I have a friend who has an ancient furnace in her crawl space. She asked me to see if I could light the pilot so she could have heat over the holidays for her kids and grandkids coming to visit. I have been in residential construction and remodel for over 35 years. When I went under the house to check out the furnace it was obvious that someone had messed with it who was not too handy. The pilot burner was affixed to the main burner with 12 volt wires with the plastic insulation still attached.
I cleaned out the pilot jet and adjusted the pilot for maximum intensity, fixed the pilot burner to the main burner with a machine screw and nut and lit the pilot. It is a dual flame pilot with one for the T-couple and one for the main burner. I checked to make sure that the main burner did not come on unless the pilot was lit. Everything checked out. The heat worked fine for about a day and then the pilot went out. I replaced the T-couple and re-lit the pilot. The next evening I got a call saying that a loud boom had shook the house. I told her to turn off the gas at the meter and headed over. It was late at night. I made sure the gas was off to the furnace, turned the gas on at the meter and re-lit the pilot on the WH. I had noticed earlier that the main burner is very sensitive to even a slight change in the position of the pilot. I made sure that with the pilot burner affixed to the main burner properly, the main burner lit smoothly and immediately when the furnace was turned on.
What I suspect happened is that the main burner pilot flame went out while the T-couple pilot flame stayed lit. I had noticed while lighting the pilot using a propane torch that the T-couple pilot flame lit first without the main burner pilot flame lighting. I had to go back to it with the propane torch to get it going. The T-couple pilot flame is not positioned to smoothly light the main burner and gas built up to the point where the T-couple pilot flame finally lit it, causing the loud whoompf. Both flames were lit before this occurred. Both flames were strong and blue and the main burner was lighting smoothly. There is no way for me to confirm my suspicion that the main burner pilot flame went out while the T-couple pilot flame remained lit, but that is the only thing I can think of that explains what happened. While checking for leaks at joints in the pipe using soapy water, I noticed the tiniest of leaks at the shutoff valve, but not enough to ever build up to explosive levels. To be on the absolute safe side I disconnected the gas line at a union, removed the section of pipe with the shutoff valve in it and plugged the gas line at an elbow using a threaded plug and plenty of pipe dope.
I suspect that whoever had been monkeying around with the furnace may have altered the pilot burner, intentionally or unintentionally, but I can't confirm that. I couldn't find a brand or model number. It could be that I didn't look hard enough. If my suspicions are correct about the main burner pilot flame going out while the T-couple pilot flame remained lit, is it possible to replace the pilot burner with universal replacement, preferably one with a single flame for both main burner and T-couple? There is not a lot of draft under the house. I suspect that the action of the main burner gas shutting off during normal operation may have extinguished the main burner pilot flame while leaving the T-couple pilot flame burning.
Or does anyone have an alternate theory to explain what happened as described above?