Gas Furnace - burners won't stay lit

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  #1  
Old 11-04-18, 03:49 PM
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Gas Furnace - burners won't stay lit

I've got an older natural gas furnace (Lennox Model G16Q3X-50-1) which refuses to stay lit.

When the thermostat calls for heat, the induction fan goes on, the igniter lights the pilot and the burners ignite for a moment. Then they go out and the process repeats. This continues until I turn the thermostat off.

The things I've tried (besides reading lots of forum posts) include vacuuming and cleaning everything I can reach. Specifically the two long metal rods that run just above the length of the burners. They were coated with rust. I added a long handle to a wire brush and cleaned them off. I then built a long skinny vacuum attachment and cleaned the burners. I couldn't remove what I believe to be the flame sensor but I wire brushed it as best as I could. My limited knowledge is nudging me to suspect the flame sensor. Any advice on how I can test if that is working with a multi-tester? Any suggestions of other things to check?

I've read GregL65's thread and tried to apply some of the advice that Pjmax has been giving to my problem but so far I haven't found anything that has helped.

Yes, I am aware it's a very old furnace and should probably be replaced. That is planned but just isn't in the cards financially right now.

Photos are here: https://imgur.com/a/lo2SEB5
Video of furnace attempting to light is here: https://youtu.be/Gdz2x0jZ0fw

1. In which area you live and ambient temperatures you usually experience. - SF Bay Area, 35 degrees in winter, 90 degrees in summer
2. House style and construction details. - Two story, stucco over wood
3. Make, model and age of equipment related to the problem. - Lennox Model G16Q3X-50-1, can't find an install date but the serial number sticker has a printed date of 07/85
4. Fuel type. - Natural gas
5. Water temperature and pressures of boiler systems. - n/a
6. What type of zoning do you have with your boiler system. - not sure
7. Thermostat type. - electronic - Lux TX-500
8. Anything else that would be useful. No detail is to small. The more information you provide the easier it becomes for the professionals in this forum to determined the cause and correction to the problem. - I think I covered it pretty well above but please let me know if any specific information or multi-meter readings would be helpful.
 
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  #2  
Old 11-04-18, 05:23 PM
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Welcome to the forums.

Has that furnace been checked for integrity lately ?
At over 30 years old it is time to consider a replacement.

That looks like a rust thru in the heat exchanger.
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The flex gas line thru the metal casing without protection is also a direct code violation.
 
  #3  
Old 11-04-18, 05:38 PM
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In looking at your video...... there is a problem there. The pilot needs to be proven first and then the gas valve opens. If the pilot doesn't prove..... the gas valve should be closed and a short time allowed for purge. It looks like that module is trying to relight the pilot and the gas valve is opening at the same time.

I would STRONGLY recommend professional servicing on this furnace.
Call your gas provider. They employ some good techs at usually affordable prices.
 
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Old 11-04-18, 07:03 PM
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Just gotta say, love the nickname... love Monty python!
 
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Old 11-04-18, 08:30 PM
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Hi Pete,

Thanks for the welcome and for the quick reply. Your screen shot freaked me out and I spent nearly 15 minutes running back and forth from computer to furnace with a bright light before I realized that the crack you'd found was just a shadow. That said, all of your points are valid, well-taken and disheartening.

No, it has not been checked for integrity. In fact this awesome little furnace has been chugging along since I bought the house in 2000 with no more than a yearly homeowner/diy style maintenance of replacing the filter and vacuuming out the furnace, the vents and the intake.

I will attempt your suggestion of properly proving the pilot light. If I'm reading your reply correctly, that would involve shutting the gas, waiting a bit and then seeing if it will light? I realize that I'm unclear on how a pilot works on a furnace. I know that a hot water heater pilot is burning all the time but my furnace gas valve doesn't have a "off", "on", "pilot" sort of setting. It's just on or off and I assumed that when the thermostat called for heat, the pilot got lit at that point. Anyway, I'll do some research, I'm pretty sure this info is well covered somewhere.

Lastly, I'll take your "I would STRONGLY recommend professional servicing on this furnace." suggestion to heart. I appreciate that applying DIY to a furnace is very different from applying it to a washing machine or a refrigerator. One might flood your washroom if you do it wrong, the other will burn down your house and kill your family.

Thanks again and I'll be sure to update here with my progress.
 
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Old 11-04-18, 08:34 PM
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>Just gotta say, love the nickname... love Monty python!

You're one of the few, the proud, the geeky that get it. Most people assume it involves 420.
 

