Amana furnace gas burner cycles on and off

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Old 02-10-13, 07:18 AM
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Amana furnace gas burner cycles on and off

During heating cycle the gas burner is cycling on and off due to the primary high limit switch. I removed one of the hold down screws and inserted and taped a small thermocouple wire through the hole. The limit switch is opening at 110 F, It is an L150-40 switch. Howevever if I tape the thermocouple to the side near the limit switch the metal is getting up to 175F before it opens. Which temp is the limit switch measuring, The air inside or the metal side? Is this switch bad?
 
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Old 02-10-13, 08:13 AM
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The switch is likely good and you have an airflow restriction. Replace the filter. make sure all registers (supply and return) are open and not blocked. If you have A/C then the coil should be checked, if the furnace uses PVC flues then the secondary heat exchanger needs checked, and the blower wheel needs checked.
 
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Old 02-10-13, 08:37 AM
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I opened all registers, two bedrooms which are not used were shut off. Did not make a difference, still cycled every five minutes. Filter is electronic. If I pull one of the cells out and leave the door off, it will cycle every 10 minutes - to me this would indicate return air is not the problem. I would have to cut open the duct to see the A/C coil. It does have PVC flue, I am not familiar with the secondary exchanger and do not see it listed in the parts diagram unless it is part of the heat exchanger. Blower is clean and seams to be operating OK. Thanks for the reply and suggestions.
 
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Old 02-10-13, 08:41 AM
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Electronic air cleaners are notorious for not working. Electronic air cleaners have a pre filter. Did you check that? Sometimes cutting a hole is what it takes to check the evap coil. The secondary HE is above the blower. You have to remove the blower assy. to view it.
 
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Old 02-10-13, 09:17 AM
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Air cleaner seems to working - pressing test button makes arcing. I clean every 2/3 months and get quite a bit of dirt in water used to clean it with. Prefilters look OK, I could try to remove them. But I removed a cell and left the door open - this bypasses the pre filter. This extended the cycle time but the limit switch still opened. Not sure I want to get into pulling the blower or opening the duct. I have ordered another limit switch. I will try that before pulling the blower or opening the duct. I was also thinking about getting a manometer to measure the static pressure and pressure drops. Shouldn't I be able to tell where the obstruction in flow is by pressure drop?
 
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Old 02-10-13, 09:38 AM
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If you can measure at that location.... yes. But often times you can't isolate one component to tell.
 
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Old 02-10-13, 09:51 AM
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I would have to drill small holes between the blower/heat exchanger, heat exchange/AC coil, and above the AC coil. I have no problem with above the coil, but am a bit uncomfortable about the other places because I'm worried about hitting something. I don't understand why they dont put small measurement ports in the cabinet when they make it.
 
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Old 02-10-13, 09:54 AM
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You are not allowed to penetrate the furnace sheet metal in the HE area.
 
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