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Old 01-20-2025, 06:34 PM   #1
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1984 27' Airstream 270
Orofino , Idaho
Join Date: Jan 2025
Posts: 5
NT30SP Furnace Advice / Help!!! 1984 Excella 270

Hi guys!

I relatively recently purchased a 1984 airstream Excella 270 motorhome. Renovations have been underway, however, one troublesome area has been the furnace. I purchased a second hand NT30SP furnace to replace the old NT30 with a bad blower motor. However, the furnace I bought had some serious issues. So I spent quite a bit of time rebuilding it. I got it to the point were it would run on a bench test with no problem. Went to install it... And it shuts off after a lengthy period of time. I haven't fully diagnosed this shut-off but am 99% confident it is the thermal switch cutting the power.

To the best of my knowledge this overheating is due to one of three things:
- Poor ventilation
- Decreased fan speed
- A hole in the burner chamber / leaking blower fan gasket

To the best of my understanding the air duct system is stock on my Excella with only three vent tubes coming out of the box. I have heard that if you are not using all the vent openings on a furnace you are likely to have problems?

My question to anyone else who owns a similar set-up, does your furnace occasionally overheat and cut-off due to the limit switch? If this is the issue, I'm happy to run another vent; however, I would hate to go to the trouble if it isn't.

Thanks for any help! I plan to post some pictures of our Excella soon
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Old 01-21-2025, 06:09 AM   #2
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1974 31' Sovereign
Sunset Beach , North Carolina
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Nt30sp

in case you need it, below is a link to the installation guide. I see it calls out a minimum of 55 square inches for a return, which is more than four-4" ducts have. I would think the supply would need as much as the return, but I really don't know. I hope you can find the root cause of the problem, but if not, I have seen some new units online, recently.

https://techsupport.pdxrvwholesale.c...structions.pdf
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Old 01-21-2025, 12:43 PM   #3
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1982 31' Airstream 310
champaign , Illinois
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So the furnace shuts down before it reaches the temp set on the thermostat?? “It shuts off after a lengthy period of time”
You can easily bypass the limit switch with a short piece of wire and alligator clips. See if the furnace will bring it to temp on stat. Did you replace the limit switch as you were rebuilding the unit?
I’ve the same furnace and had to replace the control board after just a couple years of use. Put in a Dino board and it solved all my problems. Sounds to me like you’ve got a bad limit sw or it’s the control board.
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Old 01-21-2025, 01:14 PM   #4
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1984 27' Airstream 270
Orofino , Idaho
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Responses:)

Yes, it shuts off before reaching the thermostat temp. If the thermostat is set to a higher temperature. It is very cold outside, approx. 10-30F. After it initially started shutting off without reaching the thermostat temp, I wired the thermostat input wire to constant 12v, and the furnace will run initially around 3 hours.

I did not replace the limit switch with a new one, however, I did swap it out with the one on my the old NT30 which I saved. No change. And I have heard it is rare for them to go bad. I have not wired a bypass for this yet to confirm, however.

And I already have a Dino board, that was part of the rebuild. I just purchased supplies to add a fourth vent, because I figured it can't really hurt. Afterwards, if the problem still exists I will likely use a jumper wire to confirm it is the limit switch.

I had used that manual many times, but never thought to look for airflow requirements. I am suspicious of that being the issue.

Mayco, do you have an 30k NT30SP with only 3 vent tubes?
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Old 01-21-2025, 03:10 PM   #5
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1982 31' Airstream 310
champaign , Illinois
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I have a new NT30sp(1yr old) and I’m using all 4 supply vents. We have done lots of winter camping in temps as low as 0° F and my furnace has never had to run for 3hrs to bring the rig to temp. I guess I’m saying that I’m not surprised if the limit sw kicked after running that long. You could just open up the fourth vent supply and try the furnace out, to test that theory. Never heard of that being a problem but doesn’t hurt to try it.
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Old 01-21-2025, 03:23 PM   #6
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1984 27' Airstream 270
Orofino , Idaho
Join Date: Jan 2025
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That is super helpful! My window and door seals are pretty horrible. So with a decent bit of heat loss and no under skirting at the 3 hour mark it was probably around 65-70 degrees.

I thought about just pulling the insert for the fourth vent and seeing if that did the trick. Will probably install some ducting and just run it out of the cabinet. Will update with results! Thanks for the help, it's very good to know yours is set up with four vents.
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Old 01-22-2025, 05:59 AM   #7
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1974 31' Sovereign
Sunset Beach , North Carolina
Join Date: May 2010
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vent tubes

on my previous 1973, 31' I just ran 2 tubes forward towards the entry door, pointed towards the sofa, and 2 to the mid and rear of the trailer. I had a large return behind a metal grate in the kitchen cabinet. ...Never any issues on the '73

Since NT30 was in very short supply after the Covid fiasco, I bought a Suburban 25,000 btu horizontal furnace, for the newer trailer, result is still TBD

mike
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Old 01-25-2025, 07:03 AM   #8
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1984 27' Airstream 270
West Chester , Ohio
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I also have an ‘84 270. When I replaced cabinets, I redesigned the enclosure under the kitchen. Quickly realized that the area gets pretty warm without adequate ventilation and my rebuilt furnace also shut down. I added a “return” vent right in front of the furnace and it never turned itself off again. Opening one of the lower doors to provide the unit a little more air might help on those continuous running scenarios.

Keep us updated on your adventures!
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Old 01-29-2025, 10:43 AM   #9
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1984 27' Airstream 270
Orofino , Idaho
Join Date: Jan 2025
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Problem Solved :)

Thanks for everyone's help! Added in the fourth 4in vent and the furnace ran for around 6 hours afterwards, before I shut it off. So problem seems to be fixed.

I think keeping the lower cabinet door open could definitely still further improve the ventilation, will probably do that when I think it is going to run continuously.
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