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Small Gas Explosion


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Today I was inspecting a 1986 Lennox furnace. It had 3 ribbon gas burners and an IID (Intermittent Ignition Device) I asked the buyer to go up and adjust the thermostat. I removed the front cover and the small flame shield held on by 2 screws. I do this so I can see a bit of the heat exchanger and the flame pattern and color. So the IID was click click click clicking away and I was squatted down about 4 feet from the front of the furnace. About the time I suspected something was not right because the pilot had not ignited, the pilot ignited and the gas valve opened. Two of the three ribbon burners ignited but the one on the left had no flame. About 2 seconds later flames blasted out the front of the furnace. This was not just a little flame role-out this was a small explosion. The husband and wife screamed and I reached forward to hit the power switch on the side of the furnace cabinet when another small explosion shot flames right in my face. That was the end of the furnace inspection. I could smell my burning hair, but just played it cool and finished the inspection. It was not until several hours later that my face and eye lids started hurting / burning like I have a sun burn. I?m not really injured its just my face is not use to flash burns.

I write this just to remind us all to be careful. I thought 4 feet was far enough away to avoid a problem. If I had been closer I would really have been hurt. In the future I will be on the side (not directly in front) and on the side with the power switch so I don?t have to reach in front of the furnace to flip it.

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Welcome to delayed ignition [:-bigeyes.

While there are many things that may have caused this, the most typical one is a plugged crossover (where gas gets ignited and shoots over to light the main burner). I fix these delayed ignition issues on a regular basis.

It's a great reminder to keep your hand on either the power switch or gas valve (one on the combination valve) when firing a furnace.

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I've noticed this on older Lennox furnaces a couple of times. In each case, the burners worked just fine when the flame shield was in place, but when it was removed, the last burner wouldn't ignite (at least not right away).

I suspect that the flame shield sort of "corrals" the gas and makes the crossover work better. When the flame shield is gone, it becomes more prone to delayed ignition.

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Welcome to delayed ignition.

It's a great reminder to keep your hand on either the power switch or gas valve (one on the combination valve) when firing a furnace.

Yup. I had mine 20 years ago. I stand to the side with a hand on the switch ever since.

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I got my sunburn from a conventional gas fireplace converted to burn gas. Nice ceramic log set but no control valve. Just turn on the gas and use a long match to light the burner.

Yeah, riiiiighht! That was about fifteen years ago. I never place myself in front of a burner anymore. For a while there I had no eyebrows or eyelashes and my face skin was all peeled like a bad sunburn.

Danged eyebrows never did grow back all the way!

I agree with Jim. I've taken those old puff shields off a couple of times and fired furnaces and had delayed ignition issues. Had one wher4e some idiot had taken the puff shield off and tossed it in the corner. The inside of the metal cover for that burner compartment was all scorched and the paint was blistered on the outside.

ONE TEAM - ONE FIGHT!!!

Mike

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