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! -)