Lennox Furnace, burner won't shut off

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Old 09-25-07, 08:11 AM
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Lennox Furnace, burner won't shut off

i have searched the forum, but is does not seem like anyone had this problem before, hard to believe. I have older Lennox Furnace, about 20yrs old. I had it turned off for summer with pilot light off. Yesterday i went to light up the pilot and and as soon as i turned the knob to ON the burners fired up. the thermostat was set to lower temperature to prevent this. I let it run for a while and house was getting very hot. i have turned OFF the thermostat and nothing happened. i then proceeded to remove the thermostat altogether (it's a programmable kind). but it still continued to run with thermostat cables disconnected. So i think that thermostat is not at fault. Basically the only way to turn it off now is to switch off the power switch to the furnace and then it stops (but this does not allow it to cool off properly, not good). as soon as i turn the power back ON it fires up and after it heats up the blower comes ON. So we are not too desperate, because we can heat up the house, but automation is gone. Can anyone help me to troubleshoot this beast. i really don't want to have to call a serviceman, they will just want to sell me a new one (which may not be a bad idea, just not right now). Thanks in advance.
 
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Old 09-25-07, 10:18 AM
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Disconnect the thermostat wires at the furnace connection. If this causes it to stop then you have a problem with the thermostat wire itself and it (the wire) will probably need replacement.
 
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Old 09-25-07, 10:23 AM
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I already did that and it made no difference.
 
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Old 09-25-07, 11:22 AM
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When you originally wrote that you had disconnected the thermostat cable I assumed that you made the disconnection at the thermostat and not the furnace.

At this point I would advise that you have a Lennox-certified serviceperson check it out as it may be beyond DIY.
 
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Old 09-25-07, 11:45 AM
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Wink

Basically the only way to turn it off now is to switch off the power switch to the furnace and then it stops .

If that turns it off . Then you have to have a short in the tstat wire to the gas valve. In the wire , the tstat or there at the furnace. Try and pull just one wire off the gas valve.
 
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Old 09-25-07, 12:09 PM
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when i pull the power wire off the gas valve then it tuns off. But what controls the power to the gas valve? i think that is what's busted. because the gas valve stays energized all the time.
It cannot be the t/stat wire short because i already disconnected it completely.
 
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Old 09-25-07, 09:24 PM
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It cannot be the t/stat wire short because i already disconnected it completely.

At the furnace???????
 
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Old 09-25-07, 09:43 PM
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yes, at the furnace.
 
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Old 09-26-07, 11:51 AM
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Originally Posted by rebus View Post
yes, at the furnace.

most likely you have a bad ignition module or someone has futzed with internal wiring on the furnace. What you have is a very dangerous condition because if the blower fan were to quit the furnace would start a fire. Depending on which model you have Lennox has used mostly Johnson Controls and White-Rodgers modules. It's probably best to call the local Lennox dealer.
 
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Old 09-26-07, 08:39 PM
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yeah, i already figured about this and had shut it off completely. thankfully it's not too cold yet in the northwest.
i decided to replace the whole thing rather then spending hundreds on repairs. it's 28 years old and grossly overpowered for the house (120K BTU), about double of what we actually need. Thanks for the help to all of ya. i am glad i could puzzle the pros
 
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