Heat Shuts Off After a Minute

Old 10-13-04, 07:44 AM
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Heat Shuts Off After a Minute

Im looking for some low-cost repair/ troubleshooting suggestions (Im selling the house).

I have an old gas forced air furnace w/a new-ish LUX programable thermostat that was working like a champ until this season.

Now, when I set the thermostat to a temp adequately higher than the room temp, the furnace kicks on as it should, & heat begins to come out of the registers, but it shuts off after about 1 or 2 minutes...well before the room gets up to temperature.. Any advice?
Old 10-13-04, 09:10 AM
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Several Possibilities

Hello: dgumbs. Welcome to the Do It Yourself Web Site.

As the header indicates, there could be any one or more possible causes.

Dirty air filter.
Dirty A/C coils, if unit equipped with Air Conditioning.
Fan blower speed too slow. Caused usually by unoiled bearings, excessive dust on blower cage, defective motor, slow fan speed, etc.
Fan relay is defective.
Fan on/off firebox temperature sensor may be defective.
Furnace overheating can result for any of the above reasons. Thus the unit shuts down early to prevent damages caused by over heating the firebox.
Flame sensor for pilot flame is defective or not properly or not constantly heated by pilot flame.
Burner flame sensor is defective or not properly or not constantly heated by burner flames.
Major firebox problems may be the cause. These could be smothering flames or flame roll out caused by a defective firebox, lack of air flow inside the firebox, a cracked firebox or a safety device shuting the unit off to prevent damages, etc.

Not a problem commonly caused by the wall thermostat. More likely caused by the above and or possible causes not listed.

More detailed and specific info would greatly help to determine an exact cause. Kindly use the reply button to add the additional information.
Old 10-13-04, 10:54 AM
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Wow.. that is a littany of possible causes.. Ill try to add some detail based on your suggestions..

Thanks very much
Old 10-13-04, 11:08 AM
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Jump R to W and hold it. See if the furnace keeps on running. If it does then its in the tsat if not its down to the furnace. If the tstat has batteries are they good and new?

Old 10-14-04, 07:38 AM
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Turns out the problem was in the furnace.. the wire from the fan limit control was positioned very close to the heating element of the furnace.. insulation became damp & cracked & ultimately broke. Simple wire jumper & some re-insulation & I'm back in business.. thanks all

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