Furnace runs but have to manually restart

I'm having a problem with my furnace that I hope someone can shed some light on. The furnace will run and then usually get to the desired temperature set by the T-stat and stop. Then when the T-stat thinks it should run again the the T-stat will show that the furnace is running (it's an electronic T-stat), but the furance will not kick on. If I am near the furnace, I can only hear a real faint buzzing sound (which goes away if I shut the power off to the furnace). If I either turn the T-stat off and on a few times or turn the power to the furnace off and then on, the furnace will usually kick on (until the next go around).
It seems to be a furance issue and not a T-stat issue, but I'm looking for some help. Unless it is something really easy to fix, I plan on calling a repairman, but I would still like to be knowledgable of the problem beforehand.
It is a forced air Lennox furnce about 18 years old. I will mention too that it started doing this a couple of days ago soon after I used the electrical outlet on the furnace to power a shop vac (I rarely use this outlet -- and I don't know if this would be related anyway).
Thanks.
Jim
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This is Turtle.
I wouldn't say you have a thermostat problem but a Failure to start on the furnace. I would say i would call a service company to see why it is failing to start.
TURTLE
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There is a sensor around the top of the burners usually held in by one screw with a wire hooked up, remove the wire remove the sensor, clean with steel wool or fine sand paper and replace.

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you are clueless sir, not on an 18 year old lennox. where did you get this info? Alt.home.repair ?
1991 Z28 wrote:

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You should know that I don't "know much" about furnaces but if it did have the flame sensor the gentleman could have possibly fixed it himself. Its NO reason to be such an asshole.

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If you don't "know much" about furnaces, then there's no reason for you to give the first bit of advice with such an air of authority. People who know better get most frustrated and respond accordingly when incorrect info is provided as if it is true in all cases without proper caveats. Try it like this:
"If your furnace is like those I've seen, there may be a flame sensor around the top of the burners usually held in by one screw with a wire hooked up. If there is such a sensor, remove the wire remove the sensor, clean with steel wool or fine sand paper and replace."
That way, you're covered if the OP furnace is outside your experience.
%mod%

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In your experience, does a flame sensor problem permit the system to attempt startup, but then shut down after ignition on failure to prove flame?
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wrote in message
That's not the problem. It is not the spill switch. I think you should call a qualify HVAC person to check your furnace.
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Jim... I'll try to help while the other boys are fighting it out. First if it's an oil furnace ( you didn't say) it could be what you're hearing is the motor trying to start. When you play with the stat it will at some point, start the burner depending on the inrush of current. It could also be that the motor isn't starting and what you're hearing is actually the spark from the electrodes. It'd be faint and almost sound like a hum. If this unit is gas, you'd need to continue to hope the other guys put down the gloves and try to help as I've had no training in gas furnaces. I've had since '75 on oil but up here in Malone NY, no one has gas.
Doug
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FYI, If this is Lennox Conservator 'Mid-Efficency" furnace with the Electronic ignition lite pilot lite and motorized Chimney damper there is a well known problem with the electronic controls going on them, usually made by Johnson.
Most of the ones here installed about 19 - 20 years ago have either been replaced, had the controls replaced or had the controls removed and been converted to a pilot light and no damper configeration.
How do I know? I had a Lennox Conservator G8E model and found out by speaking to both the gas company and several HVAC guys over the 4 years it took to completely give up the ghost. Lennox makes a retrofit kit but I wasn't prepared to spend $500 to have it installed on a 20 yr old furnace.
This info may relate or not.
Canadaloon
BTW I had the flame sensor (Lennox part 12F1901, Sensor: Flame sensor for penn valve) replaced 3 times in 4 years before it finally went.

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Is the furnace gas, propane, or fuel oil?
Can you tell if the furnace gets warm (but doesn't blow the hot air into the house), or doesn't get warm at all? If the furnace gets warm, but the blower doesn't blow, then one possibility is the run capacitor on the blower fan. Clicking the furnace off and on will sometimes start a fan blower with a bad capacitor.
As a couple of other posters mentioned, we would like some more information. I doubt the shopvac had much to do with the problem.
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Hi Stormin, hope you are having a nice day
On 26-Jan-04 At About 23:25:06, Stormin Mormon wrote to Stormin Mormon Subject: Re: Furnace runs but have to manually restart
SM> Is the furnace gas, propane, or fuel oil?
SM> warm, but the blower doesn't blow, then one possibility is the run SM> capacitor on the blower fan. Clicking the furnace off and on will SM> sometimes start a fan blower with a bad capacitor.
Wrong again!!
SM> As a couple of other posters mentioned, we would like some more SM> information. I doubt the shopvac had much to do with the problem.
I don't know what you would do with any more info. you don't have any idea what to do with it.
-=> HvacTech2 <=-
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