Furnace backfires from exhaust vent

[Derek]

Hi there, I have a suburban furnace that has backfires 2 times since we filled the propane. We just purchased it and it was the first time we used it. First time blew the screen off and second was 39 minutes later.  Furnace Unit is clean inside, exhaust pipes are visually clear, exhaust temp running around 438° and turned on and off what seemed to be normal. Any thoughts would be appreciated ☺️

[Kirk W]

While you didn't say the model of your furnace I feel pretty safe to guess that it is either the NT series or the SF series and that this service manual would apply. What you describe would sound like there is a buildup of propane that backs into the area outside of the burner area and then ignites. The combustion air movement must be proper as the sail switch should prevent the ignition if it were not proper, unless the sail switch has failed closed. I would check that switch for proper operation ASAP. Did you notice if the blower operated for 10-20 seconds before the gas valve opened and the ignitor fired to start it burning? There should be a noticeable delay between the blower starting and the igniting of the propane to allow the blower to clear any residual gas from the area before the propane valve opens and the ignitor makes a spark. The other thing that I think of is that perhaps your propane regulator is supplying too high a gas pressure and that might be the cause. The regulator should supply propane at 11" of water column (0.397003 psi) to the furnace so I would get that checked too.

I am also calling our resident RV tech for help as this is something that I have never come across before and I would not sleep in the RV with the furnace operating until you find the cause of this problem. Better yet, do not use the furnace. I am guessing that you bought it used and so have no warranty from the seller?

Here is a link to another tech manual with some additional information that might be helpful to you. 

Edited November 27, 2021 by Kirk W

[Wrknrvr]

  There could a blockage such as a bee’s nest in the burn chamber. Or a dead frog, yep had one several years ago.

  Just let furnace run and the smoke disappeared.

   But the first thing that I would check is that there is proper voltage in the system and that the furnace has proper voltage when it is operating. Low voltage could cause build up of gas as the blower is not moving enough air for the gasses to keep moving.

   It could also be a bug in the  opening for the gas where enters the mixing tube.

 Or it could be a combination of several things causing it to backfire.

 

    Vern  

[Derek]

Thanks Vern, no smoke but I will look at the voltage and burn chamber😁

[D&J]

Very good chance the hear exchanger has a hole in it causing the frame to be blown out and then and reignite before the control has a chance to shut it down, don't use it until its inspected repaired or replaced. 

Denny

[Wrknrvr]

  I was thinking more about what is happening.

 

 One thing that is common with suburban furnaces is for the igniter to corrode over several years. The igniter tip is stainless steel, but after a time it can turn green on the exterior. Sometimes I have seen the short piece of the end fall off as you touch it. 

  Why this is important is that the igniter board needs to see a few millivolts   of dc voltage to keep the gas valve open. So the flame will keep burning. But if the millivolts drop off the valve will close. So let’s say the valve closes for a instant then opens again. Before the control board shuts off the gas valve voltage. Possible cause of new ignition and explosion. Now let’s say the igniter is still red hot from it being in the flame to produce that millivolts it need to stay operational. Then the new gas flow is ignited by the red hot tip.

  Another thing that could cause this situation is maybe the gas valve power supply of 12 vdc. Bad connection on the positive side of just one coil on the vale and it will should the gas off. Maybe just for a instant. Then on. Boom it goes.

  Now another thing could be a bad ground on the gas valve itself. Just intermittent and boom.

 

  As for a hole in the burn chamber,,,,, I have never seen that on a Suburban furnace.  If there was a hole in the burn chamber I sorta think you mite just small fumes in the interior of the rv.

 

  Now what is really common on Suburban furnaces is the the stainless steel screws and mounting brackets for the igniter and flame diffuser has a problem registering the millivolts needed to keep the flame operational.

  This subject has really irritated me for about my first two years as a rv tech. Then I took a furnace all apart and for nothing wrong with it. Put it back together and it worked. No new parts were installed.

