What should I be looking at for my over-cycling furnace? I have already changed the thermostat setting which controls how far below temp setting the system needs to wait before restart.
The furnace is basically turning off and on while the room is coming up to setting temperature.
Here’s what happens:
Let’s say inside temperature is 62 and desired temperature is 68.
I turn on the thermostat.
The furnace comes on and fan turns on, then turns off at 63 degrees inside temperature.
The fan runs for awhile after the furnace goes off.
Just before the fan turns off, the furnace goes on again. Followed by fan.
At 65 degrees the furnace goes off. The fan goes off some time after.
This cycle continues until finally the room comes up to setting temperature.
Thanks,
Martin
Replies
Give us some info about the furnace. How old? Gas? Do you know if it's "high efficiency" or not?
This sounds like an old "standard" efficiency forced air gas furnace that needs to have its fan/limit switch adjusted or replaced. If so it's a relatively simple procedure and not too expensive.
And (if this is the case) what it's doing is not dangerous or particularly hard on the furnace, just a bit inefficient (and irritating).
It may also be that your filter's clogged. Check it.
It sounds like the high-limit switch is turning off the burner because it's (high limit) getting too hot. This could likely be because there is an obstruction somewhere restricting how much air the blower can get a hold of and move over the heat exchanger.
Try removing the air filter altogether if you don't have a new one on hand. See what happens. If that does it, then replace the filter with a new one.
If that doesn't do it, try also removing the door from the blower/filter housing. If that does it, you probably have too few and/or too small of return ducts....or some of them have become obstructed somehow. Anybody parked rugs over them?......or blocked them off in some other way?
If you do the door removal test, make sure the furnace has access to plenty of makeup air or you could/will instantly create negative pressure in the room which can/will draw exhaust gases/CO into the blower uptake. IOW, if this is in a closed up utility room or similar....open the room door and others... to give the furnace blower access to the rest of house. Kinda like turning the whole house into a return duct for the time being.
Is the blower running up to normal speed or is it running slower than normal? This could also cause the high-limit to kick out.
Fan control might be faulty.....or even the high-limit might be faulty. Eliminate possibilties of air flow problems first and if that doesn't resolve it.....look to the blower speed, fan control or faulty high-limit.
http://www.inspect-ny.com/heat/FanLimitSwitch.htm
How old is the furnace? Would recommend/hope you have a CO monitor in the house. Any chance of a hole in the heat exchanger?
Any chance your flue or chimney is obstructed? This would cause the heat exchanger to get hotter than normal and could trigger the high-limit switch. So would an obstructed heat exchanger. Gas furnace or oil-fired? If oil-fired, the flue, chimney or HE can easily become obstructed with soot if the combustion is poor.
Edited 1/31/2009 11:33 am ET by HootOwl
Pics of you system would help a great deal.
In addition to HootOwl's comments, an air flow problem could also be downstream of the furnace in the hot air ducts. If the temperature in the main duct and furnace get too high the furnace will trip off. Make sure you haven't closed any hot air registers that have normally been open in the past. I had this problem with my mid-efficiency furnace which is a bit over-sized for my ducts. I had forgotten to reopen a basement register after the summer cooling season when I normally have it closed.
Good point.
I had same thing couple of years ago. Mine was abad blower control circuit card. Replaced it and set the delay on the control and works fine. Your control may be different. Try downloading a schematic and looking at what you have. Sometimes there's a control diagram pasted inside the cover of the unit.
If you take any controls aprart always test your high limit shutoff when you are done.
If you don't feel OK doing this then call a service guy.
The most common cause of short cycling I see is overfired burners - which are inefficient and likely to have excessive CO, not to mention over-stressing the heat exchanger.
Obstructed filter is probably second most common.
Get an HVAC tech with, and who knows ow to use, a combustion analyzer.
You can't judge flames/operation by color - no matter what folks tell you.
Very low O2 readings - high CO - and high flue temps are symptomatic of over-firing.
"Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."
Howard Thurman
Thank you all for the great analysis and suggestions. Most likely I will just get a tech out to fix it, unless I can see the cause and repair very easily.This is a small home, California climate, and a heater that is less than 5 years old. We replace the filters religiously. So most likely it is one of those other things such as the high cutoff switch. It is highly unlikely that we have somehow introduced an obstruction anywhere in this very very simple system.Thanks again. I will post the model number later in the afternoon just in case something comes directly to mind with one of you guys.Martin