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Old 01-17-2011, 09:38 PM   #1
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Furnace short cycling


I have a Janitrol/Goodman GMP075-3 furnace. It operates correctly accept only runs for about 2 minutes, then the gas turns off, the inducer and blower continue to run. Then about a minute later the start sequence repeats and the runs again for about 2 minutes. (a short cycle) It repeats like this until the thermostat is satisfied. When I bypassed the limit switch it ran without the short cycling. I thought the limit switch was bad and replaced it. But it didn't change. Then, I cleaned the evaporator and let the furnace run with the accesss panel to the evaporator open so the air flow would not be restricted, but it still short cycles. I think what is happening is the limit switch is heating up (takes 2 minutes) and turning off the gas, then cooling down ( 1 minute) and resetting. I've read this furnace has had heat exchanger problems. I am mechanically inclined and know industrial controls and can make the repairs with a little guidance. I would appreciate any expert opinions regarding the probably causes and what is the next logical step. Thanks!

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Old 01-17-2011, 09:57 PM   #2
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Furnace short cycling


Another common step is to clean flame sensor by one of the burners with steel wool or sandpaper. Check it's firmly in place.

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Old 01-18-2011, 04:58 AM   #3
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Furnace short cycling


Check air filter. Is it clean. If your using a 3M, use a different brand. Make sure all registers and returns are open and not blocked.
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Old 01-18-2011, 05:56 AM   #4
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Furnace short cycling


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Originally Posted by repman55 View Post
I have a Janitrol/Goodman GMP075-3 furnace. It operates correctly accept only runs for about 2 minutes, then the gas turns off, the inducer and blower continue to run. Then about a minute later the start sequence repeats and the runs again for about 2 minutes. (a short cycle) It repeats like this until the thermostat is satisfied. When I bypassed the limit switch it ran without the short cycling. I thought the limit switch was bad and replaced it. But it didn't change. Then, I cleaned the evaporator and let the furnace run with the accesss panel to the evaporator open so the air flow would not be restricted, but it still short cycles. I think what is happening is the limit switch is heating up (takes 2 minutes) and turning off the gas, then cooling down ( 1 minute) and resetting. I've read this furnace has had heat exchanger problems. I am mechanically inclined and know industrial controls and can make the repairs with a little guidance. I would appreciate any expert opinions regarding the probably causes and what is the next logical step. Thanks!
I'm having a similar problem with my Goodman gas furnace, but mine will run longer a lot longer than 2 minutes. But, after turning off, it will immediately come back on and repeat three or four times. I have two different HVAC companies trying to figure this out, but they're coming up with different solutions. I'm also told the type of filter I'm using may be a problem, which I've yet to check out. Here is the thread. Honeywell VisionPro Short Cycling?

Hopefully, we can help each other figure this out.

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Old 01-19-2011, 12:18 PM   #5
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Furnace short cycling


I took the advice of several of you and got a carbon monoxide monitor. And thankfully it did not sense any CO. It appears the furnace was running hot, and the limit switch was doing its job. By decreasing the gas supply it now runs within its nominal range and doesn't short cycle. Thank you all for your suggestions.
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Old 01-19-2011, 03:03 PM   #6
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Furnace short cycling


So what pressure did you turn the manifold down to.
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Old 01-19-2011, 04:08 PM   #7
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I don't Know, I don't have a way to measure it.
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Old 01-19-2011, 04:25 PM   #8
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I don't Know, I don't have a way to measure it.
So now you may have made your furnace a hazard to you and your family.
Or just be shortening the furnaces life span.
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Old 01-19-2011, 04:57 PM   #9
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Why do you say that reducing the gas pressure will pose and danger and shorten the life of the equipment?
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Old 01-20-2011, 03:54 AM   #10
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Burns to cool, and causes condensation inside the heat exchanger, and will rust it out from the inside out.
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Old 01-20-2011, 07:45 PM   #11
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How do you properly check the manifold pressure?
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Old 01-21-2011, 02:44 AM   #12
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With a manometer.
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Old 01-21-2011, 04:41 AM   #13
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Easy to make your own with clear vinyl tubing and water. Make sure you read the bottom of the meniscus on each leg.

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