Gas explosion. What happened?

Bob Harper · February 2006

This is a matter for experts. Have the homeowners report it to their insurance carrier. They in turn will hire someone like Ed Carey and a licensed Cause and Origin Fire & Explosion Investigator to come out along with reps. from the boiler and possible gas controls mfrs. Let THEM do the analysis. It is driven by the cost of the repairs though.If you have only about $10K in damage, you might only get Ed or the C&O.

I would caution the choice of language. Explosion has a very negative connotation. In fire investigative terms, it represents combustion faster than the speed of sound. Most delayed ignitions are actually deflagrations. Its alot like in medicine: when I was a paramedic, I couldn't state things flatly, even when self evident but I could always quote someone else. "The homeowner claims there was a boom or explosion in the basement" as opposed to saying " the boiler blew up"-see? If you say it, you're making a diagnosis. Just report the facts and let the experts take it from there.

The main thing is no one was hurt.

As to the cause, we can sit here and speculat a whole list of failed controls, low inlet gas pressure, vent restricted, etc., etc, but it won't help.

S Ebels · February 2006

Gas explosion. What happened?

A customer called last week Friday about 6:30AM and I could tell by the tone of her voice she was scared. As in really scared. She and her husband were getting ready to leave for work when they heard a loud boom. She described it as being similar to shoooting a deer rifle inside the house. She ran into the kids room to see if they were all right and here husband hurriedly bailed out of the shower to investigate. Within a minute of the first explosion, a second one occured even louder than the first. This was when she called me because being out in the house, she could tell it was the furnace.
I told her to run outside and turn off the valve on the LP tank, which her husband was already doing, and told her I'd be there ASAP. When I arrived I could see the results of the explosion in the furnace but I can't say for sure what happened. The sequence of events that made it go boom are unclear to me and I'm wondering if anyone here cares to guess or shed some light on this puzzle.

Here's some details: The furnace is a typical modular/mobile home unit with a standing pilot and concentric vent setup going straight up from the top of the unit, through the roof. At some point in time, or maybe right from the factory, an automatic pilot relight box was installed on the furnace. I have never run across a burner setup quite like this one as it not only has a regular standing pilot assembly with a thermocouple, but also what appears to be a spark ignitor for the main burner. It's approximately 10 years old.

Here's what happened to the furnace: It has a typical round heat exchanger with a rectangular piece arcing over the top of the blower housing. The sides of the round cylindrical HX were bowed out to such an extent that the support brackets between the HX and the jacket were collapsed. The rectangular (approx. 2"x12") piece that curves over the top of the blower is now oval shaped. The chimney cap was blown right off the top of the pipe, ripping the screws out of the metal. The sides of the furnace were bowed out to such a degree that it would barely squeeze out through the opening of the furnace closet. The furnace obviously had a good load of LP in it when it ignited.

This has to be some type of delayed ignition but I'm puzzled as to how it could happen. If there is a pilot present, there shouldn't be a possibility of gas accumulating to the level that it did before igniting. If there was no pilot, there should be no possibilty of gas being introduced into the combustion chamber in the first place. So what do you think happened?

PS: I don't like LP gas and I wouldn't give a plugged nickel for any Intertherm, Coleman/Evcon, Nordyne, Miller, etc. mobile home type furnace. I'll see if I can get pics up here Saturday.

PSS: They now have a Carrier 90% furnace attached to a real live duct system in their basement.

Steamhead (in transit) · February 2006

Steve, that pilot relighter

might be part of a White-Rodgers Cycle-Pilot system. In this system when there is a call for heat, the "relighter" is powered up to create spark and the pilot portion of the gas valve opens. When the pilot is lighted, a flame sensor (which looks like a thermocouple with a three or four prong plug at the gas valve end) trips and sends power to the main portion of the gas valve, and the burners light off.

This system (at least the version I see frequently) does not shut down the pilot if it fails to light, but keeps on sparking. It's easy to see how that furnasty could get a bellyful of LP if something went wrong with the ignition to cause a delayed spark.

The Cycle-Pilot is still available as a retrofit kit but is plainly marked "NATURAL GAS ONLY" in every catalog where I've seen it. I would assume the box it comes in is marked this way also. Maybe the one you encountered was installed by a functional illiterate?

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Timco · February 2006

Since propane is heavier than air...maybe residule from a previous ignition attempt?

Tim

Larry Savino · February 2006

How many burner tubes are in it,Most of the delayed ignition I have gone out on end up being that one of the burner tupes are crooked and the lighting rail is ascue, or the lighting rail is glogged.

Do you have any photos you can share, I have an educational class every Wensday morning for our guys and like to use photos like this to point out how powerful gas products

tim smith · February 2006

explosion

I would not be surprised to see the burner tube slots corroded out and they usually corrode out near the ends which causes a lot more gas to be released out away from pilot before the gas has built up near the pilot and hence, Boom. Seen alot of older Tranes, Paynes, Tempstars etc that this has occured on. Customers will call and say there is a loud banging coming from there furnace and thats it usually. Good luck Tim.

Larry F · February 2006

Or maybe

If it is a thermocouple setup with the relighter, the pilot may have gone out and the safety failed to shut off the gas. On a call for heat, the valve would start dumping burner gas. The relighter is intended to light the pilot, not direct burner ignition. Eventually the mixture would accumulate to the explosive level and get lit off by the spark. This scenario would continue as long as the conditions remain the same. Yikes.

I've seen similar situations on older Carrier and Trane systems but with different ignition setups and not on LP. Both of these situations resulted in continual delayed ignitions. Also, these weren't mobile home furnaces either. Come to think of it, I actually sent Timmie McElwain the valve from the old Carrier. It was a Robert Shaw. It was used with Carrier's 3-wire type intermittant pilot and I've seen the same condition on several units. Not sure what, if anything he found out on that one. That particular valve incorporates some internal solid state stuff which was and still is beyond my expertise. I now know the symptoms but still don't know how it happens. As long as the pilot lights promptly, the problem goes un-noticed but whenever the pilot fails to light in a timely fashion, boom.

Larry

David Sutton_6 · February 2006

Empire_2 · February 2006

Mobile home equipment

Most I have seen are standing pilot both propane and Nat. gas. Since the GV is redundant I doubt it was a pilot still leaking into the heat exchanger although it could possibly happen. WHat I think possibly happened is the re-light kit was added at a later date due to the wind that usually causes problems due to the lo stack height off the roof. During a wind draft which blew out the pilot, the spark ignitor started to spark while main valve was energised and filling nat. gas into the chamber and eventually flashed one and then twice before the T-couple redundant valve kicked out. This probably happened on a pilot snuff out just before it called for heat.

Also, Steamhead... what I think you were talking about is a mercury switch pilot commonly by White Rogers, which I personally think are unreliable and very dangerous.

Steamhead (in transit) · February 2006

That's the one

lots of them around here on Columbia and Weil-McLain boilers, but I rarely have trouble with them.

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trainboy · October 2011

A little scared

I have a teledyne laar mini therm 2 and we are having quite a bit of delayed combustion. Just had it serviced but it is still happening. Am I in danger of a gas explosion?

lchmb · October 2011

If in doubt

Shut it down and request someone to come out and take a look at your system. Better safe than sorry.

Gas explosion. What happened?

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