lennox gas furnice will fire sometimes

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Old 01-28-13, 10:54 AM
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lennox gas furnice will fire sometimes

hey I have a Lennox furnace, that the inducer motor will come on and spin for like 2 or 3 seconds and then shuts off. it will come back on after a few minutes and do the same thing again.I check the pressure, it is kicking in the pressure switch. on the schematic it shows a limit switch after the pressure sensor, if I take the wire off of that pressure sensor the indoor blower motor starts to turn instantly. in about 1 minute the inducer motor will spend and the furnace fires like normalit will run for about 10 or 15 minutes and then shut down. when it starts up it may or may not fire, if it does the burners will come on for about 1 minute and just before the blow motor comes on the burners go out. if I pull that wire off of that limit switch again it starts all over but never runs more than 15 minutes or so. I am NOT leaving that wire off I just momentarily take it off and put it back on when the blower motor starts.

I checked the error lights and they are blinking error code "Primary or Secondary limit open" . So I shorted all switches, one at a time to find the bad switch. gave same error on all until I shorted them all at once. I inserted a temp probe to make sure it didnt get sky high.The temps did get up to 150 degrees then gave another error code that says "Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR:Open ignitor circuit."
I checked the voltage and the ground. I'm not sure about the broken ignitor because it did ignite for 45 sec.

any clues?
 
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Old 01-28-13, 01:39 PM
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The inducer motor should turn on and stay on whenever there is 24 VAC applied to the W terminal on the furnace.

The W terminal should be energized whenever the themostat is calling for heat.

So use a multimeter to see if the thermostat is switching on 24 VAC to the W terminal and keeping it energized. If it is, the inducer motor should turn on and stay on.

If the inducer motor continues to operate erratically with 24 VAC applied to the W terminal, check to see if 120 VAC is being applied to the inducer motor to keep it on. If 120 VAC is being applied to the inducer motor and it's operating eratically, replace the inducer motor.

If you have 24 VAC at the W terminal, the ignition control module should be turning 1230 VAC on to the inducer motor. If that voltage isn't being switched on by the ignition module, replace the ignition module.


The less said about the rest of you diagnostic methods, the better.
 
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Old 01-28-13, 08:36 PM
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how does this flame look to everyone?

https://www.youtube.com/watch?v=4IkS...e_gdata_player
 
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Old 01-29-13, 12:13 AM
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Looks like a UFO

Aren't the burners horizontal ?
Where you holding the camera on it's side ?
 
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Old 01-29-13, 07:03 PM
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yes they are horz but the camera is sideways. I got the plug off and checked the pressure was 3.2. I brought it up to 3.5 and of course it does the same thing. It pops the limit switch just before the fan starts, but then after the fan run it will refire and run for 45 min or so and then give the low voltage error code. This time I ran it with the furnace doors open to make sure there was not a air problem. Im at a lost! the only thing I can think of is the board. Any thing else I should try. oh yeah I did check the exhaust and it was 148 just after the fan but went down quick as I checked further up the stack, so it is not blocked
 
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Old 01-30-13, 12:09 PM
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<<
oh yeah I did check the exhaust and it was 148 just after the fan but went down quick as I checked further up the stack, so it is not blocked

Read more: http://www.doityourself.com/forum/gas-oil-home-heating-furnaces/487393-lennox-gas-furnice-will-fire-sometimes.html#ixzz2JUGx4wuH>>



Please describe in more detail what you did and where you measured the temperature.

It would be useful to post the temperature at which the limit switch opens. Usually that will be listed on tghe limit switch in the form of "L180" or whatever, which means that it's a limit switch that opens at 180 degrees F.

<<
. I got the plug off and checked the pressure was 3.2. I brought it up to 3.5 and of course it does the same thing.

Read more: http://www.doityourself.com/forum/newreply.php?p=2063846&noquote=1#ixzz2JUHbG7zN>>


I would reset the regulator pressure to what you found it at. There can be good reasons why the pressure would be less than 3.5", and changing it arbitrarily might overfire the furnace.
 
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