We have a Concord Gas Furnace that is about 3 years old. The furnace seems not to respond to the thermostat. We replaced the thermostat with a new digital one but are still experiencing the same issues.

We set the thermostat to 83 just as a test. I had my wife record when the furnace came on and when it went off along with the temp. at both times.

The furnace comes on around 70 and runs for about 30 minutes and then turns off. The temp is 71 or 72. The temp. in the house never reached 83.

The furnace has a honeywell control board ST9160A1002 on it. The thermostat is a 4 wire (G,Y,W,RH) RC & RH are jumpered together in the thermostat. The control board has G,Y,W,R,C on it. The RH wire is hooked to R.

I watched it run once and it came on at 71, the temp on the thermostat dropped 2 degrees to 69. The furnace ran for 35 minutes and turned off at 71 again. The thermostat was set at 83 degrees.

Any help would be greatly appreciated?

To eliminate a thermostat problem remove the thermostat and jumper W and RH together only do not disturb the RH/RC jumper on the thermostat leave it as is. Do not connect or jumper the other wires to anything. This will eliminate the thermostat completely from the circuit. If the unit comes up to temperature (does not shut off) then you know you have a thermostat problem. This is a easy test to perform. If it continues to shut off during the above test then we know you have a problem with the furnace itself. Post back with your results.

If the furnace comes on and runs for 30 minutes and only raises the temperature a degree or 2, something besides the thermostat is wrong. The gas is coming on? I would start with looking at your air circulation, the filters first. I am saying there isn't enough air flowing through the heat exchanger to heat the house. Instead, the high limit switch heats up and shuts the furnace off. Are you getting a flashing light from the furnace controls?

How would I go about testing the air-flow? It seems that the thermostat is not the issue but I will try by-passing it to make sure. The filters are all OK as we just replaced them.

I don't have any lights that I can see but I have not looked really close, I will later today. Is there a way to tell if "high limit switch" is turning the furnace off? That sound like what is happening from the way the furnace acts. I am not a furnace guy but I am a very analytical person (20+ as a computer programmer).

The easiest way to tell there is a problem with a high limit switch is to count the flashes of the control light and check it against the list of codes. Most newer furnaces have them. The other way is to open the furnace up right after it quits and check the terminals with a meter. The schematic shouls show it as a closed switch.

For the air flow, you just have to decide if it seems the same as when the furnace worked right. If crud slowly built up on the ''A'' coil, it could need cleaned. The heated air has to go through it to get to the house.

If the filters and A coil are clean, and the gas does come on, review the sticky at the top of the forum.

Well it turns out after we replaced the thermostat that my step-dad did not clean the filters. I thought that he had. Once we cleaned the filter the furnace started working right. So I guess the problem was the thermostat. At least at the moment all is working. The furnace come on and ran until the set temp. and turned off.

Thanks for your help. I now know a little bit more about how furnaces work.