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Old 02-12-2014, 11:33 AM   #1
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Lennox G40UH furnace stops and fails to start at nights only


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 - http://youtu.be/zGzqCzxtdMQ
Before the 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 things:
  • 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 remainings 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 far:
  • 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 after).
  • 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, the HVAC tech blamed the central humidifier, I checked myself later and found out that it was the condensate pipe, and I fixed it myself).
  • 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!

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Old 02-12-2014, 12:47 PM   #2
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Lennox G40UH furnace stops and fails to start at nights only


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.

http://www.completeheating.ca/manuals.asp

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Old 02-12-2014, 09:32 PM   #3
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Lennox G40UH furnace stops and fails to start at nights only


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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.

http://www.completeheating.ca/manuals.asp
Thank you, yuri, for your quick response. I will try your suggestions.

Do you think you have an explanation why this is happening at nights, but never occurs during day time?
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Old 02-13-2014, 09:04 AM   #4
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Lennox G40UH furnace stops and fails to start at nights only


Colder at night and more people using gas/load on their supply. I suspect your gas meter or regulator is slowly failing. I have had that B4 and it is a bu*gger to diagnose or prove to the gas co. Have to be persistent and your tech may have to catch this at night.
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Old 02-13-2014, 09:57 AM   #5
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Lennox G40UH furnace stops and fails to start at nights only


Quote:
Originally Posted by yuri View Post
Colder at night and more people using gas/load on their supply. I suspect your gas meter or regulator is slowly failing. I have had that B4 and it is a bu*gger to diagnose or prove to the gas co. Have to be persistent and your tech may have to catch this at night.
That was the original guess by the HVAC tech that the gas meter/valve has moisture in it causing the membrane to freeze at night (we had unusually cold weather this winter) and thus causing problems with gas supply. When he expressed this opinion, I called the gas company and I was lucky that their tech came over fast and checked the valve while the HVAC man was still in the house. After that both guys talked and the HVAC tech had no more questions about outside gas valve.
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Old 02-13-2014, 10:52 AM   #6
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Lennox G40UH furnace stops and fails to start at nights only


It has nothing to do with water it has to do with the fact that the meter and regulator are mechanical devices and will wear out and fail. I usually get one failure a year where it fails completely. I also had an intermittent one where it would build up pressure but after 1-2 mins of running the pressure would drop and it would fail and kept blowing out a pilot light. I had my manometer on the inlet of the gas valve and watched it happen. Gas co came and apparently the meter was failing not the regulator. They changed both. Not sure why yours does it at night but maybe the gas co is having problems maintaining their pressure due to the unusual cold weather. Everyone gets home by 6 pm and with these setback thermostats all of a sudden there is a huge demand plus the furnaces run more when it is colder. I have NEVER seen a Lennox furnace POP when it shuts down and I have seen hundreds and hundreds of those. Where I am they have to replace the meter every 7-8 yrs due to the accuracy and weights and measures laws and they do random tests on them. I would try get them to replace the meter and regulator but you need to be convincing.
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Old 02-13-2014, 11:19 AM   #7
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Lennox G40UH furnace stops and fails to start at nights only


yuri, your explanation about meter or/and regulator possible failure sounds convincing. Now I need to think of how to ask the company to check them again and ideally to replace, and how to support my request.

Thank you for taking time to explain in such details!
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Old 02-13-2014, 11:32 AM   #8
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Lennox G40UH furnace stops and fails to start at nights only


Usually they will do it as long as you are diplomatic and not blaming them. They get all kinds of wild complaints about using too much gas and faulty meters along with the elec company. If you have legit mechanical concerns and talk to a supervisor and have receipts for the work done already they should not have a problem. Diplomacy is paramount in this situation.

Pay it forward and help someone else, works 4 me.
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Old 02-13-2014, 11:44 AM   #9
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Lennox G40UH furnace stops and fails to start at nights only


Great suggestion! Simply being polite makes life easier in many cases. Thanks!
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Old 02-18-2014, 01:13 PM   #10
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Lennox G40UH furnace stops and fails to start at nights only


yuri, following your suggestion, on Friday (02/14/2014), I called the gas company (Nicor) and they sent the tech who checked the regulator and meter, found that the "readings are off" (even though the previous tech did not find anything the week before) and replaced both. I hoped that this will solve the problem, but the same night the furnace stopped again twice. The next evening, on Saturday, it stopped several times again even though we raised the temperature to 84F/29C (max). But on Sunday and Monday evening the furnace worked fine, without stopping.

