When I woke up this AM my heater was blowing cold air and the fan wouldn't turn off. All other gas appliances, etc. worked fine. The indicator light on my furnace shows an open limit switch, but I have no clue where to even look for this. A little help?

A lot of times they are sort of a diamond shaped piece of black plastic with a round metal cap in the middle and a couple of wires. There should be a couple of them inside the furnace near the burner assembly. One usually closes after the gas comes on and heats the air up to start the blower. The other one usually stays open unless the first one doesn't close. It that case, when the furnace gets too hot because the blower isn't running, it opens shutting off the gas. Don't understand why switch being open would make the blower run. Should be just the opposite. The limit switches, along with the relays they control cause most furnace problems.

You may be able to tell from a schematic on the furnace which way the limit switch should work. If you have an ohm meter or test light. Pull the thing out and put a hair dryer to it and see if it switches when the heat hits it. The problem could be a sticking relay, but unfortunately, likely it is on the board and difficult to troubleshoot.

It would be a lot easier to answer this question if I knew what type of furnace your dealing with?? For example: some late model lennox and trane furnaces have several different devices that could be open usually you have a few rollout switches a primary limit and sometimes a secondary limit they are wired in series, I would first start with finding why the furnace is overheating, dirty filter, lack of air flow? If you can tell me the model of the furnace I can help you locate/ check the problem

https://www.askmehelpdesk.com/heatin...tml#post269193

I'm so glad I spent 2 days putting this information together so that no one would read it.

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Originally Posted by hvacservicetech_07

It would be alot easier to answer this question if i knew what type of furnace your dealing with??? for example: some late model lennox and trane furnaces have several diffrent devices that could be open usually you have a few rollout switches a primary limit and sometimes a secondary limit they are wired in series, I would first start with finding why the furnace is overheating, dirty filter, lack of air flow?? if you can tell me the model of the furnace i can help you locate/ check the problem

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It's a Rheem, only about 7yrs old.