Last edited by MrSmokesTooMuch; 11-04-18 at 08:50 PM.
  #7  
Old 11-07-18, 10:26 PM
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Hey PJmax,

Hope you're still "listening" to this thread. I've been reading a bunch of articles and manuals and watching way too many "so your pilot light won't light" videos and have come to the conclusion that a. there's not a lot of info on 30 year old furnaces on the internet and b. HVAC techs are severely under appreciated. From what I have been able to learn there are roughly three ways that furnaces light.
1. Pilot light - always burning like a hot water heater
2. Pilot light and igniter - spark lights the pilot which lights the burner. When the furnace is off, the pilot is off.
3. Electronic - I didn't focus on this much cause I knew it didn't apply to my situation but basically it lights the burner electronically without a pilot light?

Anyway, I know I have number 2 but I'm not completely certain in what sequence the furnace lights. Based on your post and what I've read it should work like this:
Thermostat calls for heat
Spark lights pilot light
Pilot light heats flame sensor to the point where it trips a relay and allows the valve to releases gas to main burner
Pilot lights main burner
Is that close to correct? In your post it seemed that you were concerned that gas was getting to the main burners at the same time as the pilot, before the flame was really "proven".

I was able to get the furnace to fire up but in such a manner that you might want to stop reading right here. It's probably not recommended and possibly dangerous. I started by pulling the igniter/pilot/flame sensor assembly and making sure that there wasn't any blockage in the pilot. I re-assembled everything, checked for gas leaks and then tried firing it up again. No luck, got the same situation you saw in my earlier video. That's when I got a bit crazy. I used a long, butane lighter and kept the flame against the pilot and flame sensor while the furnace was trying to light. After about eight or ten attempts, it fired up and stayed lit. I also noticed a clicking sound (relay?) from the little electronics housing once the flame sensor got hot. I'm guessing this is all pointing towards a faulty flame sensor but it doesn't explain the fact that gas is getting to the main burner before the pilot is proven.

Sorry for the wall of text. I'm being intentionally verbose so that the next poor ******* whose furnace won't light and goes looking on the internet for answers might find something slightly useful. If you've got any more pearls of wisdom, toss them my way. I know your primary advice is to have a professional take a look at it and I'm sure that professional's advice will be to replace it but any suggestions are appreciated.

thanks!
 
  #8  
Old 11-08-18, 11:11 AM
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That ignition system is completely obsolete.

If you want to see what's really going on, you'l have to use a meter and see if it's getting consistent power and then work from there.

A pressure switch upstream could be cycling power to the module/valve.

Could be other things.

Normally, there is supposed to be a delay between proving the pilot is lit, after which the burners come on. You could have shorted gas valve and or issues with the electronics.

A tech will be required if the module or valve has failed- would have to be converted to a separate valve/module setup.

The heat exchangers in 1980s lennox furnaces crack at the start of the curve. watch https://www.youtube.com/watch?v=QNfiESSEFBs
 
  #9  
Old 11-08-18, 11:18 AM
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Here's the manual for your module: http://cgproducts.johnsoncontrols.co...df/1201136.pdf

Check for consistent 24v ac between terminal 2 and GR - it shouldn't drop out once the inducer is up to full speed.

If it's consistent, check between terminal 3 and GR.

The module shouldn't be applying any power there when the module first tries to light the pilot. The sequence may be very fast, hence it looking like both the pilot and main valve are opening at the same time.

You can also disconnect the wire powering the main valve and see if the main burner still lights off when the pilot lights. that would indicate a shorted gas valve.

Having it apply power to the main valve before pilot is proven lit would indicate bad module.

Either way, the valve and module get changed to:



and



...exact models will vary. May be able to just change the module and leave the valve if it's still good. But better just to eliminate both. Could probably use a new pilot assembly, electrode and flame sensor too.
 

Last edited by user 10; 11-08-18 at 11:51 AM.
  #10  
Old 11-08-18, 06:09 PM
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Thanks for the great info User10! I know what I'll be doing the weekend.
 
  #11  
Old 11-12-18, 12:47 AM
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The less verbose reply may go something like this, sand/polish/replace ionization detector electrode its most likely the problem and a free fix for most people. In gas furnaces the ignition cycle starts with the exhaust fan. When it builds vacuum it activates the switch that starts the ignition. Once heat from the ignitor is detected the gas valve cycles open. When flame ignites the ionization sensor triggers the board to keep gas valve open until heat signal is removed.
 
  #12  
Old 11-13-18, 09:58 AM
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The module shouldn't be applying any power there when the module first tries to light the pilot. The sequence may be very fast, hence it looking like both the pilot and main valve are opening at the same time.
to be clear, you have to disconnect wire from terminal 3 to check the module, if there's a short in the valve it'll back feed.

just start the furnace with the wire powering the main valve disconnected.
 
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