Here is how the schedule looked like (with the lowest temperature for each day):
* Fri 6F (-14C) - new regulator and meter installed, furnace failed twice
* Sat 0F (-18C) - furnace failed multiple times at night
* Sun 14F (-10C) - no failures
* Mon 21F (-6C) - no failures

It looks to me that there is a correlation between the outside temperature and the furnace failures.

Tonight I am planning to clean the burners again, even though they were cleaned by the heating company 3 weeks ago. Also I am going to check and clean the flame sensor (I installed a brand new one about two weeks ago). Next week they forecast the low temperature to come back, let's see what happens.
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Old 02-18-2014, 05:20 PM   #11
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Lennox G40UH furnace stops and fails to start at nights only


I suspect it may be draft thru the furnace and chimney related. When it is colder outside the air is denser/heavier and you may have less draft. Check your chimney for obstructions and remove the vent pipe from the top of the furnace and look for dead birds or burnt up pieces of dead birds in there. The pressure switch proves draft but maybe it is out of whack. Is the furnace in a very airtight room. If so it may need some more combustion air. Do you have any exhaust fans or a wood burning open fireplace running. Both can suck air out of the house. Clothes dryers, powerful kitchen fans and central vacs can suck a lot of air out too.
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Old 02-18-2014, 07:55 PM   #12
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Lennox G40UH furnace stops and fails to start at nights only


Make sure your drain line isn't freezing
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Old 02-19-2014, 09:54 AM   #13
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Lennox G40UH furnace stops and fails to start at nights only


It is a mid efficiency unit, no drain.
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Old 02-19-2014, 11:51 AM   #14
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Lennox G40UH furnace stops and fails to start at nights only


gotya. It would be hard for it to freeze then.
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Old 02-28-2014, 10:45 AM   #15
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Lennox G40UH furnace stops and fails to start at nights only


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I suspect it may be draft thru the furnace and chimney related. When it is colder outside the air is denser/heavier and you may have less draft. Check your chimney for obstructions and remove the vent pipe from the top of the furnace and look for dead birds or burnt up pieces of dead birds in there. The pressure switch proves draft but maybe it is out of whack. Is the furnace in a very airtight room. If so it may need some more combustion air. Do you have any exhaust fans or a wood burning open fireplace running. Both can suck air out of the house. Clothes dryers, powerful kitchen fans and central vacs can suck a lot of air out too.
The vent pipe was cleaned about two weeks ago by the HVAC tech. Two days ago I checked it all the way up from the furnace to the roof, and it was clean. The furnace is in our basement, but it's a full-size basement(range-type house) with no doors, so there is probably enough air circulation. We have a wood burning fireplace, but we did not use it for the past 2 months, also the chimney door is closed, so the air does not leak through it. We have a normal gas-powered cloth dryer standing a few feet from the furnace, but the furnace was failing even when we did not use the dryer. We don't have power kitchen fans or central vacs.

Yesterday the furnace started failing and we raised the temperature to 88F. This solution helped before, but not at this time. The furnace kept failing until my wife lit up gas in the stove and left the hot water running in the kitchen's sink. I left a video camera recording the furnace working and so I should have two failure recorded last night, I just did not have time to process it.

Here is how furnace behaved last few days, along with each day's lowest temperature:
Feb 21......23F/-5C......no failure
Feb 22......18F/-8C......no failure
Feb 23......11F/-12C....failed at 11:00 PM
Feb 24......9F/-13C......no failure
Feb 25......4F/-16C......no failure
Feb 26......-4F/-20C.....failed in the evening, but raising the temperature stopped failures
Feb 27......-6F/-21C.....failed at 5 AM (temperature was 5F/-15C at the time of failure), kept failing in the evening starting at 7 PM (4F/-16C) and until about midnight when we turned on the gas in our stove and left the hot water running all night

What confuses me is that:
(1) this problem started when the gas valve was replaced and the pressure was reduced from 3.9 to 3.5 wc. Can the problem be solved by raising the pressure? Won't it create any problem?
(2) furnace can get restarted by switching the main shut-off (next to the furnace), or the thermostat (switching between Off and Heat). Both solutions affect the circuit board (well, probably like everything else). Can it indicate that the problem is the circuit board and it should be replaced?

Today we are thinking of calling the HVAC company and ask them to increase the gas pressure from its current 3.5 (recommended by Lennox) to 3.9 (as it was before). I am a bit frustrated, and so I am tempted to replace the circuit board as I can't think of anything else that can cause the problem.

So, again, right now I am thinking of two solutions I can try (in the preference order):
1. Call the HVAC company and ask them to raise the pressure from 3.5 to 3.9 wc.
2. Replace the circuit board.

Can you think of anything else? I really appreciate your help, yuri!


Last edited by Muscovite; 02-28-2014 at 10:48 AM.
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