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Old 02-21-2012, 03:06 PM   #1
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Boat A/C Ducting

I'm installing a new self-contained a/c unit on my boat.

Marine a/c manufacturers sell "duct kits" with their units for running the duct work. These kits consist of a sheet metal plenum cube, 5 or 6 inches on each side, with instructions to install three or four 4" take-off collars (depending on BTU size) from the sides and top of the cube and run insulated flex duct to corresponding 4" outlets from there. Lot's of turns and twists and a bit spaghetti-ish.

The new a/c is 18,000 btu and has a 4-1/4" x 5-1/4" blower outlet with 555 cfm capacity (Dayton 1TDT2). The a/c will go in an enclosed fiberglass "L" shaped settee base that's 5' on the short leg of the "L" and 8' on the long leg. The settee base is 13-1/2" high and 24" wide with both ends exposed to the space. The A/C unit will be in the 5' leg with an 18" x 10" return cut in on that end (big enough also to get the A/C inside the settee space) and the supply outlet will be in middle of the 8' leg. There is no way to have outlets up high in the cabin where they should be because there's no way to get them there. The ceiling is 6'6" high. The space to be air conditioned is an enclosed pilot house with big glass all around and is about 700 cubic feet. The boat's on the Chesapeake.

Thoughts on how to improve from the industry standard approach to ducting would be appreciated.

Left to decide on my own, I would attach a round collar at the rectangular blower outlet and go from the collar, turn the corner that I need to turn to using a 90 elbow and go on to the supply outlet using an end register boot. I can also use flex hose to turn the corner and get to the supply. Total length of the duct run is short - about 3' including the 90 degree turn.

Since the outlet would be close to the deck, I want to make sure that there's sufficient velocity to get the cold air to circulate in the pilot house. For that I'd use a double vane register with the horizontal vanes pointing upward. The main questions, if this is the right approach, are: 6" vs 7" duct size, flex vs. rigid and the size of the outlet.

Finally, the plan outlined above may not be the best one and any thoughts on how to improve it would be most welcome.


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