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Old 02-06-2011, 11:08 AM   #1
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Suburban NT35 Burner adjustment

I had a problem with soot buildup on the electrodes in the combustion chamber. Once covered with a big glob of soot that enveloped all three probes the furnace would go into lockout, cut the propane and just run the fan to eternity. After cleaning it a couple of days, I removed the furnace and cleaned / polished the thermocouple, electrodes etc and reassembled unit. The burner itself is made of stainless steel so there was negligible rust on it, attempts to disassemble the burner itself was aborted since it's likely I wouldn't be able to get it back together.

Next AM the thermocouple and electrodes were covered in soot again, knocked it off by reaching through the viewing hole. NO PLACE I LOOKED DESCRIBED "HOW" TO ADJUST THE FLAME HEIGHT. The manual has pics that show what a good flame is, and what a desirable flame appearance is, but no adjustment directions.

I had an epiphany in the morning. I could adjust the flame by turning manual gas shut off valve to a level that gave a good flame shape. Results are widely varied since the furnace really put out when the manual gas valve was wide open, but sooted up the probes. Current settings give me an kitchen vent output temp of 96F with a cabin temp of 60F, when cabin temp is 70F output temp is 105F. Combustion chamber exhaust temp is 205F. Sure would be nice if these were more efficient, need some dispersion fins like a radiator.

Vents output temp in bathroom was 93F and under bed vent was 90F.

Anyone know if I used the correct adjustment technique? Is there a screw somewhere else?

Thanks,
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Old 02-06-2011, 10:19 PM   #2
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The actual problem is unlikely to be related to gas flow or, for that matter, adjustment at all.

Most sooting problems are caused be combustion air being blocked due to the presence of spider webs, mud dauber nests, or other foreign objects in the venturi or burner tube.

You can try a pipe cleaner, gun barrel brush, or something similar in the burner tube to clean it without disassembly. Compressed air can also help.

If you are absolutely sure that everything's clean there is an air shutter that can be adjusted.
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Old 02-06-2011, 10:22 PM   #3
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Also see this thread
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Old 02-07-2011, 03:25 PM   #4
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That was an informative post, actually last September I had the whole thing apart to clean the mud dobber nests out of it. There were several just like those in the other thread. That's when I put it together and left it with the manual shut off valve fully open.

But this still doesn't answer the question of how to adjust the flame to the correct shape or height. The manuals state what is correct and what is not correct for flame shape but don't describe how to adjust it. The NT35 furnace is made something like this. The manual gas shut off valve is on the incoming side of the electronic shut off valve. Once electonically opened, gas passes down a tube, around a couple corners and up into the combustion chamber where it terminates with a screw in orfice marked with a 1 and a 5. (cleaned it already). This orfice shoots directly into a funnel shaped collector that directs the gas/air mixture into the body of the burner where it exits through the top fins and is ignited by the spark.

You see my problem is how to adjust a system that has no apparent adjustment device. And, it could be likely that most people haven't had to adjust theirs or I might have more feedback!! Sorry, I got frustrated there for a second. The production of soot is due to an imbalance of gas /air ratio and incomplete burning of the propane. This would suggest either inadequate air supply or excessive gas supply. Since the air supply is not controllable, controlling the flow of gas seems to be the object here.
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Old 02-07-2011, 04:17 PM   #5
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Can you see the condition of the flame from the viewing port? Yellow not blue?
While the NT30 in my rig isn't the same as yours, are you sure there's no adjustment for the burner? The NT 24s and 30 I've seen have a airflow shutter flap on the rear of the burner that is adjusted by a long threaded rod.
#40 on the drawing.
You turn it with a screwdriver but I'm not near the trailer now so I don't know If you can reach it from the front.

That's all I can think of for now,
Tom
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Old 02-07-2011, 04:23 PM   #6
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Quote:
Originally Posted by texascanuk View Post
That was an informative post, actually last September I had the whole thing apart to clean the mud dobber nests out of it. There were several just like those in the other thread. That's when I put it together and left it with the manual shut off valve fully open.

