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Old 08-15-2007, 10:06 PM   #1
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1979 31' Excella 500
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At the end of my rope with a Suburban NT30MA

Well I thought I was going to have this problem licked. I had the furnace on my bench and found that the limit switch which is normally closed was open. I ordered a new limit switch, fan switch, and sail switch, for good measure.
Replaced them all, cleaned things up a bit and hooked up to gas (with a brand new regulator) and hooked it up to power. Then I fired it up by putting 12V on the thermostat connector pin. The fan turns on, the sail switch engages, and about 10-15 seconds later, the gas solenoid pops open (audible click) and I get a good spark from the ignitor for a good 5 seconds. Problem is I get no flame. Don't know what else could be bad. Is there some sort of gas venturi somewhere that could be plugged? I can smell unburnt gas coming out of the exhaust port so I know it's getting some gas but I don't know how much it needs to get.

Any furnace doctors out there? I really don't think it's the board that is bad but I could be wrong. Will any service centers still service these old gals or do they stay away from them with a 10' pole?

MODERATORS NOTE: LET THIS THREAD BE A WARNING TO THOSE WHO MAY ATTEMPT REPAIRS TO OLDER GAS APPLIANCES!
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Old 08-15-2007, 10:44 PM   #2
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We had a similar problem with a one year old Suburban NT30 installed on our '74 31' Sovereign.

Turns out that there was "something" in the propane line that was partially blocking the gas flow. (no, we didn't use any pipe tape on any of the pipe propane system - sometimes, pieces of improperly used pipe tape will break off & clog the lines as well as the regulator.)
Strange, as the stove did work as well as the water heater. Don't know if the furnace requires a greater gas flow than the other two appliances.

We solved the problem by removing the gas line from the heater as well as the front tanks and used an air compressor to clear the line. The Suburban worked flawlessly after that.

If your unit is original, it probably wouldn't have a computerized board.
Good luck!
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Old 08-15-2007, 11:06 PM   #3
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The problem with mine (twice) were tiny little insects that found their way into the orifice tube. The end screwed off, the little critters were tapped or blown out, and the end put back on. Some disassembly required. I have screens on the vents, but these were small enough to get through and gum up the works. Hope this helps.
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Old 08-16-2007, 01:18 AM   #4
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Quote:
Originally Posted by wacnstac
Well I thought I was going to have this problem licked. I had the furnace on my bench and found that the limit switch which is normally closed was open. I ordered a new limit switch, fan switch, and sail switch, for good measure.
Replaced them all, cleaned things up a bit and hooked up to gas (with a brand new regulator) and hooked it up to power. Then I fired it up by putting 12V on the thermostat connector pin. The fan turns on, the sail switch engages, and about 10-15 seconds later, the gas solenoid pops open (audible click) and I get a good spark from the ignitor for a good 5 seconds. Problem is I get no flame. Don't know what else could be bad. Is there some sort of gas venturi somewhere that could be plugged? I can smell unburnt gas coming out of the exhaust port so I know it's getting some gas but I don't know how much it needs to get.

Any furnace doctors out there? I really don't think it's the board that is bad but I could be wrong. Will any service centers still service these old gals or do they stay away from them with a 10' pole?
Checking the orifice is a good idea.....just use carb cleaner of compressed air to blow it out. Don't put ANYTHING thru the orifice that is harder than that.

You also might have a poorly adjusted regulator, even though it's new. If you have a manometer, connect it to the test port on the regulator and start an LP appliance (that is working). You should read 11 inches of water guage, or about 1/2 PSI. If you're not getting this, the regulator is out of adjustment.

I would think that the orifice is the problem though .
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Old 08-16-2007, 06:03 AM   #5
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Where is the orifice?
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Old 08-16-2007, 07:59 AM   #6
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I have to admit, when I read the title, I was thinking, boy this person is really having a tow vehicle issue and as I read on, finally realized it was a furnace.....
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Old 08-16-2007, 08:58 AM   #7
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You may want to check the placement of the sparker electrode.
Since you have been working in and around the insides of the unit(heater).
The spark has to travel thru the "air/fuel" mixture at a point where the mixture is in the correct ratio. If that little tiny wire electrode wire was bumped, and moved 1/16 of an inch, it won't ignite the fuel/air, cause there might not be any fuel/air where the spark is.
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Old 08-16-2007, 04:12 PM   #8
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Quote:
Originally Posted by wacnstac
Where is the orifice?
The orifice is located at the downstream end of the gas valve. You get to it by removing the gas/air mixing gate, then carefully unscrew it from the threaded end of the tube that holds it. You may have to remove the entire burner head on some models.

BTW, your igniter electrode gap is 1/8"....more or less. If you test fire the ignition without the gas on, you should have a spark jumping from the end of the electrode to the gas supply tube. When the gas hits this spark, it ignites, but if the gas orifice is blocked and no LP is exiting the end of that tube, the result is NADA!

Keep us posted on the progress..........
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Old 08-17-2007, 12:21 AM   #9
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I think the orifice was plugged with rust particles from the internal gas supply line in the furnace. I plan to bench test it again tommorrow.
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Old 08-17-2007, 07:41 PM   #10
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This ended in a very bad way. The orifice was plugged. I took the orifice off and blew the internal line out and it was chuck full of rust particles. I had fired the gas solenoid by itself and had the front inspection plate off to make sure I was getting good gas to the burner which I was. With building optimism I took battery, furnace, and propane tank out to my driveway to "bench test". No sooner than the blower motor starter the thing blew up!! Almost blew the inspection plate door off. The heat exchanger nearest the combustion chamber has it's side largely bent out. Thank God I wasn't hurt... Just scared. There must have been left over propane in the chamber and the spark from the motor must have been enough to ignite it in a bad way. Now I'm totally out a furnace and must try to retrofit it with something.
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Old 08-17-2007, 08:39 PM   #11
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Quote:
Originally Posted by wacnstac
This ended in a very bad way. No sooner than the blower motor starter the thing blew up!! Almost blew the inspection plate door off. Thank God I wasn't hurt... Just scared.
This ended in a GOOD way considering the circumstances. Neither you nor anyone else was hurt! And you now have the best possible reason to buy a new, properly functioning furnace and there is no place better than this forum for advice as to what to buy.
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Old 08-17-2007, 08:53 PM   #12
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Glad to hear that the only casualty were your wits! At least this didn't happen when you were sleeping with the furnace on.

Although many like to keep their vintage units entirely original, we've always preferred to replace "critical" appliances like the furnace & hot water heater in our vintage units.

FYI, the new Suburban NT 30 will fit in the same space as the old model - you may have to build a 3/4" platform to raise the unit to match the old outside vent holes.
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Old 08-17-2007, 08:59 PM   #13
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Yeah, I'm very lucky I wasn't looking closely through the window for a spark and got an eye blown out or worse. My replacing the furnace I assume you are referring to the NT-30SP as in this: http://www.airforums.com/forums/f207...0-a-27770.html

My question would be couldn't you use the old case with the new furnace with the addition of cutting a new slit for the gas line at the bottom instead of the top of the case and get better alignment on all the vents?
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Old 08-18-2007, 11:03 AM   #14
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Quote:
I'm very lucky I wasn't looking closely through the window for a spark and got an eye blown out or worse.
yes that is exactly what happened to one of my buddies.

other than a trip to the emergency room to get his eyes washed out nothing else happened.

later that week he crushed it with his tractor as an act of revenge!

LET THIS BE A WORD TO THE WISE! QUIT FOOLING AROUND WITH THESE THINGS AND JUST JUNK THEM!!!

he got a brand new furnace and is very happy and safe with it.

john
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