Gas furnace short cycles

Old 01-25-20, 10:48 AM
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Gas furnace short cycles

I have an old Kenmore Power Master 80 gas furnance that short cycles every 2 to 3 seconds. It sometimes runs fine for hours and then will begin shutting off and on every 2 to 3 seconds. This furnace was made before the use of flame detectors so it is not equipped with one. Before this problem began I have over the past year replaced the blower fan motor, cleaned the squirrel cage fan blade, replaced the blower motor relay, cleaned the igniter spark with pilot light assembly and replaced the vent motor air flow sensor. I also have installed a new thermostat. Any help would be greatly appreciated.

Last edited by Dennis2; 01-25-20 at 11:13 AM.
Old 01-25-20, 07:21 PM
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Several questions come to mind: (1) Is the air filter clean?
(2) Make sure all vents are open & not blocked
(3) Is the blower wheel/fan turning in the correct direction?
(4)I Is the fan turning at the correct speed?
Old 01-26-20, 12:12 PM
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Furnace short cycle

Thanks for answering Grady,
Yes I checked all of your suggestions. Since my first post I opened the furnace up while it was short cycling and while at my wits end I noticed that the pilot light was cycling on and off but the burner was not lighting. I began tapping on various components, when I tapped on the ignition igniter module the pilot light lit and the burner also lit. While it continued to short cycle the pilot light, each time that I tapped on the ignition module the burner would light. I think that the ignition module may be faulty and failing intermittently not calling for gas from the gas valve to the burner. What do you think?
Old 01-26-20, 01:15 PM
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We'd need more information to help you further but it's very possible the problem is in the intermittent ignition circuit. There needs to be a full pilot flame for the burner to light. There could be a dirty pilot orifice.
Old 01-26-20, 01:26 PM
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If your furnace has a fan and limit control, the furnace blower motor may be coming back on after the thermostat is satisfied due to residual heat causing the fan contacts to reclose. This cycle would repeat until the residual heat stays below the fan on setpoint.
Old 02-17-20, 01:09 AM
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Could be flow restrictions

It sounds like its turning off on high limit. Make sure filter is clean, also make sure there is no furniture or obstructions on any return vents throughout the house because if it doesnt get cool return air from vents the furnace overheats. Remove the filter completely and see if it runs like that if so there is a air flow issue.

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