Lennox G40UH furnace stops and fails to start at nights only

Problems related to residential installations.

Lennox G40UH furnace stops and fails to start at nights only

Postby Muscovite » Thu Feb 13, 2014 10:52 am

I have a 7 years old Lennox G40UH-36A-070-11 furnace which recently started turning off and failing to turn on until restarted. This only happens at night time starting around 9-10 PM and going on all night, 3-10 times per night. During the day, the unit works fine. When the failure occurs, the code lights read Alternating Slow Flash which means "Watchguard −− burners failed to ignite or lost flame 5 times during single heating".

I made a video of the moment when the furnace fails - w w w . youtube. com / watch ? v * zGzqCzxtdMQ (unfortunately the URL gets messed up after I post it, so please (1) delete spaces AND (2) replace * with an equal sign)
Before that failure the furnace was working fine for about an hour, no failures were seen on the video.

When the failure occurs we do one of 4 -
• restart the furnace using the main power switch on the unit;
• switch the thermostat to Off and back to Heat;
• raise a desired temperature on the thermostat from our usual 72-73 to 80 degrees (my wife's recent discovery). In this case the furnace does NOT starts immediately, but after some time (I tested yesterday and it started about 40 minutes after raising the temperature);
• about a week ago, after reading somewhere that at nights the gas pressure is different than during a day (less gas-powered appliances are used) and this somehow can affect the furnace's work, we let the hot water running in the sink all night, and that night we did not have any problem with the furnace. But yesterday I tried to repeat the same solution and it did not work - the water was flowing, but the furnace still failed.

I remember similar thing happening a year before, but I fixed it by cleaning the flame sensor with the steel wool. This year it started again, and I again could fix it with the steel wool. For a few days after I'd done that, the unit worked fine. But by that time we had a heating tech visit scheduled, and so he came, cleaned up the furnace and recommended to replace the flame sensor as the surface seemed damaged. After his visit we started smelling gas. I did smell a little gas in our basement for entire time we owned the house (6 years), but it was just very little (my wife did not sense it, so little it was) and having no experience living in a house with gas furnace before I thought it was normal (maybe some leftovers from the gas burned). But this time the smell was strong, we could even sense it on our main floor of the house, other people visiting our house reported it too. So we called the company again, another tech came over, found that the gas valve was leaking and so he replaced it. He also said that our gas pressure was too high and this probably damaged the old valve's membrane, so he reduced the pressure from 3.9 to 3.5 inches w.c.

After the gas valve was replaced, we started getting the problem above every night. We called the same company over and over again, each time they did something (see below), but it did not eliminate the problem.

Here is what we did so -
• Cleaned the gas flame sensor (myself).
• Replaced the gas valve (HVAC tech).
• Replaced the gas flame sensor with a new one (myself).
• Checked the ignitor resistance (I brought it to DYI shop and they checked it for me, the reading was 288 ohms which was within the normal range).
• Checked the pressure switch (HVAC tech).
• Checked the main valve outside of our house (Nicor's tech came over, checked the valve and confirmed that the pressure is normal and there is no moisture inside. HVAC tech was in the house at the same time, so Nicor tech explained him his results and HVAC tech had no questions about it).
• Cleaned the vent pipe (HVAC tech found that it was clogged with debris due to no vent pipe cover on the roof, I installed the cover later that day).
• Replaced the condensate pipe (myself). We have hard water, the pipe was clogged with calcium build up and as the result the water started coming from underneath the furnace. Initially the HVAC tech thought that this was the central humidifier, but I checked myself later and found out that it was the condensate pipe, and so I fixed it.
• Replaced the thermostat (myself).

I don't understand what is happening and most of all why it is happening only at the night time, and why raising the desired temperature eliminates the problem. Does anyone have any idea? I will appreciate any help. Thanks in advance! -)
Muscovite
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- Lennox G40UH furnace stops and fails to start at nights

Postby Freon » Thu Feb 13, 2014 11:08 am

Do you now still smell gas like before?

If I read your post correctly, the initial cure for your problem (undetected burner ignition or no burner ignition) was to clean the flame sensor. Then it worked correctly.

Once the gas smell developed, the furnace was working correctly until the gas valve was replaced.

If that is how things happened, I'd first read the installation manual and lean what the manufacturer says the gas pressure to the burners should be. It seems changing that pressure by the tech may be a possible cause.

