Furnace "backfires" when hot

[paddy79]

The good news is I resurrected my electronic ignition Suburban furnace ('79 Sovereign) - the P.O. told me it just didn't work. Doesn't always fire but does most of the time.
The bad news: after a few minutes of heating, it starts making a popping noise, like it's "backfiring." Starts out making the noise sporadically, then more often. When I turn the thermostat down, it immediately stops popping. I let it run 'til the fan stops, then turn the thermostat up and run it again. Am getting lots of exercise, but that doesn't work so well at night!

anybody got a clue?
Thanks,
Patrick in Virginia

[Inland RV Center, In]

Take the furnace out and disassemble it.

You should find the combustion chamber and burner log filthy dirty.

After reinstallation, check the LPG pressure when the furnace is on. Set it to 12" of water column pressure.

Also, make sure the air adjustment is set correctly.

Andy

[paddy79]

I took the furnace out and had it bench-tested. The guy blew it out, took the log out and cleaned it, renewed the log slots with a hack saw (sliding it through the slots), and tightened up all the wiring, since some of the parts had come loose from their mounting. He didn't find much dirt and thought the problem sounded electrical. After all that, it ran great in the shop, but when I came home and installed it, returned to it's old tricks.

When the furnace gets hot, it seems to be cutting the gas off, then turning it back on and re-igniting it, sometimes in rapid-fire motion, making a sound like a pop-corn popper. I took the furnace out again & cleaned all the electrical connections in the electronic control unit, but that didn't cure it.
I'm wondering if the relay or the gas vlave might be the most logical suspects. Can I check them with an ohm meter? If it malfunctions only
when hot, I suspect I'll just have to start trading out parts. Which one would you start with? And where would you suggest getting these parts?

Thanks for your time and advice!

-Patrick

Originally Posted by Inland RV Center, In

Take the furnace out and disassemble it.

You should find the combustion chamber and burner log filthy dirty.

After reinstallation, check the LPG pressure when the furnace is on. Set it to 12" of water column pressure.

Also, make sure the air adjustment is set correctly.

Andy

[john hd]

voltage low in the trailer?

voltage good on the bench?

would that explain why it works one place and not the other....?

how about gas pressure? how many inches of water ???

john

[WayWard Wind]

You might want to look closely at your venting system. Is it somewhat plugged or has a small pin hole in the tubes. I worked many years (35) for a natural gas company & we would sometimes find when -direct vent heaters- had a venting problem, it would cause the main burner flame to float inside the chamber as it looked for combustion air. As sufficient combustion fresh air became available it would re-inginite to the burner tube, causing a popping sound.Usually though if an air leak occurs in this type of venting system it causes the pilot to go out.It could also be your terminal cap on the outside is re-cycling combustion fumes back into the fresh air side because the cap could be faulty or not installed correctly. When recycled combustion gas (fumes) is reinjected to a flame, is when you get carbon monoxide, so be very careful. Without seeing it burning, but saying that it works on a bench kinda says to me, it MIGHT BE in the venting side. Whatever you do make sure you do it SAFELY & if in doubt replace it.
Best,

[paddy79]

Thanks for your advice, folks. I guess the exhaust pipe could have pin-holes. The exhaust & intake are, of course, in close proximity on the exterior of the trailer. There is no cap or other obstruction where the trim-piece for the pipes sits on the exterior skin.

Here's a little more info.

The biggest factor seems to be the operating temperature of the furnace. It always starts perfectly when it is cold and it produces the prettiest blue flame. This can continue for 3-10 minutes, and then the popping starts.

One difference - besides the possible ones mentioned of voltage and gas pressure - between the shop where it was worked on and the trailer - is that the unit was out of its case in the shop and had plenty of room to dissipate heat.

Another thing that sometimes happens - when I turn down the thermostat when the popping starts, the fan usually continues to operate as it should, but not always. I know the furnace is hot, because I saw the blue flame & then heard the popping, so the fan should continue to run until the temp sensor tells it to quit. Could that implicate the temp sensor? (I'm assuming it has a temp sensor.!!!)

Thanks to all for helping me to think through this.
-Patrick

[WayWard Wind]

Pat ---if I read your comment correctly you're saying there is NO cap on the exterior of the trailer? That might be your problem, as on most there is a terminal cap, of some sort, on the outside of the unit. Usually on a direct vented heater the fresh air intake tube is a tad shorter than the outgoing exhaust tube & will terminate on the back side of the baffle, with the exhaust tube extending on thru to the back side of the cap. If a cap is missing then the fresh air intake will just recycle the exhaust gas. NOT A GOOD THING!!!! I'd suggest you have a qualified serviceperson check it out.
Best,