HVAC Mags, Furnace continues to cycle despite house being at thermostat set temp

[rconstan]

First of all, a search didnt yield results for this particular problem. Just turned on the furnace for the first time as there was a pretty good cold snap anmd the house was too cold for for the family. Started right up as expected and heated the house to the thermostat setting of 71. However, I noticed that once the thermostat reached 71, the furnace would turn off, but then shortly thereafter (less than 1 minute) it would turn back on again for a short period of time (perhaps a minute or two) only to turn back off again. This "short cycling" would continue, despite the house being at the temperature designated on the thermostat, until I turned the furnace off on the thermostat. I know that the heat is not escaping through some hole in my house as I can heat the house to 71 and then turn off the furnace at 9pm and wake up at 7am and the house is still 66 degrees. If I was losing heat out of the house to necessitate the repeated cycles that I am seeing then I am certain the house would be colder than 66 after 10 hours of the furnace being left off. This is a Bryant 80+ 2 Stage furnace that was made in December 1998. I have had problems with the flame sensor before and thought this might be the problem, so I cleaned it with steel wool, reinstalled and tried again, but got the same result. The thermostat also controls the central air conditioning and that ran all summer with no problems, so I am hesitant to believe that the thermostat is bad, but it could be. The LED light on the furnace is not flashing any codes, just solid red as always. Not really sure where to start short of calling an HVAC company, but would prefer to have some sort of idea before doing so. Any thoughts or input would be appreciated.

[rconstan]

evening bump....

[Chowda]

I'm not an HVAC guy, but I'd start w/ troubleshooting the thermostat. Disconnect the thermostat, and jump the leads to start the furnace. After it runs for a bit, disconnect and see if the furnace cycles as described.

My guess is the thermostat is getting too much direct heat, causing it to turn off prematurely, heat dissipates into the room, thermostat kicks on, direct heat triggers it off, and so on...

You could also try running a ceiling fan or oscillating fan in the room, to get a better air mix.

[bornincolorado]

find, and clean the secondary flame sensor as well. also replace the filter (duh)
if that doesn't work, i would agree that its a thermostat problem.

[Snow Dog]

Thermostats operate with a deadband (it has different names in the HVAC industry like "anticipator"). If you set the thermostat to 71 then it turns on at 69 and off at 71. If your thermostat is a standalone unit (it's not specific to the furnace) then I'd start with a hard reset and reprogram it. Or it might have a jumper or program option to set the deadband.

[Truenilbog]

I'm an ex-HVAC contractor. Before taking anything apart, when you say you turned the furnace "off at the thermostat", do you mean that you turned a switch to "off" or that you changed the temperature setting to much lower so that it cut off?

Here's what I'd do if I were you: set the thermostat on 71. Let it heat up and cycle off. When it comes back on a minute later, turn the thermostat setting down to like 65. See if it shuts off and stays off. If you can wait until it cools off to 65, do so to see if it cycles at that temperature. Either way, now cut it up to 73. Wait for it to go up to 73 and start cycling off and on. Turn it down to 71 and see if it turns off for a while and then starts cycling.

If it does all this, it's likely the thermostat is the problem. It is either too "sensitive" in that the anticipator is either set with too small a margin or it's malfunctioning. Does any furnace air blow directly on the thermostat? Can a breeze be felt near it? That could do it also.

Regardless, if it appears the thermostat is indeed malfunctioning, replacing it yourself is really easy as long as you get the right one and match up the wires inside properly and then follow the install instructions that come with it. Seriously. Be sure to cut the breaker off to the unit. The thermostat shouldn't shock you, but it's not good for the unit.

[rconstan]

I'm an ex-HVAC contractor. Before taking anything apart, when you say you turned the furnace "off at the thermostat", do you mean that you turned a switch to "off" or that you changed the temperature setting to much lower so that it cut off?

Yeah, I turned the switch on the thermostat from the "heat" position to the "off" position. No furnace air blows directly on the thermostat. I have been in this house for 5 years, same thermostat, same furnace, everything in the same locations, just decided to do this now. Also, forgot to mention in my original post that this is all happening with a brand new filter. Thanks for the responses, I will give these things a shot.

[old goat]

I found this on the web: Second: On your thermostat there is a devse called a "heat anticipator" and it has amp markings settings on it. It has to be set to the apm ratings of the gas valve of the furnace. If you take the front cover off the thermostat it is possible that you may see this heat anticipator. The scale goes from probably .2 .3 .4 .5 so on through the scale till it gets to the whole number 1 the proper setting is going to be some where around .45 or so. There is an amp reading on the gass valve but sometimes it is hard to find. So if you do find the scale move the setting up a little if this is not enough move it up more. How this heat anticipator works is that it has a small coil of wire and when the thermostat calls for heat electricity flows through this coil and creates heat. If the setting is too low it heats fast and turns off the thermostat because it thinks the house is warm enough. But the air is still cool and so it cools off the coil rapidly and the thremostat turns on and starts the cycle over again.

I'm no expert--but I've had the problem and adjusting the anticipator worked for me. I need to send this to my next door neighbor. His AC sits right outside my bedroom window and it's been cycling multiple times a minute all night.