HVAC Experts?

tworley

tworley

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Anyone an HVAC expert?

Forced air heating. Lennox furnace manufactured sometime in 2011/12 with a Google Nest thermostat. The issue is the heat will not kick on, but only intermittently. We've woken up a handful of times in the morning and the house is 55°. Replacing a clogged filter as well as giving the igniter and flame sensor a good clean with steel wool helped immediately. As soon as I did that it fired up and worked great for about 2 weeks. Constant heartbeat on the control panel. The last two mornings--we woke up to a 55° house again.

What I know:
-The control panel is flashing 5 times which indicates a "gas burner failed to ignite" code.

-Turning the furnace off/on again typically helps, though I may have to do it 3-4 times for it to finally ignite.

-The igniter gets red hot. I am only assuming red hot is hot enough to light the gas that is being vacuumed through.

-The flame shoots across the burners for a split second--but from what I can tell it is not even long enough to hit the farthest burner that has the flame sensor in front of it. This suggests to me that the flame sensor isn't the issue, since it doesn't see the flame at all because its cut out so quickly (maybe I am wrong there?). This happens for 5 cycles and then it goes into lockout mode. Hence the hard resets that I have been having to do.

-Pressure switch is active. I can blow into that and hear the click meaning it is closing. I can hear it click with the furnace trying to start as well.

-Gas valve--How can I troubleshoot a gas valve?

Some additional background--we had the popcorn ceiling removed prior to moving into the house, as well as carpet. My popcorn contractor wasn't exactly "clean" since keeping the old carpet clean was not a concern. I only mention this because when I replaced the clogged filter it was full of white dust and particles. Is it possible that the return vents have sucked in so much popcorn dust that it has clogged the burner and/or gas valve? I took my compressor to everything and blew it out the best I could. What should I try next? I had a contractor out, but surprisingly the system was working fine when he was there. Its hard to diagnose an issue when everything is working fine.
 
I'm definitely not an HVAC expert, but I've had to troubleshoot a few systems in my day, and the path you have outlined makes sense.
I would think if the gas valve itself was bad, that you would never get a flame.
The ignitor was my issue last year, but yours is getting hot and you get flame for a short time, which suggests to me that a thermocouple may not be holding the gas valve open as it should.
 
WestCoastDan said:
I'm definitely not an HVAC expert, but I've had to troubleshoot a few systems in my day, and the path you have outlined makes sense.
I would think if the gas valve itself was bad, that you would never get a flame.
The ignitor was my issue last year, but yours is getting hot and you get flame for a short time, which suggests to me that a thermocouple may not be holding the gas valve open as it should.
Click to expand...
Are thermocouples used on a forced air system that has a flame sensor?
 
tworley said:
Are thermocouples used on a forced air system that has a flame sensor?
Click to expand...
Thermocouples are used on every gas-fueled heater to control the flow of gas. They prevent the gas from continuing to flow if the flame goes out - or if the thermocouple thinks the flame has gone out.

I recently had to replace the TC on one of our portable heaters because it would not stay lit; which sounds exactly like the issue that you are having with your furnace.

The 'flame sensor' may be the thermocouple in your system.
 
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WestCoastDan said:
Thermocouples are used on every gas-fueled heater to control the flow of gas. They prevent the gas from continuing to flow if the flame goes out - or if the thermocouple thinks the flame has gone out.

I recently had to replace the TC on one of our portable heaters because it would not stay lit; which sounds exactly like the issue that you are having with your furnace.

The 'flame sensor' may be the thermocouple in your system.
Click to expand...
I'll need to take a closer look at the furnace. But from what I recall it was gas line>gas valve>igniter>burner>flame sensor. I dont remember seeing anything around the gas valve but I cannot say with confidence.
 
I claim HVACR expert status a lot around here but unfortunately my expertise is all in the -ACR part of it. I'm inexperienced and blissfully ignorant about furnaces beyond the underlying thermodynamic principles of operation.

Your troubleshooting process seems valid and I don't have any good ideas.
 
you may not have a thermocouple if it's electric ignition, which this sounds like. however your flame sensor is likely the culprit. they have a protective coating on them that helps prevent accumulation of soot. on an in-shot burner, if you've scrubbed the flame rectifier and it resolved, i would highly suspect that it re-sooted and has failed. i'd pick up a new one and i bet it resolves it. the question i'd then look to answer is why did it soot, and my guess is that it wasn't derated for our altitude correctly
 
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Only TJ's said:
you may not have a thermocouple if it's electric ignition, which this sounds like. however your flame sensor is likely the culprit. they have a protective coating on them that helps prevent accumulation of soot. on an in-shot burner, if you've scrubbed the flame rectifier and it resolved, i would highly suspect that it re-sooted and has failed. i'd pick up a new one and i bet it resolves it. the question i'd then look to answer is why did it soot, and my guess is that it wasn't derated for our altitude correctly
Click to expand...

