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Old 11-11-2006, 12:27 PM   #1
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1979 31' Excella 500
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NT-30MA to NT-30SP furnace change '79 Excella 500

Hi, folks,

Over the weekend and the next few days untill it's done, I'm going to be changing out the furnace in my '79 Excella from the original 1979 NT-30MA to a modern NT-30SP. The "A" nomenclature appears to mean that the heater is a variant of the NT-30M made for Airstream. There may be other differences, but the main one I see is that the old heater has an opening in the bottom from for flexible ducting to the forward holding tank.

The NT-30M is described in the 1979 service manual, and it is a thermostatically controlled pilotless model. I am replacing it because it's been operating a little bit erractically and because it is twenty-seven years old.

The furnaces are not exactly identical, although they do share very close dimensions. The outside end does fit in the existing cutout in the interior skin, but the inside end needs to be raised about half an inch to make the inlet and outlet pipes line up.

Here's a picture of the new furnace pushed into place.

Lamar
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Old 11-11-2006, 12:32 PM   #2
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And a picture of the duct detail. The old furnace had the gas inlet at the top facing rearward, and the gas line came in from behind the curve in the duct and arched over the top of it to connect. (The rear gas line is to the oven. I have it pushed out of the way here.)

Lamar
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Old 11-11-2006, 12:37 PM   #3
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Here's a comparision of the inlet locations. That's it on the lower right of the picture on the new furnace. Below that is the new line that I'm going to try running UNDER the duct. Obviously, this heater is not really made to accomodate the large ductwork adapter of this application -- the inlet is too close to the duct. (We shall see. I'll put an extension pipe on it if I need to in order to get clearance from the duct.)

Lamar
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Old 11-11-2006, 12:54 PM   #4
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1979 31' Excella 500
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And here's a closeup of the duct adapter and the proposed new supply line. That's no optical illusion - that's a mighty tight clearance to that inlet.

Lamar
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Old 11-11-2006, 12:57 PM   #5
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As mentioned above, the new furnace lacks a hole on the bottom to feed the forward tank.

Lamar
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Old 11-11-2006, 01:18 PM   #6
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1979 31' Excella 500
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And, uh-oh. This may have been the cause of my problems to begin with. The installation instructions specify that three of the furnace outlets or two of the outlets and the bottom punch-out are to be used. Total outlet space should be a mininum of three 4-inch diameter outlets, and outlets to "dead space" having no air return like to holding tanks are not to be counted!

When the catalytic heater was put in for a PO, the forward duct was covered. The thought might have been that this would also push more of the heat to the back. It also might have interfered with the temperature rise in the heater and might have been the cause of the erratic operation that's been bugging me. (The old furnace often failed to cycle off after a run, sometimes went into lock-out mode, and always required several attempts to start.)

My current plan is to move the cat heater upwards, maybe (grrrr!) losing the use of the little folding extension in order to place an outlet somewhere near the original location.

It never has really made sense to me to have to run the furnace to warm the back of the trailer and the catalytic heater to warm the front. I LOVE my cat heater, but it's really better at warming objects and people than it is the air, and, of course, it doesn't do a thing in the world for the holding tanks. I also don't run it when I'm sleeping.

Lamar
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Old 11-11-2006, 04:53 PM   #7
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Trouble in paradise

This photo illustrates two problems. With the heater pushed back against the skin, the duct adapter is not lined up and the bottom will not expose an opening to the forward tank heater hose. Hmm... I can pull it back about two inches and expose the lower hole. This is almost, but not quite where the old furnace was installed, so I guess that's acceptable.

However, the geometry of the duct adapter is wrong for this furnace. The old one had the openinings near the bottom, and this one has them near the top. Also, the duct can move forward a couple of inches, but there is no clearance to move it aft -- it hits the wall.

Looks like I'll be modifying the duct some. What fun. (The party never ends.)

Lamar

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Old 11-11-2006, 04:59 PM   #8
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You can use 4" flex duct, cut the old stuff out, it weights like 75 lbs.

Or cut out part of it and adapt the flex pipe.
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Old 11-11-2006, 05:38 PM   #9
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Hi, Bob,

Thanks. Well, I pulled some duct tape off the bend in the duct and found three screws that hold it together. A 18-inch screwdriver comes in handy here. With the screws out, I can lean it and rotate it enough to make it fit without strain. Now all I need is an 18-inch drill bit to pilot in a couple of new holes and that will solve that. (I have it sitting in place now with no fasteners.)

I see what you mean. This duct is about twenty feet long and it has four right-angle turns in it. Three connections to it are made under the shower pan, though. (I've been there - had that fun.)

Oh, I found an old ink pen and a funny looking refirigerator part behind the duct. Removed them and now I can go further aft with the duct. (At least I THINK it's a refrigerator part.)

Lamar

PS - Oddly enough, that's the first time I've seen duct tape used on DUCTS.
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Old 11-11-2006, 05:50 PM   #10
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You can buy self tapping screws with hex heads, you can use standard 1/4 or 3/8" socket drive in a drill with an extention, they will go right in.
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Old 11-11-2006, 05:54 PM   #11
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Hi, Bob,

Thanks. I like to drill pilot holes even for self-tapping screws. I don't know why - I just picked it up somewhere.

Is your furnace under the stove on the curbside? Center bath or rear? Did you run 4" flex in yours?

Lamar
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Old 11-11-2006, 05:57 PM   #12
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And every job, it seems, needs a supervisor. This is Lucy.

Lamar
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Old 11-11-2006, 06:23 PM   #13
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Center bath, I'm running two 4" all the way to the rear bedroom, I put in two Y branches to the water tanks.

I think it took 20' @ $15.

More room under the skink too!
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Old 11-11-2006, 07:21 PM   #14
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Hi, Bob,

Do you get a decent puff of warm air out of the little vent under the shower opening? I checked my flex tube when I had the shower pan out. It was unobstructed, but still little or no airflow comes through that vent. (Maybe this new heater will be heartier.)

Thanks,

Lamar
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