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Tom, HVAC Technician
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Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
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My Lennox Diplomat furnace 80 MGF 3/4 100 r-2 is working intermittently.

Resolved Question:

My Lennox Diplomat furnace 80 MGF 3/4 100 r-2 is working intermittently. Limit switch fault flashing now. No fault until heat demand made by thermostat. Then the furncace blower comes on with no flame and blowing cold air. Cleaned the flame rods. Tried shutting the gas and power off for 15 minutes. This was working fine for 3 days straight after having the same issues prior to it seeming to resolve itself.
Submitted: 7 years ago.
Category: HVAC
Expert:  Tom replied 7 years ago.

Hi, Tom here. Sorry you are having furnace troubles.


A limit switch fault indicates an over temperature condition. Some things that can cause a limit switch fault would be a dirty air filter, defective blower, dirty air conditioning coil, defective heat exchanger, blocked flue. Anything that prevents proper airflow through the furnace can cause a limit switch fault.


According to my sources, your furnace would have 5 limit controls. There is the primary or main limit located in the front of the heat exchanger (above the burners sort of behind the gas valve). (automatic reset) There are 2 roll out switches located on either side of the burners. These are manual reset. And there should be 2 automatic reset limits located in the back of the blower housing.


Since you are not even getting the burners to light and the blower starts immediately, it sounds like you have a limit that has opened and not reset. At this stage you need to do a little trouble shooting to see which one is causing the trouble.


You could try jumpering them out one at a time until the furnace works normally or if you have a volt meter you can check it quickly that way. You should have 24 volts ac from ground to both sides of the limit. The ones behind the blower are going to be difficult to get at. It would be best to follow the wires and try to jump them out external to the blower.


After you get the limit thing figured out, you will need to watch the furnace and find out why the limit went bad anyway. There could be an underlying issue like one of the things I listed above. Often times a furnace with an airflow issue will cycle many times on the limit without the homeowner being aware of it. This usually happens when the weather is sort of mild and the house maintains temp. When things get really cold outside and the furnace needs to run long cycles these sort of issues can crop up and when a limit cycles enough times they often die.


If you are not comfortable checking these sort of devices, please don't hesitate to call in a furnace tech. They should be able to run down the problem quickly.


Let me know if you have further questions. Thanks, Tom

Customer: replied 7 years ago.

I am not an HVAC tech(obviously) but what do you mean by "jumper" them out. The 2 wires going into the gas limit switch for example-Just connect the 2 terminals together? Please explain more.


Expert:  Tom replied 7 years ago.

Yes, sorry about that. The limits are a simple open/closed switch. They should be closed to tell the furnace everything is OK to turn on the burners.


The limit switches are connected in series, so if any one of them opens it will kill the burner.


You can connect the 2 wires together to test. Use a piece of wire or paper clip or something to hook them together. IF you use something like a bare wire or paper clip, make sure the bare metal does not touch anything metal on the furnace. That would cause a short circuit and could damage the control board or transformer. If you are unsure, just wrap a little tape around it so that it cannot touch metal.


Just turn the power off to your furnace first, make the jumper connection and then turn it back on to try it.



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