Furnace won't come on without help

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  #1  
Old 10-05-05, 07:00 AM
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dreamchipper
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Unhappy Furnace won't come on without help

I have a Lennox gas furnace that's about 10 years old. Last fall, when I bought the house, it was in such bad shape that it would only spit out cold air and it cost me $800 to repair. No matter what the repair company did though, one thing wouldn't change; the furnace wouldn't come on unless the thermostat was pushed up to about 76 or 78. After that I could bring it down to my normal 70 degrees, and after the repairs the furnace worked great other than this one thing.

After several visits the repair company said I needed a new thermostat. I thought this was odd because the first thing I did was replace the thermostat, but I went ahead and put in a second Honeywell round thermostat (and I levelled it very carefully). By that time it was so late in spring that I didn't really need the furnace coming on anymore and I was just too tired of fighting with it.

Here we are again in fall and I'm having the same problem. The furnace will not come on unless the thermostat is run up to 76 or 78 for a moment. Then I hear the ignition come on, it fires up in a timely manner, the heat is wonderful, and whatever temperature I set it to after the intial jump is the exact temperature it stops at. Anybody have any ideas?
 
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Old 10-05-05, 04:35 PM
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Once the system is running, does it shut off at set point?

Pop the cover off, and watch the bulb of mecury bulb.. If that bulb don't "Spark" till you push it up like 75, then something is out of line..

Did this do that on the old t-stat as well?
 
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Old 10-05-05, 05:51 PM
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Heat anticipator

The round Honeywell thermostats have a variable resistor called a "heat anticipator". This needs to be adjusted to match the gas valve & ignition system amp draw. My guess is it has never been adjusted & is causing the thermostat to be "satisfied" before the burner even lights.
 
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Old 10-05-05, 08:23 PM
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dreamchipper
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Red face Thank you

I appreciate the help.

Yes, once the furnace gets going, it shuts off exactly at the set point. I haven't had any troubles getting the thing to stop. And you're right, the spark doesn't happen till around 76 or 78, which frustrates me because I've levelled it exactly and I know the wires are hooked up right; there are only two of them and it's the only configuration that actually causes the furnace to work.

I saw the heat anticipator listed in the little instruction fold-out with the thermostat. Isn't this the same little dial you push with a pencil point? I have it all the way to 1.2 because other settings weren't causing the furnace to run long enough. I haven't changed it yet this season, though. Maybe I should try again, or does anyone know of another thermostat that will be easier to deal with?

Thanks again for taking the time to lend a hand. I hope I'll be able to return the favor one of these times.
 
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Old 10-06-05, 08:42 AM
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Sounds like the spring coil is out of place when you need heat. I would suggest trying a new one.

Honewell has a Round electronic type that is non-programable.
 
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Old 10-06-05, 05:39 PM
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Thermostat

I'd go with anything electronic. My favorite non-programable is the White-Rodgers 1F78-144.
 
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Old 10-06-05, 08:53 PM
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dreamchipper
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Smile Will try a new thermostat

Thank you very much, both of you! I will try a new thermostat per your suggestions. I'll post a new thread once I've got one in place to let you know how it goes.

Thanks again for taking the time to help. Have a great week!
 
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