Marine AC using fresh instead of raw water?

[aybabtme]

I'm interested in installing a AC unit on our boat to control humidity when away, and to cool/warm the cabins every now and then. But I don't want to add a thru-hull for it if I can avoid it: I don't have a haul-out scheduled soon and would rather avoid the hole. At the same time, our water heater is defective and needs to be replaced. So I got thinking.

Could I plumb a loop in the hot water line, before the water heater, that would go through the AC unit instead of the raw water? I'd pre-warm the water headed for the water heater, and avoid all the complications of the raw water intake going in the AC unit. When no one is showering, the AC unit would loop the water until it gets progressively warmer (and my guess is it won't ever get so warm that the AC won't be able to work anymore). When someone uses hot water, the AC would be getting cold water from the tank, so no issues there.

Has anyone done this?

[ItDepends]

Originally Posted by aybabtme

I (and my guess is it won't ever get so warm that the AC won't be able to work anymore).

Your guess would be very wrong.

A very small marine AC unit moves 6000 BTU/hr. That’s enough to heat 100 lbs (about 11 gallons) of water 60F., in an hour of operation so from 70F to 130F. Most marine AC units need water less than 95F or so to work.

You will quickly shut down your system on high pressure, and most require a manual reset to get going again.

[S/V Illusion]

It's amazing the imaginative ways people find for not doing something correctly.

[aybabtme]

Originally Posted by S/V Illusion

It's amazing the imaginative ways people find for not doing something correctly.

Hey, what's the problem with trying stuff out? It's important to keep your mind open and explore stuff out, it keeps alzheimer and other brain diseases away! Also there's nothing wrong with making mistakes and sometime ending up with a cool idea that works!

[Cheechako]

While in the boat yard in the tropics I thought maybe I could hook up a 5 gallon bucket and run the onboard AC now and then through the bucket and back to the bucket.. NOPE! The water is very hot in 5 mins or less! My guess is an entire water tank wouldn't work.

[rtoner]

Some people put a house window type into their companionway/hatch. This might be challenging on a cat.

[aybabtme]

Originally Posted by ItDepends

Your guess would be very wrong.

A very small marine AC unit moves 6000 BTU/hr. That’s enough to heat 100 lbs (about 11 gallons) of water 60F., in an hour of operation so from 70F to 130F. Most marine AC units need water less than 95F or so to work.

You will quickly shut down your system on high pressure, and most require a manual reset to get going again.

Somehow I hadn't thought about running the math.

Let's assume 25ºC ambient temperature for the fresh water tank, and 95ºF is 35ºC, 6000 BTU is 6330 kilojoules. That gives me a 10ºC budget. If I go with Q=c*m*dT where

• Q=6330e3 J
• c=4.186 J/gºC
• dT=10ºC
• `m` is the mass of water in grams

Then solving for `m` in grams (`m = Q/(c * dT)`) gives 151,218g, or 151L of water. So a "small loop" of fresh water wouldn't work. However if I were cycling my entire fresh water tank (400L) I could run the AC for a little over 2h before hitting the limit. And ruin my drinking water in the process!

Ok, well. Next time I'll run the math real quick!

[CaptTom]

The math works against you this time. But nothing wrong with creative thinking.

How about just dangle the air conditioner inlet hose over the side? Most likely you'd need to prime the pump, and even then you'd still want the pump low, preferably below the water line, so it's not pulling the water up, but siphoning it level. While you're at it, you can let the hose down farther so it gets cooler water below the surface level. That should work until you can pull the boat and do a proper thru-hull.

[S/V Illusion]

Originally Posted by aybabtme

Hey, what's the problem with trying stuff out? It's important to keep your mind open and explore stuff out, it keeps alzheimer and other brain diseases away! Also there's nothing wrong with making mistakes and sometime ending up with a cool idea that works!

And I forgot - the ability to rationalize anything is even stronger than imagination. Simple first law of thermodynamics.

[aybabtme]

I'll put this idea in the "doesnt work as is" basket but I have a feeling there's something here that could work. Residential electric water heaters, the high efficiency ones, do use heat pumps as a first stage. And those heat pumps reportedly help cooling and de-moisturizing the area they're in. Hence clearly there's a way.

[aybabtme]

Originally Posted by S/V Illusion

And I forgot - the ability to rationalize anything is even stronger than imagination. Simple first law of thermodynamics.

Thanks, this is amusing and useful feedback.

[aybabtme]

Originally Posted by CaptTom

The math works against you this time. But nothing wrong with creative thinking.

How about just dangle the air conditioner inlet hose over the side? Most likely you'd need to prime the pump, and even then you'd still want the pump low, preferably below the water line, so it's not pulling the water up, but siphoning it level. While you're at it, you can let the hose down farther so it gets cooler water below the surface level. That should work until you can pull the boat and do a proper thru-hull.

That's a good idea and probably what I'll do. Our watermaker is setup in this fashion (Rainman).

[meirriba]

Originally Posted by S/V Illusion

It's amazing the imaginative ways people find for not doing something correctly.

Only imaginative people invent new things.
So no need to be negative...

[CDR Smith]

If you have salt water flushing heads, could you T off the seacock for that supply? You should not T off your engine cooling raw water, but I don't think there'd be a problem using a head or anchor washdown supply line. You could avoid adding a thru-hull that way, no?

[SeeMonstrEd]

I didn't do the math when we were on the hard in a boatyard in GA. I got a 64 gal. trash can a filled it with fresh water from the tap. My 16K BTU AC had it so hot I could not put my hand in it within 15 minutes. At least it cooled the cabin enough to sleep that night without the biting bugs.

I agree using a small window AC in the companion way or a hatch is probably a better option. I have seen several other boats do this and they said is was less expensive than running the bigger marine AC units.