Suburban NT 30

J

johnny

New member
May 10, 2011
8,287
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3
Fixing the failed new furnace. No sparks. Removes it and took the owrks out of the case and removed a pound of rat turds.
Checked the overtemp and airflow switch. Ever thing works, valve has 12V and clicks. Much fire between the electrodes, lotsa sparks, and it should
work when I put it back. As soon as the hi temo sealer dries (this weekend) I'll seef it doesn't run.
Of interest, and some concern, this thing doesn't have a flame sensor. It has no way to know of it lit or not. There's an overtemp to shut it down
if it gets too hot, and an airflow which will shut it down if there isn't sufficient combustion air and distribution air across the heat exchanger.
But there ain't chit to tell if it lit or not. The ignitor simply runs for a set period and then it happily keeps the valve open and the fan running
as long as it doesn't overtemp or the airflow slow and the 'stat calls for heat. Additionally, since the housing is plastic, I believe the cumbustion
air blower is a pressure blower rather than the scavenger it ought to be. This thing is a dangerous design - I wouldn't run one without a CO and
propane detector in the coach.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.

"Sometimes I wonder what tomorrow's gonna bring when I think about my dirty life and times" --Warren Zevon
 
I recently helped a friend try to get his NT-34 working. Everything checked out, burner would ignite, burn for a short time and then shut down. As I
understand the trouble shooting guide and other research on the net, the igniter electrode MUST stay in the blue flame or it will not keep the gas
valve open. That tells me that the igniter electrode also works as a flame sensor. I'm not sure how that could work, but that's what the net info
is telling me.

Ray
 
> I recently helped a friend try to get his NT-34 working. Everything checked out, burner would ignite, burn for a short time and then shut down.
> As I understand the trouble shooting guide and other research on the net, the igniter electrode MUST stay in the blue flame or it will not keep the
> gas valve open. That tells me that the igniter electrode also works as a flame sensor. I'm not sure how that could work, but that's what the net
> info is telling me.
>
> Ray

Using the ignitor as a flame sensor is very common in new equipment.

Matt

--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit
 
Awright, I'll buy into that - if somebody will tell me how it does it. And feel a lot better about the safety pf the thing. The positive electrode
hooks only to the top of a high voltage coil. The electrode itself doesn't look to be made of anything esoteric. Maybe the flame is ionized enough
that a voltage applied would cause a small current flow if it were in the flame? There's substantially more junk on the control board than is
absolutely necessary for what it's doing lighting the thing. One chip with the data obviously covered over with resin. Ever thing elts looks pretty
much standard components. There's a view window, I can make sure the flame is over the igniter electrodes.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.

"Sometimes I wonder what tomorrow's gonna bring when I think about my dirty life and times" --Warren Zevon
 
Per manual--- flame is sensed through spark wire and electrode.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First
 
As I recall [and others have said] the electrode is the sensor for the board. I had wondered the same thing [and about the excessive components on the
board] as well
I think it works as a thermister, reading the change in resistance with the increase in temperature through the high tension lead
--
76 Glenbrook
 
JohnL is closest to correct on this. The air ionized by the flame does have a lower impedance than air an this is easy to measure with the few parts
added. When the arc lights the flame, there is instant leakage from the electrode to ground that was not there before. So, the circuit ignores the
spike of the ignition and about the time (time constant) it takes to recover, the system sees the leakage and determines that there is a flame. It is
a pretty reliable system.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit
 
Further Furnace Follies.
I got to looking at the install. It was/is not confidence inspiring. There are four hot air outlets on the furnace case, one of which has a factory
blank flange. Two were connected to the two closest floor vents. The third was taped over, and the rear floor vent pipe merely sitting in the
cabinet open, along with the pipe to the bathroom vent. Boyz n Grllz, it's going to take a minimum of three openings flowing air to flow enough air
across the heat exchanger to keep the airflow switch engaged. Otherwise any little breeze or minor obstruction in front of a heat vent is going to
impede the flow enough to open the airflow switch and shut the furnace down. Even worse, one taped outlet was the one which connects to the bathroom
heat vent. It's 20 degrees out, you got propane, the furnace is humming away, and you settle onto a frigid throne? None for me, thanks. Almost as
bad, with hot air going only to the two closest floor vents and none towards the rear of the coach, the furnace will short cycle badly and the 20
degrees will creep into the bedroom. Or the kitchen/front area will be a zillion degrees and the back will be blue. GMC put some thought in the heat
supply, use it. With these problems corrected, I expect to be toasty warm and comfortable evenings at the January and February shows next year. And
at The Mouse in December is it gets as cold as it did two years ago.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.

"Sometimes I wonder what tomorrow's gonna bring when I think about my dirty life and times" --Warren Zevon
 
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