But this still doesn't answer the question of how to adjust the flame to the correct shape or height. The manuals state what is correct and what is not correct for flame shape but don't describe how to adjust it. The NT35 furnace is made something like this. The manual gas shut off valve is on the incoming side of the electronic shut off valve. Once electonically opened, gas passes down a tube, around a couple corners and up into the combustion chamber where it terminates with a screw in orfice marked with a 1 and a 5. (cleaned it already). This orfice shoots directly into a funnel shaped collector that directs the gas/air mixture into the body of the burner where it exits through the top fins and is ignited by the spark.

You see my problem is how to adjust a system that has no apparent adjustment device. And, it could be likely that most people haven't had to adjust theirs or I might have more feedback!! Sorry, I got frustrated there for a second. The production of soot is due to an imbalance of gas /air ratio and incomplete burning of the propane. This would suggest either inadequate air supply or excessive gas supply. Since the air supply is not controllable, controlling the flow of gas seems to be the object here.
You should make sure that the LPG pressure is correct before making any adjustments to the furnace.

The correct LPG presuure is 11 to 13 water column inches.

Setting it to 12.5 inches works a little better if you cross mountains.

If that's all ok, then the problem with your furnace is that the burner is starving for oxygen, which is easily correct by adjustment.

Andy
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Old 02-07-2011, 06:16 PM   #7
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If there's really too much gas then there's a pressure problem (regulator) or you have a damaged or wrong orfice. I doubt if that's the case. Usually there are other signs of overfiring, like the air coming out too hot, which it's not based on your temperature measurements.

So then that leaves the air adjustment. As others have described there should be some kind of shutter, but if it's as bad as you say, I would believe there's a blockage.
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Old 02-07-2011, 07:42 PM   #8
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When cleaning your furnace LP delivery system, be careful that you don't force ANYTHING thru the LP orifice other than compressed air or carb cleaner. These tiny little holes are precision drilled and easily deformed and will negatively effect the performance of your furnace or water heater.

ALso, if your fan runs continually with your original igniter board (if you have one) on lock-out, you should replace it with a Fan 50+ igniter board from Dinosaur Electronics. It has a circuit that will eliminate the fan run-on and will shut the system down after 3 unsuccessful tries at ignition.
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Old 02-12-2011, 06:48 AM   #9
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Here's a diagram of the burner parts of the NT 35K, the Installation Instruction manual version is 08-89.

The burner PN 53 is completely open on the cone end, the round pipe extends about half way down the burner body.

Actually, if you look close there may be a knob on top of the gas valve PN47 where the pipe 46 attaches. It is not accessible when furnace is installed. I control gas flow with the manual shut off valve PN 51

Suburban part number for the gas valve is 160832
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1991 34' excella 1000, bought 05/10, oak floors, granite countertops, Marble bathroom counter and floor, 2 A/C's, 2005 Hensley.
Tow With: 2006 Dodge 2500 Quad Longbox 2WD, 5.9 Cummins, Edge Insight CTS, Smarty Jr programmer, Bilstein Shocks, Ingalls balljoints.
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Old 02-12-2011, 07:46 AM   #10
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Actually, after going out to look at the valve again, there is not a knob on top but rather a screw. There are only a few threads showing on it so I doubt that it has enough range to control the gas flow. If there was an access hole to it I'd be more convinced that it is used for something, but there is no way to turn the screw without taking the furnace apart.

I'll keep on using the Manual shut off valve until I can find out if that's correct or not. The soot problem has been solved and the furnace works fine.

Does anyone know of anything that we can add between the chambers of the furnace to help dissipate heat more efficiently?
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Tow With: 2006 Dodge 2500 Quad Longbox 2WD, 5.9 Cummins, Edge Insight CTS, Smarty Jr programmer, Bilstein Shocks, Ingalls balljoints.
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