New Furnace Short-Cycling

tom_in_scJanuary 15, 2008

The newly-installed LP gas furnace in our under-construction house is cycling on and off quite a bit. The installer's theory is that the flame comes on and, when it has heated up enough, the blower comes on. Then, the cold air (about 40 degree) in the return cools the unit enough that the blower cuts off. Then the cycle repeats.

Does this make sense?

Should I just let the unit run for a while?


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Sounds like the installer didn't do such a great job. If the 'cool' return air is shutting off the blower, the blower may be running too fast (temperature rise not set properly. If the fan was running too slow, the flame would shut off, but the blower would continue to run until the unit cooled down, then might repeat.

By short cycling, does the heat come on with blower for a minute or two, shut off for 30 seconds or so, then repeat? If the return air is too cold, the thermostat should be calling for heat (shouldn't have turned off in the first place).

Just curious, what is the size (sq ft) of your house, and what is the make and model # of your furnace, and does your thermostat ever get satisfied (what is set temperature).


    Bookmark   January 15, 2008 at 3:56PM
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Is your home at "normal" temperatures? If the house is cold the blower may cycle on the low limit until the house warms up. You may be over cooling the heat exchanger. Furnaces are set up to add about 50 to 60 degrees to the incomming air. if the incomming is too cold, the blower will cycle and the fire will burn until the outgoing air can be maintained above the low limit heat exchanger temperature.

    Bookmark   January 16, 2008 at 2:28PM
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Thanks to both of you!

No, the temp is not "normal". We're just trying to keep it in the 50s during construction.

    Bookmark   January 17, 2008 at 2:28PM
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Since it's new construction, did they put a temporary t-stat on the wall?....if it's not digital, the heat anticipator may need to be set to match the furnace.

    Bookmark   January 17, 2008 at 2:39PM
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Please elaborate more on this. It is new construction. We're getting pretty close to being finished. Walls are insulated. Drywall is up. Doing finish electrical now. Ceiling is not yet insulated. They put in the regular thermostat. It is digital. They put in non-programmable, but will be changing them to programmable next week. Thanks!

    Bookmark   January 18, 2008 at 9:32AM
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Posted by tom_in_sc on Thu, Jan 17, 08 at 14:28 said...

"No, the temp is not "normal". We're just trying to keep it in the 50s during construction."

Hey fellas...his thermostat is set to 50°. It won't take long for a warm-air furnace to raise it above that level. If the outside temperature is relatively high - he mentioned 40° - it would take even less time to reach 50° inside. Under these circumstances, I'd be worried if his system DIDN'T short-cycle at such a modest set temperature and low temperature differential.

Hey Tom...after the builders go home, try setting your 'stat to a more "reasonable" temperature - say 68°. Then stick around to see how long it takes to cycle. The first cycle will be long, as raising it from 50 to 68 will need a lot more BTUs. From the the second cycle onward you'll get a much better idea of your run time. You will see your system run noticeably longer to maintain 68° than the 50° you've been running at.

The cycle time at 68° will get a bit shorter once your home is finished. Yet it should not be a drastic. If it is...than some changes are in order.

One more may want to consider adding a heat pump/AC system before the construction's finished. South Carolina has a good climate for such an install. You'll get AC in the summer and energy-efficient heat on cooler days when running a furnace would be overkill.



    Bookmark   January 18, 2008 at 10:29AM
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Since you have a digital stat, my statement about heat anticipator of no stats adjust heat anticipation automatically. With no insulation in the ceilings yet, I agree with daft, the low temp is likely causing the short cycling. 50 deg. is just allowing the walls and floor to slightly warm up, then the cool is coming right back from the warmer temp as daft suggested and watch it for a day or so.

The first few cycles will be much more frequent until you get all objects in the house up to the set temperature.

    Bookmark   January 18, 2008 at 1:15PM
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Thank you!

    Bookmark   January 18, 2008 at 3:58PM
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