I'd like to get opinions on if an idea is good, or crazy. Our aft AC is dead, has been since we purchased the boat. Luckily last year was a cool summer and it didn't really mater.

The salon AC doesn't keep up with the heat - there are many reasons for this such as no salon curtains, short cycling because it's 6" from a solid set of steps, and it's probably under rated. I've had it looked at by an AC tech and they tell me it's operating as designed. To help somewhat, we are planning to tint the salon windows and somehow reroute the flow of air around the steps to prevent the short cycling - whey they place the salon AC where they did is beyond me.

Now to my idea. The aft AC requires around 10-12k BTU. I'm going to replace the split AC system with an all-in-one, mostly due to cost considerations, but also for simplicity of trouble shooting. Since a self contained system is larger, it will not fit in the closet where the evaporator/blower unit currently lives. For a new location, I've selected the closet in the aft guest room (my son's). It will take up around 14" of the bottom so we can retain some closet space.

I'm thinking of increasing the capacity to 16k BTU and adding a Y to provide one run to the aft staterooms and the second run to the salon to assist the AC system there. During the day I can close the aft stateroom vents so the full 16k can pump into the salon (to help the undersized salon AC) and after the sun goes down, open them to cool the staterooms as well. The run to the stateroom vents would be approximately 8' and the run the salon longer, at maybe 16'. I'm thinking of using PVC to promote smooth air flow (vs flexible hose) and will insulate it in unconditioned spaces.

Thoughts?

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The factory units have a real advantage in being quiet.  By having the compressor remote and piping the refrigerant to the local unit the only fan is that for the local coil.  I have a single unit that does a good job cooling the boat but the noise of the fan travels through the duct work, cool is better than quiet and it is not disturbing but certainly noticeable.  I did not consider hard piping but used the insulated ducting and found it to be satisfactory.

http://commanderclub.com/forum/topics/ocean-breeze-air-conditioners

http://commanderclub.com/photo/albums/installation-of-seabreeze-ac-...

You will never get the full capacity of the unit down one run at that distance. As the saying goes "You can only fit so much shXX down a horn". By restricting the cfm of airflow you will only be successful in freezing up the unit. The loss of static pressure will be less by using a smooth walled pipe but its really the size of the pipe that will make a difference. look up duct sizing and you will get a better idea. Think of air flow the same as water flow from your house hose. Great pressure at the faucet. not so good 100 feet away at the end of the hose due to frictional losses. Air or water, it doesn't matter. 

Agree Jim, I'm hoping that someone on here is a pro at designing duct systems and can tell me the parameters I need to work within. The Webasto unit says it delivers 505 cfm and asks for a duct system with less than 100Pa (0.4 iw). It only has a 5" trunk connection, which seems small based on what I've read for 505 cfm.

Jim Personeni Jr said:

You will never get the full capacity of the unit down one run at that distance. As the saying goes "You can only fit so much shXX down a horn". By restricting the cfm of airflow you will only be successful in freezing up the unit. The loss of static pressure will be less by using a smooth walled pipe but its really the size of the pipe that will make a difference. look up duct sizing and you will get a better idea. Think of air flow the same as water flow from your house hose. Great pressure at the faucet. not so good 100 feet away at the end of the hose due to frictional losses. Air or water, it doesn't matter. 

Steve, Marine duct work is not my thing but I can tell you at 4 tenths drop with 500 cfm you would be looking at an 8" pipe. That's not factoring distance. The 5" outlet is not an issue, you would increase coming off of that. Always use a tapered increaser or reducer. Always use a wye instead of a regular tee if at all possible, really try to make it possible. The smaller the pipe the more velocity (to a point) there will be and that equals more noise. Just like pinching the end of that long garden hose. You're not getting more flow, just increasing the velocity of the water leaving the hose and making noise. On a long run you would want to start at least one size bigger to allow for the added frictional loss and then decrease down a size with a tapered fitting. This will allow you to get the cfm you need and keep your velocity steady. Remember that marine ducting is really not my thing so use these pointers as general speak. Air flow is air flow but I have not had to deal with marine systems yet although my time is coming!

Thanks for the pointers Jim. I can definitely use a Y instead of a T. I was thinking a Y and then a 45 to get the 90 turn complete, not sure if a Y is actually branching out at 45 or not. 8" is pretty big, I had been thinking of 6", but without any actual calculations. That said, I could probably fit 8" without much difficulty, but it wont be cheap!

If it can't be done in a practical sense, I might just have to abandon my idea.

The new (used) boat I just bought has a "T". Not as efficient as the "Y" I did on my Catalina.

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The salon ac in my boat had the same problem, the fix (done by the previous owner) is to run new ducting to a new vent in the middle of the room on the same side, and use the old vents as an extra intake. My salon unit no longer freezes up.

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