New furnace in 30R

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KYAvion
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New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 8:07 pm

The replacement furnace for the Duotherm unit in a 30R, a Suburban NT-30SP, is anything but a direct swap. To begin with, the vent on the Duotherm has intake and exhaust holes at about 5 1/2" on center, whereas the vent holes on the Suburban are at 3.5". To complicate matters further, the Suburban vent holes are about 4" higher than the Duotherm vent holes. Another issue is the Suburban is too deep for the cabinet opening, so the interior skin has to be removed to gain the needed depth.

One option to resolve the vent issue is to install a patch with the new Suburban vent. I preferred to make the existing Duotherm vent work, so that required dropping the furnace shelf about 4" and modifying a couple horizontal framing members inside the wall. I verified with Chuck Cayo that modifying these framing members wasn't a big deal, and he agreed.

This pic shows the horizontal members that are in the way.
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A pair of linemans pliers makes bending the outer edge of the aluminum easy.
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The bottom member is fine left in place with the outer edge bent down, but the upper member had to go. A cut off wheel makes quick work of the job.
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Next pic shows most of the foam removed from the inner wall and wheel well.
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KYAvion
1984 Avion 30R

KYAvion
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Posts: 464
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Re: New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 8:22 pm

Next step is the vent. I liked the outer heat shield on the exhaust of the Duotherm vent, so I decided to reuse it. The issue, however, is that the Duotherm holes are about 2" too far apart.

I fastened the exhaust vent tube that came with the Suburban on the intake side of the Duotherm vent plate. The Suburban exhaust tube is longer than the Suburban intake tube, and it is angled upward slightly, so that helps to make up for some of the 2" difference. Plus, the bottom half of the Duotherm vent is angled to go with the curvature of the trailer, so that helps a little as well. The longer tubes make getting the vents on the furnace mated up a little easier, as well as provide for extra overlap.
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As can be see though, in order to get the 3.5" on center spacing of the tubes, the intake had to be raised just past the interior of the Duotherm hole. Since the outside of the Duotherm vent is slightly raised, the restriction to the airflow for the intake appears negligible.
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KYAvion
1984 Avion 30R

KYAvion
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Posts: 464
Joined: Sun Sep 18, 2016 12:04 am

Re: New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 8:39 pm

Now for the ductwork. The opening for the main duct needs to be moved about 4.25", so tin snips take care of that.
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The vertical duct boot is also too large, so about 2.5" has to be trimmed from it as well.
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Finally, cutting slits in the bottom of the boot make attachment to the main trunk a little easier.
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Pic of boot attached and sealed with foil tape below. The top of the boot is flush with the wheel well. At some point I'll wrap the edges of the interior wall cutout with aluminum sheet for a cleaner look, but for now foil tape does the job.
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KYAvion
1984 Avion 30R

KYAvion
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Posts: 464
Joined: Sun Sep 18, 2016 12:04 am

Re: New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 9:10 pm

The furnace shelf simply rests on top of the wheel well and is supported on the opposite side with a couple fasteners. The opening of course had to be made larger to match the duct boot.

Pic below shows the duct flange going into the the duct boot.
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While overkill since the manual specifies zero clearance for the bottom, I decided to remove the flange and first cover the wood shelf with sheet metal.
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Next pic shows the flange put back in place on top of the shelf. I tried installing the flange through the furnace cage, but the furnace had a tendency to grab the flange when trying to remove it. At some point I'm going to add a wood stove rope gasket around the furnace cage opening to make the connection between the furnace cage and shelf/flange a little tighter, although once the furnace is inserted things are pretty tight as is.
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Furnace cage installed.
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KYAvion
1984 Avion 30R

KYAvion
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Posts: 464
Joined: Sun Sep 18, 2016 12:04 am

Re: New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 9:19 pm

Pic of furnace installed.
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Pic of propane connection. It's hard to see, but I also cut away the cage where the brass elbow connects to the furnace. I didn't want to have to fool with removing that elbow each time I want to remove the furnace from the cage for cleaning or repair, and the rubber grommet and washer still seals that area just fine.
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KYAvion
1984 Avion 30R

KYAvion
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Posts: 464
Joined: Sun Sep 18, 2016 12:04 am

Re: New furnace in 30R

Post by KYAvion » Mon Nov 21, 2016 9:35 pm

All in all not a bad job, but it is too bad Suburban doesn't make a direct replacement for the Duotherm. One thing I am impressed with on the NT-30SP is that it isn't nearly as loud as the Suburban furnace in my prior Airstream. You hear it, but for the most part the blower is pretty quiet.
KYAvion
1984 Avion 30R

nralifer
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Re: New furnace in 30R

Post by nralifer » Mon Nov 21, 2016 10:24 pm

Looks like you had fun!!!

I am looking at this on my phone..... can't wait to get home and really study it via the laptop!
nra1ifer
1987 Avion 34W
Every day is a good day..... some are just better than others!

KYAvion
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Posts: 464
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Re: New furnace in 30R

Post by KYAvion » Fri Dec 16, 2016 7:07 pm

Didn't care for the elbow with a flare fitting that came with the furnace because of the flared swivel connector that was needed, so swapped both out this evening.

Also replaced the door seal, and what a difference it makes on a cold night like tonight!
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KYAvion
1984 Avion 30R

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