A/C Duct Sizing

[Sailorman375]

Recently installed a 10K BTU unit on my Oday 28. I know it's a bit small, but wondering if my ducts are part of the problem. I put the unit in the hanging locker and ran two short ducts, both 4" one to the v berth and one to salon. In attitude to get below 90 on very hot days near 98 F. The outlet on the unit is 4" hence the 4" ducts. Would it hurt anything to close off one of the outlets to run more air into the salon?

[Pblais]

Would it hurt anything to close off one of the outlets to run more air into the salon?

No, but it may not do much good. The air in a 28 ft boat spreads easily and sending more to the saloon may not really change anything. While the air temperature being hot is one thing the other part is the water temp. The hull becomes the temperature of the water. Add a really hot day most boats will need to push the BTU's to stay ahead late in the afternoon.

Make an awning that covers the deck. It can get you back 10 degrees. It's that or tie up in the shade. If you can find a marina in the shade I could move there too. This summer really has been a rough one. My upstairs in my house has trouble. With dew points close to 80 and heat near 100 degrees.

[Prerequisite]

10k BTUs should be able to keep a 28' sailboat pretty cool. What is the water temp in Kemah? I remember it getting pretty warm when I lived in that area. Most units lose some efficiency when the water temp climbs above 90 degrees. Also, if I notice my A/C not working as good as it should it normally means my raw water strainer needs to be cleaned out. Ducting size is important though, as is supply and return grill size. Are the grills you installed big enough for your unit? Will you still have adequate size if you close one off?

[Sailorman375]

Water temp 87 F. Strainer is clean and good water flow. Duct size 2 at 4" return is 12 x 14. The duct size I guess is my question. The outlet on the unit is 4" so I ran 4" ducts air flow is good, but cabin temp is running high 80s in the heat if the afternoon with outside temp high 90s with high humidity.

[DeepFrz]

Is your deck shaded? How is the water flow around your boat? David_Old_Jersey insulated his ceiling this summer and said it made a significant difference. Do a search or pm him for details.

[HopCar]

10K BTU should be plenty. The duct size sounds right. You say the air flow is good, so that sounds like the return is large enough. How about the water pump? If I remember right you want about a 500 GPH pump. March LC-3C-MD and Cal MS-580 pumps are often used for this size AC unit. Is your water pump the right size? An oversized pump putting out too much water is also a bad thing.