How much air conditioning?

[b-rad]

New to me boat, it's a 43 foot very beamy pilothouse, lots of windows, lots of surface area. Need to install ac to make it a little more comfortable to liveaboard here in sunny south fl.
Currently have a 15k portable in one cabin that turns the room to a meat locker but does nothing for the rest of the boat. Trying to decide how big of unit (s) I need. Just found a 24k unit that I think could do the whole boat, or should I pony up and do a 16 and a 12 or 2 16s. Other issue is I only have a 30 amp service to the boat so if i do 2 units I will need to add an additional 30 amp power for the ac alone.

[a64pilot]

I'd do 2 16's, reason is they are not that much more in cost.
My Webasto 16K pulls 11 amps, so 1 30 amp service could pull two, but not if much else was on.
I assume the 24K is the Webasto? I almost went that way, I believe it has a built in soft start too, an issue with just one unit is running duct work, that is where the 24 would help as I think it has out of the box twin blowers or ducts?
I have a 16 and a 5 in my boat and on a hot day, that just barely cuts it, any smaller wouldn't work and I'm a 38'.
You lose cooling in long duct runs too, but could argue it's absorbing heat int he boat anyway, but I like COLD air coming from the duct, not cool air. The air from my 16K duct is right at 40 degrees, about a 40 degree drop, which is very good I think.

[Cheechako]

Maybe you need two 6000-8000 btu units instead.

[a64pilot]

I'm going to assume he is in Miami, and a 40+ boat, with a pilot house, the pilot house alone is going to be hard to cool

[Cheechako]

Yeah, it's hot down there. Last year, when we had our record heat, something like 90 days without rain and 23 days over 90 degrees I finally bought an AC for the house. 6500 BTU unit kept the house cool enough. 1100 sq ft house. I would think two of those would do the job placed well even in Fl. Boom tent over the Pilot House for sure.

[Sailmonkey]

I personally would do three units. Two large ones for most of the boat, and a small one for the stateroom. This allows you the option of AC even when at docks with inadequate shore power for your big units.


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[captjcook]

Was just about to say that SM...

I had a Formosa 44 (Spindrift 43) had it built in 1984.

If you have the galley down layout...

I suggest putting an 8k in the fwd hanging locker, a 12k (16k if ya leave the companionway open) in the pilothouse and a 5k for the aft stateroom. Lots less hole cuttin' and duct runnin'

Cheers!


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[a64pilot]

Originally Posted by Sailmonkey

I personally would do three units. Two large ones for most of the boat, and a small one for the stateroom. This allows you the option of AC even when at docks with inadequate shore power for your big units.


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That is why I have the 5K in our Stateroom, and a Honda will easily run a small unit too.


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[Pizzazz]

The key question here is one larger unit with ducts vs. a couple smaller units. I believe a larger unit is more efficient, lighter and takes less space. It is not that hard to run hidden ducts but depends on the boat. On my 31 ft boat I have just a 5,000 BTU unit in the lazarette with ducts to the saloon and the aft cabin. I am pleased with the results (Socal area). One way to keep the sun out is to put aluminium folio on the outside of the portholes (in place of the mosquito nets). It reflects the sun and keeps the windows cold. Also, you want to pick an AC with significant dehumidification power. This is as important as the BTU rating. Good luck.

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[Sailmonkey]

Originally Posted by Pizzazz

The key question here is one larger unit with ducts vs. a couple smaller units. I believe a larger unit is more efficient, lighter and takes less space. It is not that hard to run hidden ducts but depends on the boat. On my 31 ft boat I have just a 5,000 BTU unit in the lazarette with ducts to the saloon and the aft cabin. I am pleased with the results (Socal area). One way to keep the sun out is to put aluminium folio on the outside of the portholes (in place of the mosquito nets). It reflects the sun and keeps the windows cold. Also, you want to pick an AC with significant dehumidification power. This is as important as the BTU rating. Good luck.

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All AC units of equal BTU rating have equal dehumidification power. The act of cooling air lowers its ability to carry moisture, therefore as the unit cools air the moisture drops out.



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[Pizzazz]

Originally Posted by Sailmonkey

All AC units of equal BTU rating have equal dehumidification power. The act of cooling air lowers its ability to carry moisture, therefore as the unit cools air the moisture drops out.

You are probably right. I was looking at the Frigidaire website and it seems that the de-humidification ratings are similar but they are non-linear. For example,

Kitchen, Laundry, Home Appliances & Accessories | Frigidaire
5,000 btu with 1.1 pints/hr de-humidification
6,000 btu with 1.3 pints/hr de-humidification
8,000 btu with 1.5 pints/hr de-humidification
10,000 btu with 3.4 pints/hr de-humidification

There seems to be a significant jump from 8K to 10K. A 10K unit from this manufacturer will be much better than a smaller unit.

[zboss]

We used a 16,000 BTU AC unit in our hatch last summer (covered by our boom tent) and a 5000 BTU unit in our companionway this summer. We found basically no difference.

[b-rad]

Three units and give up a hanging locker, I hope not. Major issue for me for 3 units besides cost, have to have power and water in and out of each unit that would be 6 through hulls, 3 pumps and 3 strainers. I would need to add another 30 amp service to the boat. Talking about a 8-10k commitment, not sure I'm ready for that. I'm not concerned so much with the front cabin at the dock. Just need to cool the pilot house and galley (down galley ) and head and freeze the aft cabin. Thinking about trying the 24k for the boat with a smaller unit for the aft cabin, not sure where to hide it though.

[captjcook]

24k + 5k = 29k btu

16k + 8k + 5k= 29k btu

Power requirements are roughly the same. Not getting around adding more shore power unless you don't go beyond one 16k.

Very common to use 1 larger pump and strainer. A service manifold is extra nice!

Much easier to run a one inch diameter water hose than 4" (8" including insulation) or larger ducting. With a 24k, you might use two or more 25' lengths of ducting...kills storage...everywhere. Big holes through bulkheads...everywhere....

It's only the bottom of hanging locker...still hang shirts. Covers galley, fwd stateroom and head/generous separate shower.
Might be alternative locations for fwd unit.

There are also split systems where the compressor and air handler are modular...




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[GordMay]

A “rule of thumb” recommendation for sizing marine air-conditioning is about ± 14-to- 15 BTU/h per cubic foot of cabin volume.
More glazing, pilot house, etc = more heat gain; so perhaps up to 17 BTU/h per cu. ft.

A professional will size the air conditioner based on the latent cooling load (which considers the relative humidity of the air), as well as the sensible cooling load (which considers the outdoor & indoor design air temperatures) for your boat & location.
A (theoretically) perfectly-sized air conditioner will run continuously during the hottest 2.5% summer design (outdoor) temperatures.

Don't buy an oversized unit. An over-sized unit short-cycles (turns on & off too rapidly), so doesn’t properly de-humidify*, and uses more energy (starting currents).

On the other hand, make sure any ducting is sized large enough to allow low-velocity air distribution.

* The ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain. For short cycles (< 10 min, or so), the coil does not have time to operate at the low temperature, and when the unit stops, the moisture on the coil evaporates back into the indoor air. Thus, in humid climates, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units.

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