I would also check all electrical wiring to the furnace (120 volt main power) and all grounds on the furnace itself. Flame sensing usually requires very small currents to flow (flame rectification) and if any ground connection is not solid the current flow can be affected.

Lastly, control boards that do the flame rectification sensing do get old and maybe that is where the problem lies. But I would exhaust all easy, and less expensive, options first.
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- Lennox G40UH furnace stops and fails to start at nights

Postby Muscovite » Thu Feb 13, 2014 11:40 am

Freon, thank you for your quick response!

> Do you now still smell gas like before?
No.

> If I read your post correctly, the initial cure for your problem (undetected burner ignition or no burner ignition) was to clean the flame sensor. Then it worked correctly.
Yes.

> Once the gas smell developed, the furnace was working correctly until the gas valve was replaced.
Yes.

> If that is how things happened, I'd first read the installation manual and lean what the manufacturer says the gas pressure to the burners should be. It seems changing that pressure by the tech may be a possible cause.
That's what our thoughts were too. It all started happening after the valve and the pressure were changed. But the installation manual says that the Manifold Pressure (Outlet) should be 3.5 inches w.c. for a Natural Gas fuel. The heating tech said the same. He also said that if he raises the pressure to the previous level of 3.9, this can damage the new valve too.

> I would also check all electrical wiring to the furnace (120 volt main power) and all grounds on the furnace itself. Flame sensing usually requires very small currents to flow (flame rectification) and if any ground connection is not solid the current flow can be affected.
I will do this tonight. Thank you.

> Lastly, control boards that do the flame rectification sensing do get old and maybe that is where the problem lies. But I would exhaust all easy, and less expensive, options first.
Are you talking about the circuit board? It is not extremely expensive ($100-130), but I agree with you that it is better to check other options before investing in a new board.

I've got a reply to my question on another board, maybe it will give some extra -
***
You have a unstable flame which is jumping around the flame sensor and should not POP when it drops out. The tech needs to check the manifold pressure to make sure it is a STEADY 3.5" WC and meter clock the furnace to make sure it is not overfired, also he needs to check the flame sensor current in microamps to make sure it is stable. you also need to clean the face of the burner that the flame sensor sits in front of with emory paper, not steel wool as it gets rusty and corroded and will give you a poor flame signal. try that first. the sensor also needs to be PERFECTLY vertical and in line with the face of the burner. I have had a few with a burr in the sheet metal where it goes thru (screw goes thru) and that can cause it to move out of line and give you a poor signal. if it is there then file it down so the metal is perfectly smooth. you cannot run that furnace with the blower door off and sucking air or it will disturb the flame, if the blower is not on yet then that is OK. I would also check the inlet pressure to the valve, should be 7"WC to make sure it is steady as the gas co regulator or meter could be failing.
***
Oh, and I updated a link to my video in my original post. Unfortunately the URL gets messed up after being posted, so please (1) delete spaces AND (2) replace * with an equal sign
w w w . youtube. com / watch ? v * zGzqCzxtdMQ

(The problem with URL formatting can probably be solved by enabling the BBCode feature for my profile, but it looks like only Admins can do that, but did not find a way how to contact them.)
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- Lennox G40UH furnace stops and fails to start at nights

Postby Freon » Thu Feb 13, 2014 3:28 pm

It seems the other reply is also directing you to really check the flame sensing circuit. It is important the flame sensing electrode is always in a good flame. His/her point about a possible dirty burner interfering with the flame direction is another possibility. Also the flame sensing electrode circuit really needs solid electrical connections since we're talking very few milli-amps of current being sensed by the control board.

I would go to your furnace and remove the front panel so you can observe the burners when they light. If you have a jumper wire between the R and W terminals at the furnace control board (where the thermostat wires connect) you can have the furnace light when you connect the R and W terminals. That way you can have many ignitions while you observe the flame that engulfs the flame sensor probe. You want to see that every ignition is consistent and the probe is engulfed by the burner flame in the same way each time.

The last thing to do is let the furnace run (with the jumper) until the main blower motor starts circulating air in the house. Watch carefully as the blower gains speed. You are looking for any kind of change in the flame for each burner. Sometimes a heat exchanger crack will let the circulating air escape int the combustion chamber distorting the flame. If it distorts the flame away from the flame sensor probe, your furnace will shut off.

Since this problem seems to be at night and started with the new gas valve I doubt you have a heat exchanger crack. But it is possible even if not probable.

If all looks good from a burner flame stability standpoint, then we're closing in on the control board... PC board.
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