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Furnace shutting down on limit

Patrick_ Member Posts: 4
So i just replaced the inducer motor on a ruud furnace because the plastic housing was cracked. the old motor was working fine besides being cracked and being a little loud. After putting the new motor on, the furnace proceeded to shut off on limit. it shut down 2 or 3 times before i decided to try putting the old motor on again, and the furnace once again worked fine with the old motor. I warrantied the motor and put the new one on again and am still having the same problem. i jumped out the main limit and the furnace stays on, and the furnace works fine with the old motor. ive doublechecked that its the right replacement. does anyone have any ideas? this is very bizzare.


  • RED76
    RED76 Member Posts: 8
    bum motor

    it sounds like your new motor runs fast enough to pull in the air proven switch but doesn't pull the flue gases through the heat exchanger fast enough so it over heats and shuts off on limit.you could check the amp draw on the motor while its running and see if it matches the name plate rating.
  • Ironman
    Ironman Member Posts: 7,181
    Draft Fan

    You said the furnace had a plastic draft inducer, so I'm assuming it's a 90%+.

    Most manufacturers put a donut shaped orifice at the inlet of the inducer that is sized to match the firing rate of the furnace. You have to re-install the correct size orifice or else the burner will not be drafted properly and the heat exchanger may over heat.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Patrick_
    Patrick_ Member Posts: 4
    thought that too

    Hey thanks for answering back. I actually thought the same thing but i didnt have my meter with me at the time. but i took the motor back to the supply house, they gave me a new motor and it did the same thing. so ive tried two different new motors with the same result. its almost impossible for two new motors to be bad. ive also made sure that it was the right replacement. i honestly have no idea what to do at this point besides just throw the old cracked motor back on.
  • Patrick_
    Patrick_ Member Posts: 4
    edited March 2011

    thanks for getting back. the furnace is an 80% ruud upflow natural gas forced air furnace model #ugdg-10eamer. I have never run across something like this and noone ive talked to has either.
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    Ironman is correct

    some of these motors are designed for several different models of furnaces. In order to do that they put a restricter in for the smaller models and it has to be removed or in some cases a different restricter used. Get in touch with the manufacturer not the supply house.
  • Patrick_
    Patrick_ Member Posts: 4
    no restrictor

    This makes sense. Unfortunately there is no restrictor on it, the back of the inducer is wide open all the way to the screw holes. im going to check the amp draw of the motor tonight, then try replacing the limit and jumping out the thermostat. There is an old t-87 there that might be causing a problem we will see.
  • RobbieDo
    RobbieDo Member Posts: 131

    Check amperage on motor with TRMS not averaging meter, check RPM's on motor and see if it's the same as the old one, make sure there are not restrictions anywhere. What is the temp in the heat exchanger when it trips the high limit?
  • Eric_32
    Eric_32 Member Posts: 267
    good advice...

    I'm with Tim on this one, I'd be on the horn with the manufacture, and get to the bottom of it, instead of the counter guys.
  • Techman
    Techman Member Posts: 2,144

    You replaced JUST the motor?You got lucky or your good. Those flue gases make most of my inducer blower wheels part of the motor shaft.I replace the entire induced draft blower assembly,and on my last change-out a new donut and instructions came w/ it.
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