  And it worked,  what the heck did I do that made it work. What I found was if the furnace ignites but shuts off, then most likely it is those ground screws.

  Since that experience if I remove a Suburban furnace I will disassemble that diffuser and all attachments, sand all flat surfaces and reassemble it.

 

  Just my thoughts,   Vern

[Kirk W]

55 minutes ago, Wrknrvr said:

Just my thoughts,   Vern

So what do you think might happen if the propane supply pressure was too high? I can't recall ever having seen a propane regulator fail high but suspect that it is possible.

As I think about the symptoms, the blowing off of the screen would seem to indicate that the explosive mixture was inside of the combustion chamber, most likely. But the sail switch should prevent ignition without clearing of the chamber and it only tries3 times just to prevent an explosive mixture. I am also wondering if this is one of the Suburban furnaces that has an outside access door or the ones that only have the exhaust ports on the outside? 

          or 

Edited November 28, 2021 by Kirk W

[Wrknrvr]

   I think the furnace would burner warmer than normal. The burn chamber thermostat would shut it off before real problems occurred. At least I think so. Any real amount of higher pressure surely would make the stovetop flames be excessive.

 Vern   

[Derek]

Whew, lots of knowledge and help in this group. Thank you. Ours is outside vent port only, the furnace is nestled snuggly under the fridge🤔.  I am going to check the stove top today. We have propane on our house also so I should be able to compare.  I took the inside plate of the furnace and it is clean as a whistle and no components visible. I think I'm gonna have to pull it to check all this stuff. 

[Pat & Pete]

Without reading the above , sometimes the furnace can poof back when the propane supply is low . 

Maybe that's due to less pressure ???

[Kirk W]

1 hour ago, Derek said:

Ours is outside vent port only, the furnace is nestled snuggly under the fridge

That is what I had in my first 2 self-contained RVs but more recently we have had the Atwood models that have outside access. After I had to remove ours to do some simple work on it I learned to prefer the Atwood but none of them are that easy to work on. 

In reading on the internet I did find that if the vent in a propane regulator is blocked it can cause the pressure to be high or to swing up and down erratically. I still think that it is worth checking the pressure just to be sure. 

[Wrknrvr]

If your furnace has a access door there is only one screw holding the furnace in place.

  You do not need to pull the cage that has all the ducts on it.

  You just need to unhook the gas line after turning the gas off and burn the excess of at the range and wiring. A picture of the wires may hep us help you.

 

 

   Vern

 

  you should have a 5/8” crows foot wrench for this furnace job. If not u will wish you did.

Edited November 28, 2021 by Wrknrvr

[GlennWest]

Had to pull my suburban off several times due to trash in orifice. Simple job. Surprised it didn't clog up this trip from Tennessee. Works fine now. 

[Kirk W]

On 11/27/2021 at 3:38 PM, Derek said:

Any thoughts would be appreciated

Are you the first owner of this RV, or did you buy it used? The reason that I asked is that I once saw a used RV that a previous owner had shorted across the sail switch to allow his furnace to operate with lower than normal battery voltage. Such could cause something of this nature to happen.

[RV_]

If the bug screens are too restrictive they can cause that. Does it do it with the screens off? If not then try slightly bigger screen material. I had that issue with the retail bug screens so I made my own out of expanded aluminum. I used a ball peen hammer gently to form it and it worked great in both my rigs. Kept out the the mud/dirt daubers and wasps. The stainless retail screens looked bug enough but they were the cause of mine backfiring. If it does not backfire with the screen/s off odds are there's your problem. Occam's razor.

 

[Kirk W]

8 hours ago, RV_ said:

If the bug screens are too restrictive they can cause that

That is a possibility that none of us had thought of. The only case that I know about of that sort or problem was more than 20 years ago when I knew a fellow that made his own bug screens from metal window screen. As his began to corrode they restricted air movement to where his sail switch prevented operation, but if somewhat less it might be a possibility. Most commercial bug screens now are of stainless steel and supposed to not do that. 

