Lennox 80ugh no heat shuts down after running

Problems related to residential installations.

Lennox 80ugh no heat shuts down after running

Postby Blown383 » Sun Nov 20, 2016 7:40 pm

Ive done my best to read through the forums to figure this out but Im kinda stumped.

The furnace ran for a sort time today and shut down. Went to check on it and the LED lights were flashing alternately slowly so I pulled the ignitor and flame sensor to clean but now it won't run at all and the ignitor doesn't glow. The difference this time is that only the secondary led stays lit and the first led is off which -

"pressure switch is open or has opened 5 times during a singe call for heat"
- blocked inlet/exhaust vent; OR condensate line blocked;
- Pressure switch closed prior to activation of combustion air blower

When I try to start the furnace by jumping R & W it'll start the inducer motor, I can hear the pressure switch engage, and then it'll shut down after running for a while and giving me that slow flashing secondary led.

Things I've checked/done so far.

1) Cleaned ignitor and flame sensor with steel wool.
2) Ignitor ohm'd out at 17ohms. Is that normal?
3) Transformer gets 120v and puts out 27v
4) I get 120v to the inducer motor
5) I get 25V between C & R & C & W
6) Checked the vacuum line for obstructions
7) Used a drill bit to clear the nozzle on the inducer motor
8) I get continuity on the pressure switch and can hear it click when the inducer motor spools up to speed

I still have to take the vent pipe off to see if there is anything stuck in the vent pipe but from the outside it doesn't look like anything could have gotten inside due to the exterior screen.

Any ideas?

-B
Blown383
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- Lennox 80ugh no heat shuts down after running

Postby Freon » Mon Nov 21, 2016 2:33 pm

From your description the problem appears to lie in the inducer assembly. Unless the inducer fan developes the correct flue pressure, the ignitor will never light.
Turn off the power to the furnace to reset the circuit board. You can TEMPORARILY jump the pressure sensor and then restore power and see if the furnace works.

If it does work you have shown the inducer system to be the problem. Next, AFTER you remove the jumper, have someone call for heat (have someone do that at the thermostat) while you need to be at the furnace to listen to the draft inducer motor. Does it make any loud noises either grinding or squeeling? If so the bearings are bad and you need to fix those or replace the motor.

If the motor sounds normal (no odd noises) then check all the hoses for any signs of cracks, bad conections or water in the hose. If the hose is defective the pressure will be affected.

If all checks okay so far, you'll need to remove the inducer assembly and check the small squirrel cage for debris/dust.
Freon
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- Lennox 80ugh no heat shuts down after running

Postby Blown383 » Fri Nov 25, 2016 4:32 am

Thank you for the tips! I tried jumping the pressure sensor and somehow blew the 3 amp fuse. After replacing the fuse and double checking all of the connections I noticed that the terminal block for the thermostat wires literally came off of the board due to a weak soldering job. I re-soldered the terminal block back on to the board and it ended up giving me some other error message.

I said screw it....and ordered a new board. Problem SOLVED!!!! I have heat again!!!

Thank you for the diagnostic tips.

-B

Freon -
> From your description the problem appears to lie in the inducer assembly.
> Unless the inducer fan developes the correct flue pressure, the ignitor
> will never light.
> Turn off the power to the furnace to reset the circuit board. You can
> TEMPORARILY jump the pressure sensor and then restore power and see if the
> furnace works.
>
> If it does work you have shown the inducer system to be the problem. Next,
> AFTER you remove the jumper, have someone call for heat (have someone do
> that at the thermostat) while you need to be at the furnace to listen to
> the draft inducer motor. Does it make any loud noises either grinding or
> squeeling? If so the bearings are bad and you need to fix those or replace
> the motor.
>
> If the motor sounds normal (no odd noises) then check all the hoses for any
> signs of cracks, bad conections or water in the hose. If the hose is
> defective the pressure will be affected.
>
> If all checks okay so far, you'll need to remove the inducer assembly and
> check the small squirrel cage for debris/dust.
Blown383
Minimal
 
Posts: 2
Joined: Sun Nov 20, 2016 7:19 pm


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