Furnace has mild explosion inside when it fires up!

Rex · March 22, 2015, 10:31 AM

I have a late 70's early 80's suburban nt-30 with direct spark ignition. I cannot get it to fire up without a mild explosion in the firebox!! I cleaned the burner tube slots first, no change. So I took the guts out and checked it over and I can't find a reason it does this!!!! I've cleaned every inch of this thing and no change.

The flame color doesn't look quite right to me but adjusting the air makes no difference. There is no gasket on the inspection cover, would this have anything to with it? (I know it needs it, it's in the mail) the blower spins fast enough to trip the sail switch, but is is possible that it doesn't move enough air through the combustion chamber?
Anyone have any ideas? Or should I toss it into the scrap pile and buy a new one? I'd really hate to dump a bunch of money into this thing and still end up replacing it

RANGERRICK · March 22, 2015, 12:21 PM

Make sure you have the correct gas pressure at the gas valve, nothing blocking the main burner orfice, correct gap at the electrodes and nothing blocking the intake or the exhaust.

Rex · March 24, 2015, 07:04 AM

Gas pressure is the only thing I haven't checked yet, no air blockages, I double checked the electrode gap. Even tested that outside the furnace. Burner tube is clean, orfice is clean.

Mr. T · March 24, 2015, 07:57 AM

I think your furnace control board is about to expire. It is slow to ignite, thus a build up of gas before the spark hits, and thus a little explosion. I would replace the board first.

Don T.

bluebird · March 24, 2015, 03:28 PM

I agree. It should be sparking before the gas valve is opened, if it isn't, it won't belong and that furnace will be toast, and maybe the coach. Sequence is out of order.

RANGERRICK · March 24, 2015, 06:31 PM

I have a 3 try suburban control board part# 520741 for 60.00.This is new in the box.

Rex · March 24, 2015, 08:38 PM

I didn't think to check when the gas valve opens, so I just gave it a quick test (no gas of course) and when you call for heat, the the fan runs for a few seconds, and now I've heard it do this before, it opened the gas valve sparked 3 times and shut the gas valve then after a few seconds it started sparking again and the valve opened immediately with the sparking. Sounds like the control board is my problem!

I'll take that board rangerrick!!!

TerryH · March 24, 2015, 11:32 PM

My understanding is that the fan will run for a few seconds before ignition to clear any residual gas.
After ignition/heating/burning is completed (as per thermostat) the fan will continue to run for a few seconds - same reason.
Something to consider is that should you run out of propane while the fire box is lit, the fan will run until there is ignition. No fuel= fan running until there is fuel=possible battery issues.
I probably didn't explain this well, but I know that there are lot of members that can.

Rex · March 25, 2015, 07:15 PM

Well I ordered a board, and I think I may put a new fan motor in it while it's out. It squeaks when it slows down. I don't want to have to pull this thing again!!! I'll post my results when I test it!

Rex · April 05, 2015, 11:45 AM

New board, all sealed up, and it still pops when it starts. Next weekend I think I will order a replacement fan motor. Otherwise I'm out of options. At least now it starts every time

lngfish · August 14, 2015, 07:42 AM

I had this problem and the Converter was low on DC voltage output. After renewing it AOK.

lilwil99 · February 15, 2017, 04:18 PM

I have a 87 windcruiser 37ft that i've been working on for a while; I had the same problem,cause was burner chamber was almost completely clogged with dirt dobber nest. removed furnace and cleaned it out; now it works just fine. I recommend when furnaces are not being used cover intake and exhaust vents with duct tape, as disassembly and cleaning is a real pain.

HVACguy · February 16, 2017, 10:12 AM

Sounds like a gas valve issue to me. I do this for a living on residential/light commercial homes/businesses. The gas valve is no longer opening properly.

DRMousseau · February 17, 2017, 07:39 PM

That "WOOMP!" ya hear is usually due to inadequate air flow or a dirty/rusted burner grate near the ignitor end of the burner. Living aboard in "The Land of Snow & Ice", I gotta REALLY keep up on mine!!!

You noted the flame doesn't look quite right to you. It should be blue and uniform along the entire length of the burner. Anything unusual should be addressed by a through cleaning,... easy to do too, as you already found the burner assy is as easy to remove as the ignitor assy is, and both often need attending to. But you've seen no change in doing this, and have assured that burner is clean, right?

So now you seem to suspect the flow of air as being maybe inadequate, although enough to trigger sail switch. AND, you have concerns of missing gaskets. YES,.... those missing gaskets CAN contribute to air flow problems, as not all the air is being retained in the chamber and flame path it should be. Bad leaks due to gasket seals can not only reduce the air flow in this path, but the resultant incomplete combustion can release CO that can leak out dangerously thru these openings into the coach area. REPLACE those ASAP!!! And your SURE you have a clean air path from inlet to outlet right??? I use a good shop vac with a loose bag of cheese cloth to catch vacuumed particles. I expect nothing more than some rust flakes, but if I see signs of paper wasps or mud daubers,.... then I gotta a full removal and disassembly coming. I wanna make SURE I got 'em all!!!

ALSO,... make sure no other leaks exist in the intake and exhaust path, such as the connections from the outside manifold to the furnace. These too can cause issues in much the same way. BTW,... air flow thru the outside inlet should nearly equal that which comes out the exhaust, with a slight, nearly indiscernible difference. Temporarily obstructing the inlet by covering with a heavy towel or cloth, should cause sail-switch to instantly activate and the flame should go out immediately, and no airflow should be detected at the exhaust even though fan remains on for a minute or two. Covering the exhaust (caution, it is HOT!), will have the same effect, triggering sail-switch, extinguishing flame, and should have no detectable air flowing into inlet. IF sufficient air IS detected at inlet with exhaust covered,... then you have too many air leaks in the system and they should be found and sealed.

And yes, a weak fan motor and/or low voltage will also reduce airflow below minimum for adequate combustion. This is noticed most when running on a coach battery only. As battery gets lower, the fan gets slower. Switching to AC sources activates the converter/charger and increases that voltage, often with a noticeable increase in fan speed. This difference CAN BE quit significant, and the system does have a broad range of operation. But I have seen problems when a high charging voltage causes too much air flow,... blowing out flame or even preventing ignition.

Blower motor should draw about 1.5-2A @ 12.5VDC (not easy to measure)
Voltage drop should only be about a .1-.2VDC with a fully charged battery resting at 12.5VDC or so. A significant voltage drop (or high amp draw)when fan comes on would point to a failing motor or bearings.

Rex · July 01, 2017, 11:27 AM

Well I never did get around to figuring this furnace out, didn't get any time to use it last summer. But it my pondering, would my e bay special gas regulator cause my poor flame issue? I don't currently have a gauge to check gas pressure. I will check everything that has been suggested. Thanks guys!

loneviper · August 27, 2017, 09:20 PM

I have a couple thoughts here, 1 gas pressure is important so check it. 2 make sure there isn't a cracked heat exchanger the flame would be blowing around a lot. 3 make sure the blower fan is in fact CLEAN a dirty fan blade won't move enough air. It seems to me you should have spark before the gas valve opens.