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Old 05-24-2013, 01:16 PM   #1
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Furnace not lighting


title almost sounds like a broken record...

I did review the forums and got some good insight, but I just need to confirm the problem before I plunk down $70 on the part.

I have a Lennox 80UHG 3/4 that does not light. For the most part the diagnostic code is not helpful except for once. The diagnostic was one off and the other slow blink, which points to the the pressure switch. However, that only happened once and hasn't shown the same signal since.

The sequence of events that I can tell is call is being made for heat. Induction fan briefly turns on. I did at one point see the heating plate turn on, so I know the induction fan works and the thermal plate. I have checked the hose from pressure switch to induction fan and that is clear. I even blew into the pressure switch port.

Flame sensor is working and everything has signal going to it as far as I can tell.

So, new pressure switch? or am I missing something? (which, is of course, entirely possible.)

Chris

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Old 05-24-2013, 04:14 PM   #2
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Furnace not lighting


Quote:
Originally Posted by cscpianoman View Post
title almost sounds like a broken record...

I did review the forums and got some good insight, but I just need to confirm the problem before I plunk down $70 on the part.

I have a Lennox 80UHG 3/4 that does not light. For the most part the diagnostic code is not helpful except for once. The diagnostic was one off and the other slow blink, which points to the the pressure switch. However, that only happened once and hasn't shown the same signal since.

The sequence of events that I can tell is call is being made for heat. Induction fan briefly turns on what do you mean briefly turns on? if it does not stay on then it creates no pressure on the switch and will shut off and give you that code. if it won't stay on then the thermostat is shutting off or the circuit board is faulty or the inducer is faulty. you need to see if it is getting 120 volts, if not then the board is faulty or you got a bad wiring connection to it on the board in one of the wiring harnesses. most likely it is the inducer fan relay on the board that is worn out/faulty. cozyparts.com for Lennox parts
Chris
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Last edited by yuri; 05-24-2013 at 04:19 PM.
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Old 05-25-2013, 04:07 AM   #3
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Furnace not lighting


Could be the pressure switch. or an obstruction in the flue pipe, or bad heat exchanger.
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Old 05-25-2013, 06:15 AM   #4
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Furnace not lighting


So, I threw the multimeter on the inducer fan and watched the process of starting up.

1) signal makes it from thermostat
2) inducer fan turns on and reads at 120V
3) Strange signal happens from control board - fan turns off/on about 3 times before the fan just stayed off. The voltage on the inducer fan dipped at various times, but came back up.
4) Thermal plate doesn't turn on.

The A/C works fine, btw. Don't know if that helps or not.

Ironically, my wife and I have already been in discussion about replacing the furnace/AC several months prior to this. Previously, the whole unit hasn't been well taken care of before we moved in about 1 year ago. The units are about 14 years old. The A/C is sitting on brackets that are out of level and leveled by me with various bricks and so forth. There is clear indications of rust in the furnace. Considering, the above mentioned stuff and the wonderful expense of a control board, I am almost half-tempted to just push for a new unit.

Thoughts?

Last edited by cscpianoman; 05-25-2013 at 06:19 AM.
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Old 05-25-2013, 07:39 AM   #5
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Furnace not lighting


that may be a good idea. nowadays parts last about 15 yrs before you have to start replacing boards, gas valves etc so it has had it's best years of service and if it was neglected then it may have 18-20 yrs of wear and tear. after 15 yrs in my climate lots of people don't want to spend big $$ on repairs or have unreliable units so they upgrade to better ones or more high efficient units. same with the AC as compressors wear like car engines. going to be spending more $$ for the next 5 yrs to keep them going.
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