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Whitewater, WI | I am assuming it is a high efficiency/ pvc vented furnace? If it is a standard efficiency/ metal vented, this can change some diagnostic procedures. The limit that you are talking about is mounted to the inducer housing? This is an auto reset limit and not a manual one? There should also be an automatic reset high limit switch in the center of the furnace, above the inducer if it is a high efficiency up flow. There is also usually two manual reset rollout/ limits mounted to the burner rail. When it fails, is it throwing a code on the board? It will be a series of blinks that you should be able to see through the clear window in the bottom door. Sometimes it can be hard to see but its there. On a goodman it should be red. There will be a chart on the inside of the bottom door that will tell you what it is.
How did you determine that limit on the inducer is what was opening? Just checking bases.
Here are a couple ways to work through the troubleshooting to figure out what area it might be. I am assuming the short cycling is a new problem that hasn't been ongoing for the last 20 years? How many btus in the furnace and roughly how many square feet is the home? If it is tripping the limit on the inducer, I would lean more towards a plugged secondary issue rather than a home airflow problem. Start by removing the top door of the furnace, allowing unrestricted combustion air into the burners and try it. Does it still short cycle? If it does not, you may have a restriction in the intake, however, that usually won't let the furnace run at all because it won't make the pressure switch. If it still short cycles, and this I want you to be careful with, remove the exhaust pipe from the top of the furnace. Run it for a normal period of time that it would typically start to trip the limit. If this stops it from short cycling, it leads me to believe that your secondary heat exchanger is plugged or your exhaust has something in it. Check to make sure both intake and exhaust pipes are clear before assuming bad heat exchanger. Is your outside pvc vent black or sooted inside? Does it smell like sulfur or rotten eggs when it runs? A cracked heat exchanger will typically cause a roll out condition that will cause the manual limits on the burner rail to trip. You can diagnose that by watching it turn on the burners. All the burners should have a nice blue flame cone into the heat exchanger. If when the fan starts to run, one or multiple burners becomes erratic and starts to push out of the cell, you have a crack.
If doing those items does not stop the short cycling here is a couple airflow things to check. Remove the filter. Does it still do it? Do you have a cased A-coil for air conditioning? If you do, remove the door for it and then remove the screws for the triangular panel on the end of the A-coil. This will let you see the heat exchanger and allow for unrestricted airflow. Is the coil clean? Try running it again and see if it still trips with the top plenum open unrestricted. You can also pull the bottom door off the furnace so it pulls air out of your basement or wherever allowing unrestricted return air.
The other thing to check, which shouldn't affect the limit switch, is the drain. I have had furnaces in the past that have similar symptoms that are caused by the drain. You should have a gray "cup" mounted to one side of the furnace with two 90-degree plastic pipes that come out of the furnace into it. Remove the cup and make sure the bottom of it doesn't have any sludge on it. Those furnaces like to build sludge on the bottom and restrict draining. Make sure the two drains are clear. The one into the bottom of the inducer and the one into the 90-degree rubber elbow going up to the top of the furnace. Make sure that the barbs are clear into both the inducer and into that elbow. What can happen is that the furnace will start and run for a while but isn't draining fast enough. The condensate will build up until it opens a pressure switch shutting the furnace off. It will then slowly drain out allowing the furnace to restart and will operate on that on and off cycle until the temp is met.
Those are my thoughts. Good luck.
Edit: Just re-read that it is a high efficiency furnace. Typed too long I forgot.
Edited by Round Prairie Farms 11/13/2024 17:43
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Complicated furnace problem
