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High limit switch open- but only faults in night and extreme cold

I have an Armstrong Air furnace that is not blowing hot air. Noticed the LED light is flashing four times, which, according to the alarm code means I have a high limit switch open. Turned the furnace off and back on but that didn't do any good. Strange thing is, it only only faults in night and extreme cold. Already replaced air filter ans reset entire system. problem still persists.

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  • 2 more comments 
  • tonalligroup Jan 02, 2009

    I drained but the problem persists. There was less than 1 cubic in water in the line. Any other suggestions?
    Thank you

  • tonalligroup Jan 02, 2009

    I drained but the problem still persists. There was a bout 2 fl oz of water in the line. Any other suggestions?
    Thank you

  • tonalligroup Jan 11, 2009

    No, it does not have any other listings. It has since been serviced by a HVAC technician and he replaced the high high limit switch. $80 including parts and labor. Thank you

  • jmrfxit May 11, 2010

    does the code four have any other listings? (secondary limit,or flame rollout)



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  • 242 Answers

Hello , I can try to help you out on this one , It sounds like what is happening is that your exhaust vented up is condensating from the heat in the pipe and the cool in the attic or wherever the exterior of the exhaust pipe travels. It is very common because your exhaust/flu pipe is not insulated.
To fix this issue you must drain the water/condensation from the small 5/16th hoses in front of the blower assembly , just pull them loose and angle them downward to drain. This should fix the problem if this is the problem.
Let me know how it turns out or if you have anymore questions. Phil

Posted on Dec 31, 2008

  • Phil Brantley
    Phil Brantley Jan 02, 2009

    If the unit is still giving you the same problem and code-reset the unit turn on the heat and while the ignitor starts glowing tap on the gas valve soft then harder , the gas valve is in front of the blower assembly and may have a on/off toggle they are normally 4" by 4" square and the gas line feeds right into it. These valves are known for sticking and if you get it going this way or not its your problem , somtimes the tapping works but only temporarily .

    You will need to order a gas valve assembly for your model armstrong. They are not hard to replace , you need a cresent wrench and a pipe wrench and gas line thread sealant. Getting that one out will be most difficult , turn the gas off before the unit of course. Check your connections when finished by putting soapy water on them. To get the valve you may find it on line or your local A/C warehouse. If you have any questions let me know.




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Normally if you have a limit switch problem on a hot air furnace it is the manual high limit switch that should be located near the blower. If you lost power while the unit was operating the heat build up will trip this switch.
Normally they are about 2 inches round with a red button in the center. Firmly push the button till it clicks.
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The best way to find the problem is to trace the low voltage control circuit till you find the open switch. This can be challenging and time consuming, especially if you are not used to doing it.

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