Boat A/C Ducting

[ACD22]

Boat A/C manufacturers sell "duct kits" with their A/Cs 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.

I'm installing an 18,000 btu unit that has a 4-1/4" x 5-1/4" blower outlet with 555 cfm capacity. This will go in an enclosed fiberglass L shaped settee that's 5' x 8' long, 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(s) will be in the 8' leg. There is no way to have the 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 my own, I would install a transition take-off from the A/C blower to go to a 7" or 8" round insulated flex duct, turn the corner with a baffled 90 degree elbow and run flex to one large outlet on the side of the 8' part. Another way would use a streamline reducing 866 wye or sharp angle lateral after turning the corner and have two outlets, one on the side and the second in the end. I'm trying to keep the ducting compact so as to allow the inside of the settee to continue to provide storage space through various gasketed hatches that are in it.

Comments most appreciated and I hope this somewhat unusual question doesn't run afoul of the "no diy rule".

[skippedover]

Boats are unique. No way for us to provide any useful information, even if the DIY rules didn't apply, which they do.

[Shophound]

How is the condenser on this boat a/c cooled? Via sea water or whatever water the boat is in?

Bottom line regarding ducting is that it should not overly restrict airflow through the air handler, especially if you're using a "DX" (direct expansion or refrigerant based) vs. a chilled water fan coil, which I have seen on larger boats and ships.

Have you also considered how to get condensate out of the unit when it's installed inside the settee? Boat interiors are often humid since they're on the water most of their lives. Additionally, when the boat begins rocking in heavier chop, does the air handler you selected have a way to keep the condensate from slopping out of the unit and onto the deck?

One more thing: make sure that air handler can be serviced if you're going to squeeze it into a confined space like beneath a settee. It would be a real drag if the air handler clogged up or a blower gave out, and you would be forced to dismantle the settee to access the air handler. I've worked on fan coils on boats that were almost impossible to get to...and I was there to fix a chilled water leak. Made it very hard to do the job right the first time.

[ACD22]

The condenser is sea water cooled. I'll run three hoses to the unit: sea water in, sea water out and condensate out. There's a drip pan to contain condensate and direct it to the condensate out drain. The settee has several hatches for access to the unit for service and of course it can be taken out of the return cut-out through which it will be put in should the need arise.

My main concern is the ducting size. Based on my questioning of them, the manufacturer is suggesting a 6" round take-off from the rectangular blower in lieu of their usual 4" spaghetti arrangement. But based on the dimensioned drawing they just sent me of this take-off, it was designed for their 12,000 btu unit which has the same size blower duct, but the blower itself in the 12k unit puts out 485 cfm rather than the 555 cfm in the 16k and 18k units.

On reflection, I don't think I can fit an 8" take-off due to adjacent items, but I can fit a 7". The trade off I'm unable to consider between 6" and 7", because I don't know how this stuff works, is higher static pressure with the 6" vs. decreased air velocity with the 7".

[Shophound]

The higher static pressure means less air flows over the cooling coil. Less air flowing over the cooling coil can bring the surface temperature of that coil closer to the freezing point of water. Meaning, in short, the coil can freeze over if it does not have enough air moving over it.

If you use a solid plenum (take-off) at the unit and short, straight as possible, kink-free runs of ducting to the supply outlets you have planned, you might be okay regarding not freezing the coil using the sizes you've mentioned. Something you would need to watch carefully...is this your own boat or a client's? I would want to know the blower curve of the air handler in question; meaning how much air it will actually deliver according to the static pressure imposed on it.

Also hope you have a strainer for the sea water cooling the condenser coil. Especially if the unit has a coaxial condenser coil. I remember punching the tubes of chillers on Navy ships and pushing a lot of barnacles and so forth from the tubes.

[ACD22]

This is my boat and there's a strainer. And thanks for your help.

The specs on the blower (Dayton 1TDT2) for cfm@ "x" inches of water are: 549@0", 538@0.1", 510@0.2", 500@0.3", 480@0.4", 450@0.5", 435@0.6", 390@0.7" and 360@0.8".

I have two ideas about the duct plan. Plan 1 is to attach a round collar at the rectangular blower outlet and go from the collar, using flex hose, to one supply outlet (10 x 6 or ???) using a 90 elbow to turn the corner that I need to turn. Plan 2 is the same as plan 1 but to go with two outlets (10 x 4 or ???) using a reducing lateral (i.e 6x5x4 or 7x5x5) in addition to the 90 elbow to get to the two outlets. I can also skip the 90 elbow and use the flex hose itself to turn the corner. Since the outlets will 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. I'll use double vane registers with the horizontal vanes pointing upward.

The main questions are: 6" vs 7" duct size, one outlet vs. two and size(s) of the outlet(s).