Thermostat poblem?

[BWGreen]

I'm wondering if I have a problem here or not. I have a gas furnace, about 12 years old, and a Honeywell electronic thermostat that's about 7.5 years old. Recently, I've noticed some weird behaviour. During the night, I've got the thermostat set so that it brings the temperature in the house right down. Over this winter I've noticed that at times during the night the thermostat will turn the furnace on and off repeatedly - for how long I'm not sure. If I set the thermostat warmer, or a new time setting kicks in it works properly. I don't notice it doing anything during the day. By turning on and off I mean that it sounds like it starts the burner, then shuts off before it starts the fan.

Is this a problem? And if it is, could it be the thermostat?

Thanks,

Brian

[davidg73]

on a gas furnace the tstat does not call for the fan, it calls the burner to ignite and once the burner reaches a certain temp IT turns on the fan. sounds like you have a problem in the furnace itself.

[BWGreen]

But why I'm leaning towads the thermostat is that the burner turns on and off, and when the t-stat is changed to a higher temperature, the furnace seems to work properly. The burner, at the lower temp., seems to get called to turn on and off. Looking at the display on the t-stat, the temperature is where it should be for that time of night.

[AmeriServ]

Sounds like the stat may be set up wrong or going bad. The night set back option should not make it short cycle, which is what it is doing at night. The furnace has nothing to do with how long the burners fire unless it does this all the time. The stat will allow the temperature to drop below the set point (usually called the differential) usually 1 deg F before it calls for heat and it usually will shut off the heat at the set point or just below. Sometimes people set the differential too low (at 1/2 deg F) and this causes short cycling.

I would just replace the stat, you can get fairly inexpensive programmable stats these days and I am more than sure the problem will go away.

BTW, it is not a great thing to short cycle the furnace as it adds like cycles to the unit. All parts are made to last a general amount of cycles (on - off) including the gas valve, ignition, heat exchanger etc.

[mike n]

Don't replace anything without deciding it is defective unless you just want more features or attractive stuff. If you want I can show you how to tell if your thermostat is bad or not otherwise start replacing components until the problem stops

[BWGreen]

Let me know how to tell if it is faulty! I don't really want to get a new one if I don't have to.

Thanks,

Brian

[AmeriServ]

Get an electronic meter, meaure the voltage a the W terminal at the furnace while the stat is in night set back mode and catch it in action. You will see 24 or so volts while it is calling and the burners should light up. The 24 volts should drop off and the burners shut off. If the 24 volts stays up and the burners shut off then the furnace has the problem.

[mike n]

If you take the wires completely off your thermostat and tie the wire from the r terminal to the wire from the w terminal the furnace should run until you seperate these wires if it does not then you will have a problem with your furnace if it works fine then your controller is malfunctioning and you need to replace it, if the problem is with the furnace email me at mike@howtorepairairconditioners.com and I will walk you through it