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Thread: furrnace not starting

  1. #1
    Join Date
    Jan 2017
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    Default furrnace not starting

    might be a long story. I have a tempstar NUGM100EHB1 furnace. It started acting up this winter. No power to exhaust blower but while checking it out it started working. Work find for a few days then quite again. I changed the fan control board and it worked for one day and then the main blower wouldn't shut off. Put the old board back in and it worked find. I'm not sure why but the furnace seems to work find until I put the doors back on. Than it runs for a few cycles and stops and won't start again. It has new filter . When I put the main blower motor cover back on and the furnace is running there is a lot of suction.
    Last edited by troubled; 01-26-2017 at 03:01 PM. Reason: wanted to give more facts

  2. #2
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    Feb 2012
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    Lala land, OH
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    Quote Originally Posted by troubled View Post
    No power to exhaust blower but while checking it out it started working. Work find for a few days then quite again.
    Could be multiple issues. Bad bearings on the inducer motor (what this is) will cause it to seize up.. It is replaced as an assembly. Mark wire locations and remove it. Spin the blower by hand. It should spin freely and not stop quickly when you take your hand off the wheel. Before putting it back in make sure the port where the vacuum hose connect is clear. Clean it out carefully with a paperclip if needed. If this is plugged your burner will not light. (it's a safety device)

    If it spins freely then you will need a schematic diagram and a voltmeter to test further (as well as knowledge on how to use both). There are several safety switches throughout the furnace that will cut power if it detects that something is not safe. One thing to test before you tear into it.. flip your fan switch on your thermostat from auto to run and see if your blower kicks on. If it doesn't check your thermostat wiring and the low voltage transformer.

    I changed the fan control board and it worked for one day and then the main blower wouldn't shut off. Put the old board back in and it worked find.
    There are DIP switches on the control board. They need to be set to match your existing ones. Make sure you have the correct board. Looks like the other one is not enough... It may be the same basic board but with a different microprocessor/programming. The temp/limit switch shuts down the blower after a heat cycle. They can stick. You can carefully tap on the switch and it sometimes will click off..if below the off temp. You would need to replace this if it's acting up. It's not serviceable. Sometimes once you free it up they work fine.. There is also a relay on the control board that can fail and cause the same thing. The temp/limit switch is usually the troublemaker.

    I'm not sure why but the furnace seems to work find until I put the doors back on. Than it runs for a few cycles and stops and won't start again. It has new filter .
    Sounds like it's starved for air. This can create a carbon monoxide risk.. Very dangerous! The furnace is designed to run with all the covers on. You may have something that is restricting airflow (blockage or failing inducer motor) and opening a cover reduces the restriction of air (but creates a less then ideal or safe situation). If the above tests don't find the problem it's time to call someone out. Newer furnaces are not DIY friendly.


    When I put the main blower motor cover back on and the furnace is running there is a lot of suction.
    This is normal. It's on the low pressure side of the blower. There should be a interlock on this door so that the furnace wont run unless it's on.
    Last edited by Mr T; 01-26-2017 at 06:01 PM.

  3. #3
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    don't think its the inducer motor because it runs freely after it shuts off. cleaned out vacuum lines. didn't find anything in them changed out thermostat he when fan does not come on when you switch it to manual when the furnace is acting up. always had voltage(29 volts ) at control board when calling for heat. I'm thinking maybe the main limit switch might be going bad. It ohms out when I check it . Could it be getting to hot when everything is closed up and opening up when in turns shuts down the inducer motor but when the covers are off there is enough air flow to keep it cool. I installed a carbon monoxide detector a couple weeks ago and it hasn't gone off yet. I think you for your help I have been fighting this for about a month now. Thinking about putting in a new furnace this one is almost 20 years old.

  4. #4
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    Feb 2012
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    Kent, WA
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    Does it work if you remove the air filter and put the furnace cover back on? If so, your filter is dirty or is overly restrictive (MERV number too high).
    Mark
    Kent, WA

  5. #5
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    Quote Originally Posted by troubled View Post
    I'm thinking maybe the main limit switch might be going bad.
    You need to look at your schematic diagram to see what is in line with it. Unless you get very lucky you will only waste money by guessing.
    Look on the backsides of your covers. There is sometimes a schematic diagram and sometimes a troubleshooting guide on one of them.

  6. #6
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    Jan 2017
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    still having trouble with furnace. looked for a schematic on the internet and all I can find is connection diagram and ladder diagram. When I pulled off the hoses on the pressure switch a plug came off with it. It was flat with a stem with holes in it. It was stuck in the end of the hose . the other one didn't have anything like that. Wondering what it's for and should it come out of the switch .

  7. #7
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    Feb 2012
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    Lala land, OH
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    Can you post a pic of it?

    Also please post a pic of the ladder diagram.

  8. #8
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    Oct 2017
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    A pic can give the clear illustration of your problem.

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