Good info--Ill see if I can find a replacement one locally. I'll hit the burners again too with compressed air and see if I can lodge anything loose.
qslim said:
I'm not an HVAC expert by any means, but I used the site linked below to troubleshoot what ended up being an intermittent faulty igniter. Here's an article on how to check out the gas valve

https://www.foxfamilyhvac.com/troubleshooting-a-furnace-gas-valve/
Click to expand...
Ill grab a new mutlimeter today on my way home and see if I can get a reading from the valve. Thanks!
 
tworley said:
Good info--Ill see if I can find a replacement one locally. I'll hit the burners again too with compressed air and see if I can lodge anything loose.

Ill grab a new mutlimeter today on my way home and see if I can get a reading from the valve. Thanks!
Click to expand...
you should be able to pull the connector from your board/valve, put your microAmp meter in series, and see ~4 microamps ish
 
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The gas valve should be 24 volts. If the flames are not making it across the burners to the flame sensor, you may want t to check the gas outlet pressure. There should be a brass cap with a flat screw diver slot. Remove the cap and there will be a plastic adjuster inside. Screw that in to increase gas to the burner and back it out to decrease. It's most likely a gas valve sticking.
 
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Is the flame going across the burners moving fairly fast? But the gas valve shuts down be fore flames reach sensor? Could be a bad circuit board. If voltage drops when flames go out, possibly the board.
 
Rob L. said:
A short video starting before anything turns on would help
Click to expand...

Typical sequence of a gas furnace. The heat gets called from the thermostat and I can hear the inducer motor come on, the pressure switch closing (clicks), the igniter gets red hot, then this is where I get issues. The flame comes on for a split second and fails to stay lit. Then the cycle again for 4 more tries and lock out.

B252 said:
Is the flame going across the burners moving fairly fast? But the gas valve shuts down be fore flames reach sensor? Could be a bad circuit board. If voltage drops when flames go out, possibly the board.
Click to expand...

The flame is quick--I'd say less than a second. I dont see the flame reaching the flame sensor before it begins to try again.
 
tworley said:
Typical sequence of a gas furnace. The heat gets called from the thermostat and I can hear the inducer motor come on, the pressure switch closing (clicks), the igniter gets red hot, then this is where I get issues. The flame comes on for a split second and fails to stay lit. Then the cycle again for 4 more tries and lock out.



The flame is quick--I'd say less than a second. I dont see the flame reaching the flame sensor before it begins to try again.
Click to expand...
FWIW . This does sound like a faulty / sticking gas valve. Since you don't have enough flame duration to allow the flame sensor to "make" or prove, the safety system is doing it's function as intended. It sucks that it's Friday night and it's snowing but If I was in your shoes I would look real hard at the sticky gas valve.
Hopefully you have another safe source of heat until you can work this out.
Hang in there man!
 
have you attempted to bypass the nest thermostat by wiring in a different tstat or manually jumping just the heating circuit wires yet?

there has been times in the past where ive been stumped and spent quite some time diagnosing a unusual problem with no heats which ultimately turned into being a faulty thermostat... rare but it really does happen.

could also check the pressure switch with a ohmmeter to see if its correctly operating. let the inducer motor spin up and measure the terminals on it to see if your switch stays closed or if it opens circuit after a few seconds. though if your inducer stays running and the ignition sequence continues attempting to light I wouldn't expect this to be the issue.

also if adjusting your gas valve setting by removing the cap and turning the screw, you kinda want to have a manometer available. too low or too high of a setting is harmful to your heat exchanger & your wallet.
 
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its always the flame sensor/thermocouple on my old furnace and my new one. cheap and easy to replace.
I do It every 2 years now, needed or not.
 
tworley said:
the igniter gets red hot, then this is where I get issues. The flame comes on for a split second and fails to stay lit. Then the cycle again for 4 more tries and lock out.
Click to expand...
them suckers get white hot, but as long as it ignites. then that flame should come on like a mini jet engine for IDK 6-10s until it sparks the rails.
i'd be lookin at the supply for the ignitor jet or the rails.
 
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