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Old 21-11-2011, 08:15   #1
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Sizing A/C

I already know the general formula for sizing an a/c unit. What I want to know, and can't find, is what temperature or temperature drop does the formula hope to achieve? I have talked to a/c people and they recommend x btu and say "you could hang meat". Well i don't want to hang meat I just want to get the temperature to where I don't sweat at night, around 75F. My thinking is if the recommended 16k btu lets me hang meat then I should be comfortable with 6-10k btu. I suspect my thinking is wrong but don't know why. Thanks.
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Old 21-11-2011, 09:18   #2
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Re: sizing a/c

The typical “Tropical” rule of thumb probably assumes a Delta T of about 35 degrees F.
The Temperate rule of thumb probably assumes a Delta T of about 20 degrees F.

Sizing Air-Conditioning:

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

* For Tropical climates: 105F (41C) air, 95F (35C) water, high humidity.
You can reduce the figures to about 75% of above for Temperate climates: 95F (35C) air, 85F (35C) water, moderate humidity; hence 10 - 12 BTU/Hr per Cu. Ft.

A professional (which I am not) 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 Delta T between 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.

See the formulae ➥ Cooling and Heating Equations
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Old 21-11-2011, 09:30   #3
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Re: sizing a/c

I have a34' pilot house trawler and am currently in south fl looks like I have 952cf in the house with 12 windows including the windshield not counting the v berth at the lower level I keep the curtains closed most times I dont think I can get the duct work up high probably just out of the floor I suspect most boats have this issue Thoughts? Size of unit?
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Old 21-11-2011, 09:52   #4
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Re: sizing a/c

On a boat with a lot of window area, as it sounds like you have, can see most of the heat gains resulting from solar gains - sunshine coming in through windows. Need to get very good window shading to reduce these gains and give air conditioner any chance of keeping up. Curtains help, but much more effective to shade windows on the outside. Think solar screens. Screening materials that mount outside of the windows can reflect more than half the solar energy. Make sure these, or a full awning, are in your plans.
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Old 22-11-2011, 12:29   #5
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Re: Sizing A/C

Thanks Gordmay but that still does not tell me what most a/c guys hope to accomplish with the recommended btu's. I believe they talk in degrees of temperature drop, i.e. "you will get 20 degrees drop from outside temperature with this setup", but I am not sure. The point is if someone says I need x with the intent of dropping the temperature to 65 degrees then I think I should be able to use y instead because I only want to go to 75 degrees. Right or not?
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Old 22-11-2011, 16:34   #6
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Re: Sizing A/C

Quote:
Originally Posted by clayzone View Post
... I believe they talk in degrees of temperature drop, i.e. "you will get 20 degrees drop from outside temperature with this setup", ...
Right.
Outside to inside temperature drop is what I referred to as "Delta T".

Quote:
The typical “Tropical” rule of thumb probably assumes a Delta T of about 35 degrees F.
The Temperate rule of thumb probably assumes a Delta T of about 20 degrees F.

Sizing Air-Conditioning:

A “rule of thumb” recommendation* for sizing (Tropical) marine air-conditioning is about 14-to- 15 BTU/Hr per cubic foot of cabin volume.
More glazing, pilot house, etc = more heat gain; so perhaps up to 17 BTU/Hr per cu. ft of volume...
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Old 23-11-2011, 13:27   #7
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Re: Sizing A/C

Thanks for clarifying the Delta T. That is the information I was looking for.
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