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Discussion Starter #1
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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If it is short cycling, you have problems. You should always have maintenance done on hvac equipment on a regular basis. More people are killed from Carbon Monoxide, due to improperly maintained hvac equipment, than anything else in the home.

Also, heat exchangers can crack if the system is not maintained, which when the system overheats, it causes the short cycling problem. So how deep is your pocketbook, and can you afford to replace the system, due to it could be worst case scenario.
 

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Discussion Starter #3
Thanks for the quick reply. If there is a serious problem, I will the money for repair one way or another, but it won't be easy while paying for two kids in college.

I posted question because we have only two HVAC firms in town. #1 came out a few years ago to do a routine service and recommended I have the exterior of the heat exchanger cleaned. The cost was $1200 plus $99 to replace a small part (don't remember what it was but found online for $5) and he estimated the job would require 3-4 hours. That would mean the labor cost would be $300-$400 per hour. I did not hire him. When I noticed the restarting problem last year I had firm #2 come out to look at it. In spite of my repeated suggestion, the technician did not let the furnace run through its cycle uninterrupted even once. He kept using the thermostat to force start and stop. He finally concluded that he did not know what the problem was but that maybe replacing the thermostat would work. I had no confidence in him and did nothing.

I would like to get an idea of what the problem may be so that when I have it looked at again (either firm #2 or someone from neighboring town) I will have an idea if what they say makes sense.

If you took the time to read through all this, thanks, and thanks for any ideas you may have.

Tom
 

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What is more important? Getting the hvac system fixed, or worrying about paying for your kids college, and not being there if the system is leaking CO, if the exchanger is cracked. But without a technician going on site and looking at it, along with inspecting the heat exchanger for cracks, which could be one of the issues, you will not be around if that is the case, to pay for your kids college.

Hope you do have CO detectors in your house.
 

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Some causes of short cycling are but not limited to air flow restriction because of wrong filter for the particular system, dirty filter, blocked or closed registers, dirty A/C coils restricting, faulty limit switch.
 

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is hi limit opening..... you need a meter to check it..... are filters clean and a/c coil clean if you have one.... check the obvious:thumbsup:
 

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Discussion Starter #7
Greg, as I said, I will find the money, it just won't be easy. That is why I want an idea of likely causes so I can make an informed decision on what is needed.

Fairview and carmon, I thought about air flow restriction from the filter as I had switched from the original, reusable, low-impedance filter to a Filtrete 300 due to daughters allergies. However, switching back to a low-impedance filter did not help. There are a few registers closed (like over Xmas tree) so I will open them up and see if that helps.

Checking the obvious is a very short list for me - I know next to nothing about HVAC. Is "high limit" / 'limit switch" in thermostat? Wherever, that is something I will have a technician check.

Thanks again for the input.
 

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Check the heat exchanger.

1. Take the cover off and look at it.


This image may not be it, but it will probably be similar. Check seams and turns especially. If there are any holes, you are putting everyone in the home at risk of death.

2. Another way to test is with the heat OFF and the fan ON on the thermostat. Hold or tie an air-freshener at the openings where the flames enter the heat exchanger. If you can smell it inside the home through the vents, the carbon monoxide is escaping through the same place as the scent. Again, positive scent = possible death.

3. Please check this site: http://www.carbon-monoxide-survivor.com
especially the symptoms.

Good luck and be safe.
 

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Check the heat exchanger.

1. Take the cover off and look at it.


This image may not be it, but it will probably be similar. Check seams and turns especially. If there are any holes, you are putting everyone in the home at risk of death.

2. Another way to test is with the heat OFF and the fan ON on the thermostat. Hold or tie an air-freshener at the openings where the flames enter the heat exchanger. If you can smell it inside the home through the vents, the carbon monoxide is escaping through the same place as the scent. Again, positive scent = possible death.

3. Please check this site: http://www.carbon-monoxide-survivor.com
especially the symptoms.

Good luck and be safe.

WT Hell:huh:?

That's a unique view point.
 

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I'd eliminate the thermostat as the potential problem first. It's easy to do.

Turn off power to furnace.
Remove the thermostat wires in the furnace from R & W.
Connect R & W with a pigtail or short piece of wire.
Turn power back on.
If it continues to short cycle, its not the thermostat circuit. Turn off power & Return everything back the way it was.
If the furnace doesn't short cycle. Turn off power, return furnace wiring the way it was. Replace thermostat and turn power back on.

Does your furnace have a diagnostic flashing light on the board. Usually visable through a viewing port on the furnace door?.
 

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Discussion Starter #12
Gregzoll, I double checked the model number (dots stamped into soft metal - hard to read) and it very well could be the 8 in my original post should have been a "B", or 2BRC042000. Can't be certain but he first zero could be an "O". The only other info on the manufacturers plate is the serial number which I presume would not help. I just did some research myself and don't find anything with this number - or anything at all like it. I'm guessing that at 20 yrs old Carrier does not have it on their web site.

From my web search, I think what I have is a split system. The AC compressor is outside but it shares the blower and other components with the furnace.

For all commenting on carbon monoxide risk, the first thing I did after getting Greg's post was to buy a CO monitor. After about 12 hrs the peak level recorded is 0.

Johnny'sHotDogs, digging that far into the unit is daunting to me but I will see if our library has a how-to book that gives me enough confidence to give it a whirl.

how, that sounds like really good advice. If I can find a how-to book I will try this first.

Thanks to all! I will let you know what happens next.

Tom
 

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Dirty evap coil and or blower wheel. It looks to be tripping the high limit.
 

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Tom the elder said:
For all commenting on carbon monoxide risk, the first thing I did after getting Greg's post was to buy a CO monitor. After about 12 hrs the peak level recorded is 0.
I don't want to cause you any concern, and I'm sure everything is fine as long as no one is showing any signs of CO poisoning (headache, nausea, confusion, dizziness, faster pulse or breathing), but what you have is a CO monitor, or a better description, a CO alarm. It is NOT a CO detector. Readings of zero do not mean that there is no CO present, all that it means us that the CO levels aren't high enough for the unit to alarm. There are minimum CO levels for a defined period of time required for the unit to alarm. This is to minimize nuisance alarms, of which I have experienced many. Levels of 70 ppm for 60-240 min, 150 ppm for 10-50 min, 400 for 4-15 min will alarm. 30 ppm for 30 days are ignored.

A CO alarm is no substitute for proper service and maintenance!
 

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From one home owner to another, listen to the pros and find a way to get that 20 year old unit replaced.
 

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JFYI the Hi Limit is part of the furnace control system and has a sensor mounted near the heat exchanger on the front of the unit...looks like small disc....If you have Air Conditioning there is a coil mounted in the duct work above or just downstream from the furnace..It could be plugged up thereby causing high heat rise due to low flow resulting in the hi/low limit to shut the furnace off..the coil should be checked and cleaned if required...
 
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