Expert: Blake Ballard (A/C - Heat - IAQ Expert) - 11/23/2006

Question
 I have a Lennox G25Q2-50-2 gas furnace with a Johnson controller led read out. The green led reads normal when the furnace is running. I have installed a new Honeywell RTH7000 thermostat on tuesday as it seemed the best place to start. When the thermostat calls for heat after a setback period it will go on for about 5 mins. than shut down except for the co inducer fan which will keep on running and running. If I go down and shut off the main power switch and wait about 30 secs. turn it back on it will work normally until the next setback period. At this point it starts all over. Furnace repairman could find nothing wrong. Thanks for any help you can give

Answer
Gordon,

The problem you describe could be caused by a variety of different things. I would suspect that there may be a problem with a limit because it is happening after a setback period, although there could also be a problem with the pressure switch for the draft inducer, or a problem with flame sensing.

Make sure that all your supply registers are open and that the air filter is clean. Clean your flame sensor rod with a scotchbrite pad or steel wool.

If you cannot reproduce the problem when the service tech is there, you will need to install some devices called "tattletales" on the limits. If one of the limits is tripping, the tattletale will pop and indicate which one is having the problem. Other things that can be checked will require instruments like a magnahelic or manometer for the pressure switch, a manometer or pressure gauge to measure the gas pressure, etc.

I hope this helps,

Blake