Lennox furnace will not provide gas.

Old 01-06-22, 10:20 AM
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Lennox furnace will not provide gas.

It's a Lennox EL180UHO70E36B-01 unit. This morning the house was cold so I started looking into it and here I am now.

As I am watching it, the error light gives a slow steady blink indicating it is functioning normally. The induction fan kicks on for a while, then a click and the sid(?) starts to glow, stays on for about 5 or 10seconds, then another click and everything stops. Then the error light starts it's slow steady blinking and the whole thing repeats continuously. It doesn't stop after 3 tries, it keeps trying with no error code. Each cycle takes about 30 seconds and the gas never kicks on. No solenoid sounds or gas flow sounds from the gas valve either.

Other observations from troubleshooting:
If I switch off the gas valve the fan does not stop and I will get the 3 blink code "burner failed to light or lost flame", but when I switch it back the above cycle continues and the code goes away.
Voltage across the gas valve is 0Vac until the final click (I understand this is normal), then it goes up to .6Vac momentarily and back to 0.If I disconnect the leads from the gas valve and check voltage, I will get 28Vac after the sid has been on for a bit and the fan will continue to run and I will get the 3 flash error code again.
I also removed and cleaned the flame sensor and ignitor because it's easy and quick and was hoping that would fix it, but it didn't.
The second click, just before the fan stops and the cycle starts over, is coming from the K1 relay on the control panel. I don't have the full wiring schematic, just what's posted on the back of the panel, but it looks like the K1 relay activates the gas valve.

My theory is when the K1 relay activates something causes an overload and the system shut down and restarts, I don't know what it could be or how to test it. It could be the gas valve, the control panel or even the main blower for the house. If I had a full wiring diagram (something that explained what the relay connects to) I could start narrowing things down, but I got nothing.

I'm an electrician, nothing with HVAC, valves scare me, especially gas valves. Money is tight, so if this is something I can fix myself I would be greatly appreciated. Please help, it's so cold out there.
Old 01-06-22, 12:46 PM
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Welcome to the forums.

I can't find that model number. Several similar. Check and repost it.

Error codes and wiring diagram should be on the back of the blower door.
Typically the HSI (hot surface igniter) heats for at least 15 seconds and upwards to 30 seconds.

After the igniter warms up.... the click you hear just before the igniter goes out is the gas valve relay.
The gas valve only gets powered a few seconds and if there is no flame detected it starts the cycle again. It should only go thru three cycles and then lockout. I would need the part number off the control board. It's down in the blower area.

You had the meter on the gas valve and saw a brief voltage spike.
Then you disconnected the gas valve and monitored the wiring. You saw 24v.

That means one of two things.....
1) bad gas valve relay.
2) corroded loose wiring at the 9-12 pin connector.

The relay is the typical problem requiring a board replacement.
Old 01-10-22, 06:13 AM
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The model number is straight off the plate, the O might actually be a zero or vice versa. Doesn't matter. The fact that I'm getting 24V (28V actually) means the relay is doing it's job. I disconnected the plugs and inspected the wiring, I am well aware of what corroded wiring looks like, and everything looks good with solid connections. Also still wouldn't get the correct voltage if that was the case. I figured it had to be the gas valve itself.

I explained what was happening to the local hvac repair guys. They confirmed it was the gas valve solenoid, which means it needed a new gas valve. Fortunately the system had a 20 year parts warranty so it was a relatively cheap fix. New valve, warm house, happy ending.

Thanks Pete

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