Suburban furnace "backfire"

[PlumCrazy]

I have a suburban 30,000 btu furnace in my 5307 transit conversion.

It was a used unit I salvaged from an RV at the wreckers.

I inspected the unit as carefully as I could, it seemed in good shape, no rust at all.
I inspected the burner and replaced the seals on the access door.

It has been working very well for a few weeks, and lights and cycles with no problem.

But this evening I was out working on the bus and it got a little cold.  I turned the thermostat on, the blower came on as usual, but when it lit the burner there was a loud boom and a smell of smoke.
I immediately shut off the propane and killed the breaker.

I had a real good look at everything.  Everything was intact, no seals blown, no sign of split seams etc.
I got my fire extinguisher ready and fired it back up again.
This time, no problem, normal operation.  Blower starts, then 10 seconds or so later the igniter starts and burner lights.

I cycled it several times and seems normal again.

I am a little nervous about the unit now.

Is this something that can happen, or a sign that the unit is unsafe?

Sorry for the long winded post.

Thanks,
Mike.

[Dale MC8]

Mike, this sounds to me like you have a marginal and/or erratic gas valve (the electric one.) Another possibility is a marginal and/or erratic igniter.

What can happen is the valve opens too soon and fills the combustion chamber with gas and when the igniter circuit times out and ignites there is too much gas and BOOM! Or the igniter waits too long to time out, letting the gas build up with the same result.

You could take it to the shop and they will probably tell you that it is 'beyond economical repair' which means the labor charge plus the parts price are very close to the price of a new one and the new one will have a one year warranty while their repair has a 30 day warranty.

Unless the combustion chamber has burned through, almost all furnaces can be repaired economically IF you can do the trouble-shooting and replace the parts yourself. Your labor is no charge to you.

Check for a circuit diagram that is probably glued someplace inside, get out your test-light and meter and check it out.

Its been awhile since I did one of these myself and can't tell you point-by-point how to check it out but as I remember it wasn't really too hard. It will soon start to be the slow season at your local RV repair shop, maybe you can drop by and talk a tech into a 'side job' or at least some point-to-point advice.

HTH

Dale MC8

[Nick Badame Refrig/ACC]

Hi Mike,

Dale is right. Sounds like a corroded ignighter... Clean it with sand paper, and the metal surfaces around it. Then with the gas off, make

sure you see a good spark in the chamber. You may also want to check all the terminal connections to the board also.

Be careful.
Nick-

[wrench]

Check the gap at the electrode inside the combustion chamber, if too large it won't spark till there is gas in between the air gap (as gas is conductor) & then there is too much & explode, adjust the gap at 1/4" & it will work ok & make sure the electrode is secure so it stay adjust under coach vibration.
           good luck    wrench

[PlumCrazy]

I looked in the window on the burner access door and the igniter has a very small gap, almost touching.
Maybe there's not enough gap for a decent spark.

I'll adjust as you suggested and keep an extra close eye on it.

I did find a manual on the internet and it has a lot of good info, specifics on checking the valve etc.

Thanks,
Mike.

[pvcces]

Mike, the factory furnace boards were none too reliable, either, so if you have any trouble after getting the ignitor electrodes set, you might consider a Dinosaur board. The design seems to be much better and the cost is about the same as the factory board.

Good luck with your furnace.

Tom Caffrey
Suncatcher
Ketchikan, Alaska

[ryoo]

I'm not sure about using sandpaper on the thermodoupling. Nothing more than the grit of a dollar bill should be used on that.  Additionally I would check your air proving (or sail switch) switch. I have in the past used a vacuum cleaner on the out exhuast side of the furnace.  I think 1/16 of an inch is the gap for the spark. After that I would check the sequence of the board.