'68 Ambassador Duct Replacement

[mashburn.nick]

I'm doing a shell-off restoration of a '68 Ambassador and I'm looking to place a couple of gray tanks from VTS between the subfloor and belly pan (5" of room, gray tank is 4"). However, I've encountered a bit of an issue with my furnace ducts: they're large, very large.

My question is can these large ducts be replaced with smaller ones so as to permit a 50" gray tank to fit in a 55" space between the main steel frame. I currently have 45.5" of space with the ducts there. Or is there another way to duct the furnace to get the ducts out of the way down there?

Looking forward to any suggestions and help. Hoping to move quickly on this project and this is standing in the way of ordering the new tanks.

Thanks!

Nick

[mashburn.nick]

Sorry just need to bump this up because I can't find any info about this.

Would I be able to use semi-rigid aluminum round ducts

[TennTex]

Hi Nick,

The minimum total duct size is determined by the output requirements of your furnace, the specs of which are available from manufacturer online, I think. Total duct size can't be below this value.

If you can come up with a different type/shape of duct (I changed mine to round and threw away the old box ones) or have an HVAC company fabricate one for that space (shouldn't be that hard;mine fabbed some custom stuff for me), you might be able to stay within size requirements. The only other option would be ducting above subfloor within cabinets, etc. like later models.

Goods luck and have fun,
Alan

[thesignlady]

I wouldn't recommend tinkering with your ducts to downsize air flow any. I recently had to add another furnace duct to our newly installed Suburban NT30SP 30,000 BTU

With my factory existing ducting the trailer failed the propane certification, the inspector said more ducts were needed as it was triggering off to soon, apparently it burns out your furnace if the airflow doesn't have the required space to flow and the new furnace was bigger than the original

[mashburn.nick]

Alan -

Thanks for the input. From what I can tell, if you meet or exceed the factory requirements for square inches of ventilation then you should be fine (I think it's 36in. squared.

The Sign Lady -

That's a real bummer, having to go back in and add ducts. I hope it wasn't too much trouble!

By the way, how/why did you have your trailer inspected? Would you recommend it?

[thesignlady]

I went through the recertification process as we put in a completely new LP system up to 2016 standards, including rebuilding the original stove and installed the new furnace. I am planning to sell it and having it up to modern codes and proven safe was important to me. It actually wasn't that hard to add more ducts in my case as well the furnace we bought ducted straight down it also had popout circular ports on the sides so we just added the flexy furnace grade ducts and attached the ducts to the underside of the lower kitchen cabinet shelves so storage space wasn't too affected, the trailer now has heated closets and the plumbing under the plastic bathroom fixtures is now heated. So its actually accidentally kinda awesome. I am sooo going to do that in my future restorations. Warm clothes in the morning camping, what beats that :-)

I highly recommend have your propane system recertified. Couple hundred bucks for peace of mind is well worth it. It's not required anymore but it used to be here in BC you had to be recertified before you can sell a used trailer and since I am planning to sell stateside I want to help the future buyer avoid as many hurdles as possible just in case their state has that rule.

PS Furnace ducting is different than dryer ducting. I just pulled some from someone who was parting their trailer out including some really cool duct ports

[newas4mark]

On my '68 Sovereign, I removed the ducts from below the floor entirely. My local airstream dealer advised that I could use the round ducts as described above, in fact I was able to see several of their installations using this method. You can then distribute heat to the living room (front), under the sink cabinet in the galley, under the closet kick in the bedroom, and into the bathroom at the far rear of the trailer.
I do not anticipate camping when below freezing so did not feel the need to heat my tank area.
I used the larger VTS tanks for my gray water (2 tanks) and one for my black water. That way I have 42 gallons gray, 21 gallons black. They just fit between the rails....
Good luck,
Mark

[mashburn.nick]

Good to know about the certification!

Mark -

How did you measure the square inches of ventilation required for the safe operation of the furnace? The model I'm looking at says 36" squared, but I'm not sure where that measurement is supposed to come from.

Also, how did you install the gray tanks and where did you have the custom fittings placed to allow for proper flow into each other and out of the trailer, as well as proper ventilation?

[newas4mark]

The VTS site shows how to meet the ventilation requirements for each furnace. I am getting the 34,000 btu unit (NT-34SP), so it says :
NT-30SP requires a minimum of 36 square inches of duct coming off the furnace.
NT-34SP and NT40 require a minimum of 48 square inches coming off the furnace.
When you receive the NT-30SP you will see that 3 of the 4 round ducts are open and the bottom rectangular duct is closed. This is the minimum ducting required. Or you can close the round ducts and duct solely from the bottom rectangular duct.
When you receive the NT-34SP or NT40, you will see that all 4 of the round ducts are open and the bottom is closed. This is the minimum ducting required. Or you can close the sides and use the bottom rectangle.

So I plan to use the rectangle opening for the living room, and 3 of the 4 round ducts as described previously. My only question is how to get the round duct on the side of the furnace that is on the living room side to the rear for use in the kitchen, bedroom, or bath.... My furnace will be here next week, and I will sort it out then. I may end up using all 4 round openings and leaving the rectangular one closed.

As to the fittings on the tanks, the guys at VTS are very good at talking you through it, and where to place female and male inlet/outlets. My gray tanks connect below the belly pan via the drainlines to the cleanout valve. (Each has a male outlet on the bottom on the streetside). The tops of the tanks have inlet/outlets, the front tank on the curbside to collect the kitchen and bathroom sinks. The aft tank has its inlet on the top on the streetside to collect the shower water.

[mashburn.nick]

I'm going to have to crunch some numbers then to see where I need to place vents. Originally there was just the rectangular one under the front of the furnace and one in the bathroom, and that was for a 30,000 BTU furnace, so just replacing those should be sufficient. I'm having trouble getting info out of Suburban (since it's a pretty risky business and giving advice over the phone to mere mortals about propane isn't recommended). So, I'm hoping someone with the same size furnace can outline exactly what they needed to do for ductwork as well.

I should be placing the order for custom tanks with VTS very soon, so I'll keep you updated on that!

Nick

[mashburn.nick]

Here's a rough "sketch" of what I plan on doing. It's 4" round galvanized steel ductwork, which I was told I could compress into ovals at the cross members so that it can pass through. I will of course have the large rectangular ducts on either end, too.

This also just barely allows the 50" gray tank to fit, with about 1" of room!

Any thoughts?

[65Streamer]

Good topic since I'll be installing my VTS sourced furnace in my 65 Sovereign this spring. I'm thinking the ducts should be above floor due to: critters, future service and cleaning and less heat loss. No tanks planned in the way-at this time. I know corrugated ductwork is a no-no. I'm hopeful to minimize storage loss by running rectangular duct above floor under kitchen shelf and under twin bed with outlet near bath in rear. Hoping for added benefit of warm mattress!

I'll follow along with your posts to learn what works-thanks!