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Old 12-29-2012, 03:56 PM  
TomtheElder
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Default Re: Furnace shuts off and restarts back up repeatedly

There is a post with this subject from 2011, and last reply yesterday. Old but I have a similar problem. I know this forum is DIY, and I usually do, but in this case I am looking for info to help decide if whatever technician I have look at it is on the level. Here is my story:

Short version:
20 y.o. Carrier Crusader furnace (model 28CR042000) runs through its on-to off cycle repeatedly, with a new cycle starting immediately – no pause at all – after the end of the previous cycle (see video at http://www.youtube.com/watch?v=zQp0uULMqnw&feature=youtu.be ).

I assume this is hugely inefficient – any ideas what could be the problem? Thanks.

Long Version:
The cycle in the video completed in 4:14 but the duration is not consistent. I timed another as follows:
TIME ACTION
0:00 Furnace starts
0:25 Pilot light glows
0:34 Burner on
1:41 Blower on
2:34 Burner off
4:03 Blower off

Immediately upon finishing the cycle a new cycle will start; there is zero pause between the end of one cycle and the start of the next. I think it typically will run through 8-10 cycles before shutting off. There may be times of as few as 4-5 cycles. This morning I counted 20+ cycles in one run – but that might have been affected by things like doors being opened.

Near the end of the 20+ cycles this morning I timed two consecutive cycles; each took 3:39. After being off for a while it restarted and I timed the first cycle at 4:11. Clearly the length of the cycles varies, possibly becoming shorter through the sequence of cycles.

HVAC system was installed in 1991 and has not had any repairs. Here is the embarrassing part – this problem has occurred since at least fall of last year (I timed one cycle then at 4:52) but I can’t say for certain when it started. Any ideas as to what may be causing the problem will be hugely, hugely appreciated. Thanks.

Tom



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Old 12-29-2012, 06:20 PM  
rogerknapp
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sounds like it might being going off on high limit. furnace get to hot and cycles off but t state is not satisfied so it cycles through over and over untill t stat is satisfied .clean filter ,clean blower; if thats clean pull blower and look up and see if lower coil (secondary heat ex) is plugged or your ac coil could also be plugged causing a air restiction check all return airs and supply airs for blockages. That's where I would start.

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Old 12-29-2012, 11:41 PM  
TomtheElder
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rogerknapp, thanks for the insight. I have never worked on HVAC before, and had planned to hire a technician, but I think I will see if I can find a how-to book that gives me enough confidence to try your suggestions. Thanks.

Tom

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