Short cycling?

DonnyJ

Having an issue with my furnace. It works fine bringing the house up to the temperature set but once it gets to that temperature, it will turn off, then almost immediately turn back on and run for 90 seconds and turn off. It turns back on again after a few seconds and again runs for 90 seconds. It will do this until either I set the temp higher and it reaches that temp(when it starts all over again) or turn it off completely.

It will also do that if I set the thermostat, say 10 degrees below the current house temperature because it's too warm in the house and once the house temp drops to that setting, the furnace will turn on and start that quick on/off, on/off cycle.

I've read about short cycling that occurs as soon as you turn on the heat, but not this type of situation where it only short cycles after the set temperature is reached.

Suggestions? Thank you.

Harvey Ramer

When you say it turns back on, does the burner kick back in or only the fan?

If the burner does kick in on these short cycles; where is the thermostat located? Is it in close proximity to a supply vent? If so you should move it to a different location.

JUGHNE

Is the gas burner/fire actually cycling or is just the fan blower cycling?

DonnyJ

When it turns back on, the burner kicks in for about ten seconds then the fans go on. Both turn off at the same time.
The thermostat is in the "great room" with 16' ceilings that open to a loft area. The furnace room is just off the loft area where the loft area starts (looking down into the great room).

DonnyJ

Thermostat is not by a heat vent but almost directly above a return vent.

JUGHNE

Older t-stats have a heat anticipator adjustment inside under the cover which if out of adjustment can cause short cycling. maybe a lever, try moving it one way or the other and see if things change.

John Mills_5

Old clunker furnace or something modern?

JUGHNE

Is this upflow or downflow air delivery....does the hot air come out of the top or bottom of furnace? Is this just a recent problem?

DonnyJ

6 year old furnace. New with new construction house. Recent issue. Here's the setup.

JUGHNE

I guess that eliminates the old clunker--down flow--heat anticipator guess. How about if immediately after a normal heat run cycle and furnace shuts off and before it starts the extra cycles, you move the t-stat function switch to off and see what happens.

Harvey Ramer

I'm not a Carrier expert but I believe this furnace has led diagnostics. The manual will show you how to retrieve a fault code and how many flashes = a specific fault.

Note, if you turn off power to the furnace it will erase any stored fault codes from the board.

Follow the instructions and see what you find.

What brand/model thermostat do you have? It could possibly be bad.

DonnyJ

Harvey, I'll check out the diagnostics. The thermostat is also a Carrier, model TB-PAC.

4Johnpipe

I have seen improperly wired humidifiers cause this condition. Make sure yours is wired correctly.

JUGHNE

The little t-stat test would remove the t-stat from the equation?

unclejohn

Does the thermostat take batteries? Also you said that the burners and the fans go off together. By fans do you mean both the inducer fan and the blower motor.

DonnyJ

Unclejohn, I changed the thermostat batteries and yes, the inducer fan and blower motor go off together.
Jughne, confused about your suggestion. You're saying just turn the thermostat off at the end of the heating cycle? What will that accomplish besides turning it off?

JUGHNE

It would tell you if the source/cause of post fire short cycling originates from the t-stat throwing out a call for heat or If the furnace is cycling on its own without influence from t-stat.
And as 4johnpipe says you could be sure the humidifier is powered down as a test also.
Has the furnace control board ever been wet? Say because of AC condensate overflow or coil freeze up.

unclejohn

Seems to me I had a Carrier furnace that had a bad pressure switch that would open and close so quickly that the burners would drop out but the board would not pick it up and hence no fault code would appear. I do not remember about the fans stopping. I don't think they did. I caught the problem by running the furnace and using my meter across PS terminals. The numbers constantly flashed around. New pressure switch solved the problem. Both motors shutting down together though is weird.