Lennox furnace help please

[Coloshaver]

I have a Lennox G50UH furnace that has quit heating. With the thermostat calling for heat, the Combustion Air Inducer starts up and runs for about 2 seconds getting almost up to speed then I hear a relay click and it shuts down with the two green LEDs slow blinking indicating normal operation. The motor sounds fine as it comes up to speed and turns easily by hand. I don't think it is a motor problem. It seems like the control board is shutting it down.

I put a meter on the Combustion Air Proving Switch. The digital meter reads open until just before the motor shuts down when it flickers, but never settles on a reading. I think the switch is starting to close, but the meter isn't fast enough. I jumpered the Combustion Air Proving Switch and there was no change in the behavior.

The start up sequence says the Primary Gas Limit switch and the Flame Rollout switch need to be closed before the Combustion Air Induction Motor starts so I assume they are OK.

At this point, I am leaning toward the Control Board being the culprit, but at $250, I'd take any suggestions a Lennox expert might have. Anything else to check before I pull the trigger on a new Control Board?

Thanks in advance for your help,

 

[Duke74]

Could your rollout switch need to be reset? It doesn’t take much to trip one. Try resetting it. If you don’t know if it reset or not, you could jumper across the rollout switch, and see if it starts up then. Just my 2 cents worth.

 

[Gizzi]

If the pressure switch isn't closing, and no other safeties are tripped, you could have an obstructed flue pipe. To test, pull the flue pipe off the furnace but don't leave it that way. It's only for testing. If it fires, there is an obstruction.

 

[Jim greengo]

Is the thermostat calling for heat?

 

[PoorUB]

I would suspect if the LED's on the board are blinking normal operation the board is most likely bad.
Which board is it? Part number? I might be able to get you one wholesale., but not returnable!

 

[Coloshaver]

I would suspect if the LED's on the board are blinking normal operation the board is most likely bad.
Which board is it? Part number? I might be able to get you one wholesale., but not returnable!

PM sent. Thank you

 

[rharman]

Does it have a hot surface igniter? Symptoms sound similar to what I experienced and that was the problem. It wouldn't heat up so the start sequence timed out.
Mine was under $30 from Amazon.

 

[PoorUB]

Does it have a hot surface igniter? Symptoms sound similar to what I experienced and that was the problem. It wouldn't heat up so the start sequence timed out.
Mine was under $30 from Amazon.

Read again, it doesn't get that far. The inducer turns on, and in a second or two shuts off. The inducer should run for 10-15 seconds, maybe longer while the igniter warms up.

 

[Jim greengo]

Did you try jumping red/white at the board to eliminate the simple things?

 

[Coloshaver]

Conclusion

I thought I'd update you all on what I found.

Convinced it was the Control Board I ordered one. While I was waiting for it to arrive, I worried that the control board might not really be the problem (I really hate throwing parts at problems and not finding the root cause) so I did more research and found the the double slow blink of the LEDs indicated "normal" operation, but when heat is called for it should have been a fast blink.

This was puzzling because the thermostat indicated "Heating". Jim greengo asked the question and I thought, "of course." I thought maybe the batteries might be low, so I pulled the thermostat off its wall mount and found none of the wires were connected. We've owned the property for 3 years and the furnace has worked - just not recently. The wires haven't been recently disconnected. Turns out it was a wireless thermostat - not WiFi, but Honeywell's proprietary Red Link. I poked around in the crawl space for the receiver and found it. It still thought the thermostat was connected - the connect light was ON (not blinking indicating it was waiting for a connection.)

Now I really wondered if the furnace was seeing the call for heat. Jim's question, "Did you try jumping red/white at the board to eliminate the simple things?" really nagged me. I bypassed the door switch and jumpered Red to white - Eureka!! the thing fired up and ran. At that point, I was REALLY glad I hadn't even opened the box with the new control board.

The next hour was spent trying to get the stupid thermostat to connect to the furnace. It turns out the with the Vision Pro thermostat, it needs 24 AC to power it when it is trying to connect wirelessly. So I had to find the T-stat cable in the crawl space, find a 24VAC source on the furnace and hook everything up. Once I had 24 VAC to the thermostat mount, the wireless connected just like the instructions indicated and everything works.

Huge Thank You to Jim greengo. He had it all along. The control board is on its way back to the vendor.

 

[Jim greengo]

I've learned lots of things the hard way over the years. Hahaha