I have an older Lennox G12 furnace. This is the sequence of events when I go to light it. The pilot lights as usual with no issue. I allow the pilot to burn for 60 seconds with the knob turned to the pilot light setting. I then turn it to on. Once I set the thermostat above the current room temperature the burners ignite but then die out over about a 5 to 10 second time frame. If I switch the thermostat off the pilot will stay lit but will be weak and slowly gain back "strength". If I allow it to continue the pilot will go out as well. I have a residential tank that has a sufficient amount of gas in the tank. It seems to me that the pressure lowers which results in the burners going out. Any ideas? I can provide pictures of the inside of the furnace if needed.

 

Yes pictures would be good. I would suspect the gas valve might have a problem.

 

I have been trying to upload photos that I took with my phone but I am having trouble getting them to load. I will try again this evening.

 

Watch the pilot when the burner comes on. If the pilot shrinks it sounds like a restriction in the gas supply. Is the line valve at the furnace in the full open position. You may want to call your fuel supplier on this one. Pictures or a video would be great. I had a G-12 for 30+ years.

 

Please refer to the following link........ http://www.doityourself.com/forum/el...-pictures.html

 

Here is a video of what is happening. I uploaded it to youtube. Also attached are several pictures of the furnace

https://youtu.be/6t0WyGcIzHM

 

Most likely cause is either the regulator in the valve is failing or the regulator in the gas meter. Either way you have to call someone out for sure.

Do you have any other gas appliances? Do they work fine?

You have to call a pro out to check the gas pressure at the valve inlet and at the outlet.

For a gas meter issue you have to call your utility. A gas meter regulator issue may not show up without running the furnace if all your other gas appliances are low BTU input, like 30 000 btu on a water heater, a gas stove with a under 10 000 btu burner.

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Older inefficient furnace, and if it comes to needing a new valve compare the cost of repair with replacement. The Replacement could take 20-33% off your gas bill. The gas use may not be important if you don't get "real winter"; ie temps below freezing consistently in january and February.

Also get the heat exchanger inspected; it may be cracked by now. I see flex gas line into the furnace which I don't like at all.