I am new to the board and I hope that I am not repeating this situation. I have looked through many of the situations that others have had, but couldn't find the answer to my problem. Here it goes.

I have a Honeywell programmable thermostat (not sure the model number, but it is approx. 5 years old) and a Carrier furnace (same age). I have the heat temperatures preset throughout the day (low when I leave for work and sleep and higher when I wake up and get home from work). The issue I am dealing with is that the thermostat sometimes will read a lower temperature than is set, have the flame indicator on, but the furnace will not be on.

For instance, when I sleep the thermostat is set for 66 degrees and when I wake up it is set for 69 degrees. Most of the time when I wake up the heat will be on, but sometimes (and occurring more frequently) when I wake up the thermostat will show 67 degrees and the heat indicator will be on, but the furnace will not be running. This happens also when I come home from work.

What I will do is take the thermostat off the wall (to reset it), switch the furnace off and then back on, put the thermostat back on and reset the time and temp. settings. I then switch the thermostat over to "heat" and push "run program". The thermostat heat indicator will kick on and the furnace will ignite and run perfectly. However, there are occasions where I have to repeat this process a few times before getting back up to temperature. The reason being is that after the furnace begins to work it's way back up to the desired temp. (66 start and moving up to 69 degrees) it will shut off at around 67 or 68 degrees. Sometimes the heat indicator will even shut off (telling the furnace to shut off), but sometimes the furnace will shut off with the thermostat heat indicator still on. When the furnace shuts off (before it reaches the desired temp. and the heat indicator is still on), it will start up shortly after. The furnace turns on, the pilot glows orange, and the blower will begin to start for around 5 or 10 seconds - then the furnace shuts off. It then restarts again after a short delay and repeats the same scenario. The furnace does this around 5 times and then just completely shuts off - with the thermostat still reading a lower room temp. than the desired heat temp. If I then go through the reset process, the furnace will again work perfectly.

Sometimes, there are days when everything works perfectly and I have no problems, and there are also some temperature settings that will work fine and some that have problems. For instance, the thermostat is set at 68 degrees and working fine, but if I increase the temperature to say 69 or 70 degrees, the above described problems will occur. There are also days when I have to reset the thermostat and furnace a number of times. So I am wondering what the problem is, whether the problem is the thermostat or the furnace or both and what needs to be done. Thanks so much!

It sounds to me like the flame sensor may be dirty and not detecting flame as it should. This would cause the unit to try to ignite several times and then lock out until you turn the power off and then back on again. I would try cleaning that first and see if the problem gets better... A little emery cloth will clean it off...

Thank you so much for your response. What does the flame sensor look like and where is it usually located (I will look in the manual)? Is there a certain proper way to clean the sensor or can you just wipe out the dust? Also, if the flame sensor was dirty would the furnace even be able to work at all or could a dirty flame sensor cause sporadic "problems" to occur (not every day)? My furnace (and thermostat) works perfectly some days and has the issues I described on other days. I will try to clean the flame sensor tonight and see if that helps anything. If it doesn't help, what other things could be causing these issues? Thanks again!

Rick, thank you so much for your help! I cleaned the flame sensor last night and the furnace worked perfectly all night and this morning (got up to temperature with no problems). Going to keep monitoring it, but (knock on wood) I think I fixed the problem thanks to your advice. Thanks so much again!