?

    Without me blocking some of the air flow coming out of the furnace, the elements are only slightly red at the tight bends and the high limit switches are getting hot enough to cycle the elements on and off, approx every 30 seconds.

    When I partially block some of the air flow coming out of the furnace, the elements will glow red hot over their entire length, but they will run continuously without the high limit switches turning them off.

    I'm assuming the air flow restriction I am adding is causing the air flow to reach the back of the elements/plenum where high limit switches are located and cooling them better....even tho the elements are getting much hotter??

    The schematic is glued to the access door so I don't think I'll be able to copy or scan it to send it.....thanks for offering tho.

    The squirrel cage looks clean from underneath, but I'm about to go and remove it along with the blower motor to check it again and get the motor spec's.
    Ha...no small chore as the furnace is hanging from a ceiling.

    I'll post once I have it out and closely looked over.

    Thanks again

  7. #7
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    Quote Originally Posted by Canuck View Post
    Sorry, I may not be describing what I'm doing very well...I'll try again.

    The limit switches are the original switches, I have not replaced them. Probably 20 yrs old?

    Without me blocking some of the air flow coming out of the furnace, the elements are only slightly red at the tight bends and the high limit switches are getting hot enough to cycle the elements on and off, approx every 30 seconds.

    When I partially block some of the air flow coming out of the furnace, the elements will glow red hot over their entire length, but they will run continuously without the high limit switches turning them off.

    I'm assuming the air flow restriction I am adding is causing the air flow to reach the back of the elements/plenum where high limit switches are located and cooling them better....even tho the elements are getting much hotter??

    The schematic is glued to the access door so I don't think I'll be able to copy or scan it to send it.....thanks for offering tho.

    The squirrel cage looks clean from underneath, but I'm about to go and remove it along with the blower motor to check it again and get the motor spec's.
    Ha...no small chore as the furnace is hanging from a ceiling.

    I'll post once I have it out and closely looked over.

    Thanks again
    If these limit switches are 20 years old they very well could be weak. Another factor we have now is maybe the position of the unit.
    Some unit are made for up flow, some for down flow, and some horizontal left or horizontal right. Not all unit can be use in all positions. Some unit you have to change the position of the control parts inside the unit depending on how you are installing it. Do you have the booklet for that unit? if so read it all the way through. Remember heat goes up and if the limit switch is not in the right place then it can cause your problem. Later Paul

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