New Air/Heat Duct Work?

Columbia34 · Post by **Columbia34** » Tue Jan 23, 2007 12:49 pm

As most of you know we are installing a heating and cooling system in our Columbia 34. I have having a hard time on how to run the duct work I am considering using the wall cavity (like used in houses) as my duct run since it will be difficult to run any ducts or flex pipe to this location.

I my picture below you can see a hatch pattern the represents a board that I would like to install to make the air run in the wall cavity, and how the air will be let out of the space through some custom window boxes in the areas the previous owner made to install some port windows. The unit is gettting installed in a locker fwd (to the right in these pictures) of the couch, there is no problem running the duct work fwd into the v-berth.

Is this a bad plan, anyone see a better way to to this? I want to do it the best way possible.

Figment · Post by **Figment** » Tue Jan 23, 2007 4:41 pm

using a wall cavity (like they do in houses)

Buildings use wall and ceiling cavities as a plenum for return air. I don't know of any who use this approach for distribution.
Well, ok, there are commercial-application pressurized floor systems that essentially use the whole floor plenum for distribution, but they're a really bad idea. I don't think anyone is doing them anymore.
The long and the short of it is that it's just a really sloppy way to move air. The surface:volume ratio is too high, and you invariably get a lot of pressure loss due to leakage.

But if you can't run ducts you can't run ducts.
Still, I'd encourage you to find a way to run ducts. Part of the beauty of ductwork is that it can be replaced when it gets all cruddy and nasty. I'm of the opinion that flexduct on boats should be replaced every 5 years. Everyone thinks I'm crazy, until they pull one of their ducts and see that it's either falling apart in their hands, or that it's just disgusting inside.

$0.02

Ceasar Choppy · Post by **Ceasar Choppy** » Tue Jan 23, 2007 4:57 pm

Here's a question:I've seen plenty of supply vents that have long ovals where the ducting should connect to the vent box. i.e. instead of a 4" round connection, it is a 1" x 5" oval.

So the question is: Do they make ductwork in "flat" oval shapes or do you just take regular round ductwork and flatten it to fit?

Columbia34, this might be your solution. I am considering the above for the A/C on my P39.

Columbia34 · Post by **Columbia34** » Tue Jan 23, 2007 5:02 pm

I like you thinking but I just drew it up and a 1x5 oval has about 4 sq in of area where as a 4" circle has about 12 sq in. so there is alot less volume.

Scott

Figment · Post by **Figment** » Tue Jan 23, 2007 5:03 pm

Yes, "flat-oval" ductwork (elliptical in section) is generally held to be the best compromise between depth and efficiency. Of course, that also makes it the most costly per material foot, but this shouldn't add much to the bottom line of the job as a whole.

Ceasar Choppy · Post by **Ceasar Choppy** » Tue Jan 23, 2007 5:04 pm

I was just guesstimating poorly at dimensions... :)

See picture I attached for an example.

Ceasar Choppy · Post by **Ceasar Choppy** » Tue Jan 23, 2007 5:08 pm

Figment wrote:Yes, "flat-oval" ductwork (elliptical in section) is generally held to be the best compromise between depth and efficiency. Of course, that also makes it the most costly per material foot, but this shouldn't add much to the bottom line of the job as a whole.

I've looked all over McMaster a while back, but never found it. Any idea where I can get some?

As an additional note, I've noticed fancy transition boxes and grills are very hard to come by. The two places I've found them are at http://www.mmair.com and http://coastalclimatecontrol.com.