Bus Conversions dot Com Bulletin Board
September 01, 2014, 04:24:29 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
News: If you had an E-Mag Subscription: It will not get torn up or crushed if you back over it with your bus.
   Home   Help Forum Rules Search Calendar Login Register BCM Home Page Contact BCM  
Pages: [1]   Go Down
  Print  
Author Topic: Suburban furnace "backfire"  (Read 3145 times)
PlumCrazy
Newbie
*
Offline Offline

Posts: 13



WWW

Ignore
« on: August 31, 2006, 11:15:48 PM »

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.
Logged

1982 GMC T6H-5307
Dale MC8
Jr. Member
**
Offline Offline

Posts: 81





Ignore
« Reply #1 on: September 01, 2006, 12:55:56 AM »

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
Logged

Dale MC8

In Theory, theory and practice are the same.
In Practice, they aren't.
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
Global Moderator
Hero Member
*****
Offline Offline

Posts: 4863


Nick & Michelle Badame


WWW
« Reply #2 on: September 01, 2006, 03:36:30 AM »

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-
Logged

Whatever it takes!-GITIT DONE! 
Commercial Refrigeration- Ice machines- Heating & Air/ Atlantic Custom Coach Inc.
Master Mason- Cannon Lodge #104
https://www.facebook.com/atlanticcustomcoach
www.atlanticcustomcoach.com
wrench
Full Member
***
Offline Offline

Posts: 166





Ignore
« Reply #3 on: September 01, 2006, 04:26:15 AM »

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
Logged
PlumCrazy
Newbie
*
Offline Offline

Posts: 13



WWW

Ignore
« Reply #4 on: September 01, 2006, 10:49:57 AM »

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.
Logged

1982 GMC T6H-5307
pvcces
Hero Member
*****
Offline Offline

Posts: 750





Ignore
« Reply #5 on: September 01, 2006, 06:34:48 PM »

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
Logged

Tom Caffrey PD4106-2576
Suncatcher
Ketchikan, Alaska
ryoo
Newbie
*
Offline Offline

Posts: 34




Ignore
« Reply #6 on: September 02, 2006, 06:22:35 AM »

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.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.18 | SMF © 2013, Simple Machines Valid XHTML 1.0! Valid CSS!