Need help with furnace issues!! Please!

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Need help with furnace issues!! Please!

Postby Martygross » Sun Nov 13, 2016 9:14 pm

Hi,
I have an older model (approx. 15 yrs old), furnace and I am having issues with keeping it going. It is a Lennox Model 80UHG 3/4 100A-3.
One day I noticed that it just blows cold air and will not ignite. I did some troubleshooting, and found out that the diagnostic code that it was giving (#1 light slow blinking and #2 light steady on), means that a Primary or Secondary Limit switch is stuck in the open position. By now...I have already changed out both secondary limit switches, the Primary limit switch, AND the 2 rollout switches!! Every time I have switched one of these out, the furnace works perfectly and I always think I have solved the issue. But after 3 or 4 days, it goes back to doing this same thing and blinking that same code.
I am getting quite frustrated now and don't know what to do next. Can anyone please help? Could it be the circuit board? The blower motor? the air inducer? Or just an electrical problem somewhere. Any help is GREATLY appreciated. Thanks!
Martygross
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- Need help with furnace issues!! Please!

Postby Freon » Tue Nov 15, 2016 8:31 pm

When you call for heat, does the ignition (pilot light or hot surface ignitor) try to light the main burner? Or is there no ignition attempt at all? If the inducer doesn't generate the correct pressure, there will be no ignition attempt. Once the furnace stops working correctly, will it ever, after a few attempts, work again for a few cycles?
Freon
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- Need help with furnace issues!! Please!

Postby Martygross » Tue Nov 15, 2016 8:59 pm

Thanks for the reply. Much appreciated!

Once the furnace goes into the "limit switch stuck open" code, it never works correctly again. It keeps blowing cold air forever. It doesn't attempt to light at all once its in that mode. When I switch the power off, wait a while, and then turn it back on, the inducer spins for a quick second, it sounds like it wants to start its normal cycle, but then after a series of clicks from the relays on the control board, it goes into that same "cold air" mode. This all happens even though I have the thermostat cranked!
At this point, I'm thinking of changing out the Control board and see if that does it.

But just so you know...Once in a while, (especially if I shut everything down overnight),it will actually do the correct thing after I turn the power switch on The ignitor works perfectly and the gas and blower motor all seem to work correctly. But eventually...it funks out again.
Martygross
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- Need help with furnace issues!! Please!

Postby Freon » Wed Nov 16, 2016 4:36 pm

It seems the inducer is not giving the correct pressure for the furnace to light. Check any and all hoses to the inducer assembly. To verify the inducer as the problem, TEMPORARILY jump the inducer safety/pressure switch. If the furnace lights successfully many times, you have the problem. Let us know.
Freon
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- Need help with furnace issues!! Please!

Postby Martygross » Thu Nov 17, 2016 8:41 pm

You were correct! I found out that the inducer was not getting power, which was caused by a faulty relay on the control board. I replaced the board and the furnace fired up perfectly. Now, if this furnace doesn't repeat what its done in the past, (conking out again after 3 or 4 days), then I would say we have solved this issue. If that happens, I will surely post my problem.
Many thanks for your help. Much appreciated!
Martygross
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Postby Freon » Sat Nov 19, 2016 2:07 pm

Check where that relay attaches to the control board. Given the intermittent behavior you may see a cold solder joint at the relay connection points. Resolder those connections and you may have a good spare control board.
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Postby Martygross » Mon Nov 21, 2016 1:56 pm

Thanks again! I did check this and found a "cracked" solder joint. I will re-solder it and keep as a spare. Hopefully, I never need it. lol
Thanks again for all the help.
Martygross
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