Heater runs, then shuts off, then starts right up again

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Old 10-11-08, 09:30 AM
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Heater runs, then shuts off, then starts right up again

My seven year old Ruud gas furnace starts up in the morning and runs for about 20 minutes. It then shuts off (prior to reaching wall thermo setting). It then starts up again immediately and will run for another 20 minutes. This cycle repeats over and over until the house reaches the thermo setting. Why is it doing this?
 
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Old 10-11-08, 03:13 PM
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Probably because the furnace is reaching it's high limit, after running 20 minutes. Running 20 minutes at this time of year, without reaching set point? And still not reaching it in another 20 minutes, soon thereafter? Hmmm. What temp are you starting out at?: 50? Is your thermostat on an interior wall where heat should be able to pass by it? Or, maybe you have a close to undersized furnace? Or registers are shut down? Or house is drafty/poorly insulated? Or you are pushing cold air out of bedrooms past the thermostat, and the thermostat is taking too long to warm up because of that?

Have you ever experienced this problem before? If not, we can discount a lot of what I said. (I often think and post at the same moment.) How MUCH short of reaching set temp does your stat say, that we can be certain it is not reaching it, and you have stat issues?
 
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Old 10-13-08, 12:42 PM
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Start and Stop Furnace, Reply

We had this problem before, but replacing a dirty filter fixed the problem.

The thermostat IS located in a hallway where no there is no register and therefore cooler than other areas of the house. It is also located directly above a return-air floor vent.

Is the furnace getting too hot maybe? Is there a sensor in the furnace that might be broken?
 
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Old 10-13-08, 05:13 PM
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Two things to check: To make certain the thermostat is not shutting off. You really need to check voltage at the wires to confirm. Not go by just what the stat says. There is an anticipator involved and maybe shutting off premature, for all we know. Maybe the swing cycles are too long because of this, for all I do not know - and it takes an extra long time to heat, to return the house back to your thermostat set point. And due to various other things, the heat rise slowly creeps up in that time period, and shuts it off on "limit".

And we do not know how cold that air is that is coming down the hall from who knows where? And are heat registers open that are in areas and rooms nearest that thermostat? What are some temperature readings taken by the stat, and in the hall other places and in rooms off that hall?

IF there indeed still is a call for heat, then likely it is overheating. At least the limit thinks it is. We have no idea what that limit is set at or if it has a problem limiting prematurely, either.
 
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