Short Cycle? - http://www.hvacmechanic.com/ Forums
Original message
| stench
 |  "Short Cycle?"
, posted Fri 27 Oct 20:52  
Central gas heating and my furnace will go for 10min then turn off and immdiately start again. I would assume a cycles per hour set at 5 would mean it would cycle on/off every 10 min. or so, is this correct? If not what other things can I look into?
It has no error codes, tech cant find anything wrong, new thermostat. I have tried other cycle rates 2- 10 with same result of around 10 min runtime. I have set the temp to 90 and the room temp is around 65. Also new filter and vents are open.
Is this a normal runtime? or should it remain on until around 90 degrees before turning off?
Thanks in advance
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Replies:
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| stench
| "Re(1):Short Cycle?" , posted Sun 5 Nov 20:48
Update:
Seems like it was the limit switch, I missed the tech when he came out but wanted to see what and where it was. seems to work ok now.
So for an additional question: What then is a cycle rate? such as when you program your thermostat cycle rate. Does it have to do with the furnace going on and off?
Anyways I want to say thanks for everyones time and help. I really appreciate it.
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| Freon
| "Re(1):Short Cycle?" , posted Fri 27 Oct 21:05
"...furnace will go for 10min then turn off and immdiately start again" How long is "immediate"? 5 seconds, 1 minute or 5 minutes? When the burner is off, does the main blower still run and circulate air in the house?
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| | stench
| "Re(2):Short Cycle?" , posted Fri 27 Oct 21:15
after about 10mins everything shuts off as normal (no blower), then after about 5-10 sec. you hear the gas light.. then shortly thereafter the blower will come on and the hot air will resume for another 10 min.
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| | Freon
| "Re(3):Short Cycle?" , posted Fri 27 Oct 22:11
To be sure it's a thermostat issue and not a furnace issue, turn off the power, remove the thermostat face plate and jumper the R and W terminals on the thermostat mounting base. Now turn on the power and see if the furnace behaves the same (short cycles). If the furnace runs continuously (no short cycling) then something's amiss with the thermostat programming or the thermostat itself.
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| | stench
| "Re(4):Short Cycle?" , posted Fri 27 Oct 22:39
Red and White jumpered, blower on for about 1 min then gas on and after a minute or so the blower on. after 7 min of heat the gas off and then 1-2 min after gas off the blower shut off. being near the furnace I was able to hear that there was no delay between blower off and a typical noise prior to the gas lighting (sorry not sure what it is but usally hear it so i assume it is part of the cycling up) so basically right after blower off, it starts to cycle on.
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| | Freon
| "Re(5):Short Cycle?" , posted Sat 28 Oct 12:28
It appears there is a safety switch that may be turning off your furnace, as Tom r pointed out. You'll need to jumper each one, one at a time, until you find the source. Having the furnace run for 7 minutes makes me think it's more likely a high temperature limit switch but then usually the furnace will still run (inducer and main blowers) while the main gas burner cycles on and off. But your control board may handle high limit situations differently. Inducer blower sensors and small tubing, if present, are always good candidates to check and clean. If you can, CAREFULLY watch the flame of the main burner to see if there may be something blowing the flames about.
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| | Tom r
| "Re(5):Short Cycle?" , posted Sat 28 Oct 10:21:
Limit is cutting out. Why?
[this message was edited by Tom r on Sat 28 Oct 10:23] |
| | stench
| "Re(6):Short Cycle?" , posted Sat 28 Oct 13:41
If it was a high temp situation wouldnt there be an LED code? I have not seen it personally but 2 people have said there are no error codes.
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| | Freon
| "Re(7):Short Cycle?" , posted Sat 28 Oct 14:11
Depending on the specific control board there may be a LED code. I don't know your furnace specifically but you should definitely look for any LEDs on the board.
The best way to determine a high limit situation is to measure the temperature rise through the heat exchanger and compare that value to the manufacturer's recommended range on the furnace data plate. Measure the temperature of the air as it leaves the furnace--the supply plenum is usually a good place. Subtract the house air temperature and that should give you what you need.
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