Delayed ignition issue

Old 09-25-08, 05:31 AM
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Angry Delayed ignition issue

Hello all,

I've got a problem with my furnace, and I hope this fine community might be able to help. Actually, it's not my furnace; I'm renting the lower half of a duplex, and the offending furnace actually belongs to the upper half, who are also renting.

The problem is this: every once in a while, the furnace produces a loud bang, and by "loud" I mean floor-shaking. And by "every once in a while" I mean one to two times a day. This was happening all of last winter and spring. The furnace hasn't been turned on yet this season, and I'm genuinely concerned for my safety.

Yes, I've read about delayed ignition, and I'm sure that's what's going on here. However, here's some more information that doesn't seem to add up:

- The furnace doesn't produce the explosion on every start (it's only 1-2 times a day)
- The furnace was examined by technicians on two occasions. Both times, it was cleaned thoroughly (according to my upstairs neighbor; I wasn't there), but it continued to produce the bang with the same frequency. The second technician supposedly said that this behavior is expected in an aging furnace (Of course I don't believe this for a second). The landlord, who lives in a different house, refuses to replace the furnace, or have any more technicians examine it.

For your viewing pleasure, I captured one of the explosions using a webcam (this was last spring):

My theory so far is this: When the furnace shuts off after a successful run, the gas valve closes down, but not all the way, allowing a trickle of gas to flow through. This gas gradually fills up the chamber, and the next time the furnace starts, it ignites the gas explosively. Does this sound plausible?

Does anyone have an idea of what else could be repaired / cleaned / replaced in this furnace?

P.S. It's a direct spark ignition furnace (Amana Air Command).

Last edited by dbrant; 09-25-08 at 06:10 AM.
Old 09-25-08, 06:42 AM
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Has the landlord see this happen!?

This seem to happen after the system has been shut down for a while?

I would have the neighbor call the gas company right a way to get them to get the landlord get his act in gear before he has a rental house blown up, and dead bodies.
Old 09-25-08, 08:37 AM
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Depending on the burner type used, it is possible in an old unit to have say a hole in the burner and all the gas is not getting directly to the spark until after enough gas fills the chamber, and boom. Somehow or another gas is not in sufficient quantity in the beginning right at the spark. That is a given. it is determining why. Either the gas is entering not by the spark, or it is seeping in too slowly, at least in the beginning, and air movement inside is allowing gas to move away from the sparker and to fill too much of the chamber before it finally is set off.

That is, IF truly the burners by the spark are truly clean, and nothing in any crossover tube like spiders/webs.

Your theory woud make sense if it primarily did it during rapid calls for heat where the furnace shut down, then within say a number of seconds, but not hardly more than a minute, started back up again. Otherwise it be doubtful (I'd think) that the gas would be lingering in the chamber for that long without going up the chimney.

If say the gas valve is never completely shutting down all the way, then go out and look at the meter 1/2 foot dial and see it it turns when no gas being used. I would think though if the gas valve was hung up, that upon shutdown, you woud get a lot of lingering flame lapping going on, that might not ever fully extinguish. I have witrnessed this. But check your meter to be sure.
Old 09-25-08, 01:27 PM
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Since this has been documented for all the civilized world to see I would think the gas company would red tag the unit and shut off the gas.
Old 09-25-08, 01:42 PM
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I'm assuming "red tag" would mean that it'll be illegal to turn on the unit, right? That would be good, but will the video be enough evidence for the gas company to do this? What if they don't detect an actual gas leak when they examine it?
Old 10-06-08, 02:33 PM
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A couple of reasons for delayed ignition:
Pilot orifice restricted (most common) causes small flame to ignite burner(s)
Pilot assembly had been removed and improperly re-installed
Spider nest in one or both burner orifices, causing undergassed situation.
Spider nest in pilot or dirty pilot, causes lazy flame with poor direction

I would call the gas co to take a look. I've worked many many of these calls

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