On 11/28/2021 at 10:22 AM, Derek said:

Ours is outside vent port only, the furnace is nestled snuggly under the fridge

What bug screen do you have and what condition is it in? Definitely worth considering.

               

Edited November 30, 2021 by Kirk W

[RV_]

Kirk, if the sail switch was involved do you think it would cause a backfire explosion or just cut off the gas? 

He is getting a backfire, as I had. I had read about the guy using way too small window screen and having the issue so I took off the Camco or whatever bug screen it came with and . . . No backfire. If his doesn't backfire without the screen problem solved. Get a better flow screen. Mine looked big enough too.

So I made the one in the pic, along with screens from the same material for the upper refrigerator vent, the sidewall refrigerator vent, the heater vent, and the black water rooftop vent pipe, all with that same material. i even used it for the rear bumper sewer hose rubber end cap vents as wasps got in there once in Louisiana when we were stationary waiting for the new house and had a macerator pump and a 100 foot 3/4" hose to empty so the regular hose was undisturbed for most of 2015. No problems from that point on.

[Jinx & Wayne]

On 11/28/2021 at 9:50 AM, Kirk W said:

So what do you think might happen if the propane supply pressure was too high? I can't recall ever having seen a propane regulator fail high but suspect that it is possible.

let's hope not.  One of the folks here in FL has been trying to order a regulator for three months. He says no one has regulators. My guess is it will get here for summer.

Wayne & Jinx
2017 F-350 diesel, dually
2006 Carriage Carri-Lite 36KSQ

[Derek]

Hey folks thank you for all the ideas,. I now have a broken water line at our ranch so my next few days will be busy 9n something else. I will write this stuff down and check it when we get done.  We are in Colorado and it is so dry we have ground settling and causing lines to break. Good times😜. Thanks again

[Kirk W]

16 hours ago, RV_ said:

Kirk, if the sail switch was involved do you think it would cause a backfire explosion or just cut off the gas? 

The sail switch failing to close will prevent power ever getting to the propane supply valve or the ignition probe. 

[RV_]

I know that is why I questioned your answer about the sail switch with another issue.

Derek, Deek here too - I had the stainless steel one and it restricted enough to backfire and blow out the flame, although i would not have believed it until the next generation came out like you show with much bigger holes in the stainless mesh.

I used a ball peen hammer peen side gently to shape and expand the screen area that stuck out over the exhaust port on the heater. It was still small enough openings to keep out the mud daubers and wasps.

Like I said, just take off the screen/s and if the backfire stops then just get or fabricate a less restrictive screen.

Derek, this Derek is in CO too.

 

[Kirk W]

20 hours ago, Derek said:

I now have a broken water line at our ranch so my next few days will be busy 9n something else. I will write this stuff down and check it when we get done. 

Sound like the old "Murphy's Law" is working there! Let us know what happens with the furnace once you get back to it. 

[podwerkz]

On 12/1/2021 at 6:24 AM, Jinx & Wayne said:

He says no one has regulators. 

 

Have him inquire at a local propane retailer. They certainly keep them in stock around here.

[Jinx & Wayne]

On 12/2/2021 at 5:50 PM, podwerkz said:

Have him inquire at a local propane retailer. They certainly keep them in stock around here.

He's tried everywhere around here.  Do you have a dealer near you that can provide one?

Wayne & Jinx
2017 F-350 diesel, dually
2006 Carriage Carri-Lite 36KSQ

[podwerkz]

This is the company I do business with (when buying LP and LP parts and fittings) you might give them a call or fire off an email.

https://www.wtgfuels.com/about-wtg-fuels/

https://www.wtgfuels.com/wtg-locations/#wpsl-search-wrap 

The main office is in Midland/Odessa area along I-20.

I have no idea if they will ship out a regulator...if you find they will only sell locally here in Texas, I can probably (POSSIBLY?) buy the one you need and send it along.

And of course the one you need will probably not be in stock anywhere in the company...but you never know! 🛒

Good luck!

 

 

 

 

Edited December 7, 2021 by podwerkz