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I have a lennox furnace that is about 25 years old and for the last few days it doesn't want to kick on. I replaced the thermostat thinking that was the problem, but it wasn't. The problem that I'm having is that when i turn the funace on it will click a couple of times and then turn off. I keep tuning the thermostat on and off until the furnace finally clicks on and gets the heater going. Im not sure if the electric start that lights the pilot light and then the furnace is going bad. Once i have the furnace going, it will warm up the house and after it reaches the set temp. it will turn off and not come on again. Any suggestion on what the problem could be?

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I would suggest that you have someone turn thermostat up to call for heat while you are watching. Being that old it may or may not have a induced draft motor on it. It it does the draft blower should come on first, then spark igniter,then pilot ,next the main burner,and after a short period the main blower. If you do not have induced draft motor the spark igniter should come on as soon as thermostat calls for heat. If it doesn't I would check all electrical connections making sure they are all clean and tight fitting. Just follow it through step by step . You can use volt meter to check across red and white wires from thermostat to where they hook into furnace. When you are calling for heat you should read about 24 volts. I would be inclined to replace spark igniter but if it does what it's is suppose to then I would go to gas valve. It might be worth your time to check connections on t-stat. I have occasionally seen wires that were broke under screw and some time make connection and other times not. You can also remove red and white wires from t-stat and temporairly touch them together. this should turn heater on. Hope this helps. Thanks.

Posted on Nov 14, 2010

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