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Old 23-01-2021, 03:33   #1
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air cond unit size for masters cabin

Over on another thread the forum was discussing running air cond off batteries (lets try not get into why you would/should not), but an interesting comment was made regarding a 3500 BTU system not cooling a cabin.

Has anyone used a 3500 BTU (or similar) size unit in the masters cabin ONLY on a cruising cat, 38-42ft size? does it cool sufficiently?

My back-of-napkin estimates say its possible with enough $$ thrown at it as the smaller units draw a few amps (as an example Dormaire 3500BTU tech specs are 'cooling 1.3A AC', so approx 26 A, 220VAC>12 VDC inverter). Can any one confirm this is possible in the real world?

Note: Im not talking about cooling the entire boat here, just the masters cabin.
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Old 23-01-2021, 05:09   #2
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Re: air cond unit size for masters cabin

How large is the master's cabin?
I'd expect a 3500 BTU/H Air-Con' to cool on the order of about ±35 Sq. Ft.

A rough "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 (below decks) to 17 (above decks, glazed) BTU/H for each cubic foot of interior conditioned volume.

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).

Make sure any ducting and grillework is sized large enough to allow low-velocity air distribution.
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Old 23-01-2021, 05:46   #3
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Re: air cond unit size for masters cabin

My aft berth master's has a 9,000 BTU unit.

When you figure the power draw don't forget the seawater pump draw!
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Old 23-01-2021, 07:05   #4
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Re: air cond unit size for masters cabin

Most of the small cabins we do end up with a 6k, 7k, or 8k.with a single set of grills.
If the cabin is full width, and a bit larger, we will go with a 12k and 2 6k air handlers to balance the airflow/keep noise down/keep grills small, and eliminate dead zones.
The actual size we select is determined by a number of factors that vary from each boat.
We use a standard heat load formula that can be found online at Dometic's site.
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Old 23-01-2021, 08:48   #5
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Re: air cond unit size for masters cabin

A 3.5K BTU unit is very small.

It might work for a tiny cabin, that is well shaded from the sun, in a moderate climate, that is well sealed from air infiltration, and fairly low ambient humidity levels.

One of the key issues is the sun loading. If you can rig a shade cloth over the cabin you'll reduce the A/C load a LOT.
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Old 23-01-2021, 10:16   #6
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Re: air cond unit size for masters cabin

I can confirm that 7000 BTUs cools the owners cabin, which is about 80 sq ft. It has two glass hatches which can be covered with reflective sunshades. The cabin cools down to about 65F when the unit runs continuously in hot weather.

I don't think a smaller unit would be effective in my case.
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Old 23-01-2021, 10:27   #7
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Re: air cond unit size for masters cabin

Quote:
Originally Posted by Fendell View Post
Over on another thread the forum was discussing running air cond off batteries (lets try not get into why you would/should not), but an interesting comment was made regarding a 3500 BTU system not cooling a cabin.

Has anyone used a 3500 BTU (or similar) size unit in the masters cabin ONLY on a cruising cat, 38-42ft size? does it cool sufficiently?

My back-of-napkin estimates say its possible with enough $$ thrown at it as the smaller units draw a few amps (as an example Dormaire 3500BTU tech specs are 'cooling 1.3A AC', so approx 26 A, 220VAC>12 VDC inverter). Can any one confirm this is possible in the real world?

Note: Im not talking about cooling the entire boat here, just the masters cabin.
It will depend on a lot of things. How cool? just to keep it reasonable, I would think 3500 btu would be very adequate. A master sleeping cabin would be what... less than 500 cu ft? 7 ft x 7 ft x 10 ft?

I have a small 6500 btu window unit for my small 1140 sq ft house. It keeps the kitchen and living room fine on 90-100 degree days in this well insulated house. Back bedrooms not cool. Houses have much higher ceilings than boats, much more cubic feet, but are better insulated.

I'm not sure why the focus on AC though, but I dont know where you are or your plans. Throughout Mexico and the Caribe I never used AC once on the water. One boat had it already installed too. With the boat stored on land I had an AC as it was hot/muggy there. If you are living at the dock then a house unit in the overhead hatch is overkill but will keep you cool/cold in any weather.
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Old 24-01-2021, 08:33   #8
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Re: air cond unit size for masters cabin

We had an 8000 btu Dometic Turbo unit for the port hull of a Seawind 1000 catamaran. We would close the companionway, and air con the entire hull including the aft head. We used it in South Texas summer, which is pretty warm, and it could achieve 72 deg F setpoint during the day. Problem was that it didnt run long enough to dehumidify well.
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Old 25-01-2021, 07:03   #9
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Re: air cond unit size for masters cabin

Quote:
Originally Posted by sailorboy1 View Post
My aft berth master's has a 9,000 BTU unit.

When you figure the power draw don't forget the seawater pump draw!
Yeah true, do you know if they tend to run 24/7 or do they cycle on/off with the compressor?

Quote:
Originally Posted by GordMay View Post
How large is the master's cabin?
I'd expect a 3500 BTU/H Air-Con' to cool on the order of about ±35 Sq. Ft.

A rough "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 (below decks) to 17 (above decks, glazed) BTU/H for each cubic foot of interior conditioned volume.

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).

Make sure any ducting and grillework is sized large enough to allow low-velocity air distribution.
Thanks Gord

Quote:
Originally Posted by Coolerking View Post
Most of the small cabins we do end up with a 6k, 7k, or 8k.with a single set of grills.
If the cabin is full width, and a bit larger, we will go with a 12k and 2 6k air handlers to balance the airflow/keep noise down/keep grills small, and eliminate dead zones.
The actual size we select is determined by a number of factors that vary from each boat.
We use a standard heat load formula that can be found online at Dometic's site.
My thought is to install the unit as small as practical to be able to run off batts for the night, obviously 6K would draw to much for that.

Quote:
Originally Posted by BillKny View Post
A 3.5K BTU unit is very small.

It might work for a tiny cabin, that is well shaded from the sun, in a moderate climate, that is well sealed from air infiltration, and fairly low ambient humidity levels.

One of the key issues is the sun loading. If you can rig a shade cloth over the cabin you'll reduce the A/C load a LOT.
Good point, about the sun shading. large shade over that side of the boat may help with that.

Quote:
Originally Posted by dcarratt View Post
I can confirm that 7000 BTUs cools the owners cabin, which is about 80 sq ft. It has two glass hatches which can be covered with reflective sunshades. The cabin cools down to about 65F when the unit runs continuously in hot weather.

I don't think a smaller unit would be effective in my case.
65F (18'C) is cold, colder than I would have expected. I was more thinking 22'C (71'F) which is what i currently have my thermostat set at.

Quote:
Originally Posted by Cheechako View Post
It will depend on a lot of things. How cool? just to keep it reasonable, I would think 3500 btu would be very adequate. A master sleeping cabin would be what... less than 500 cu ft? 7 ft x 7 ft x 10 ft?

I have a small 6500 btu window unit for my small 1140 sq ft house. It keeps the kitchen and living room fine on 90-100 degree days in this well insulated house. Back bedrooms not cool. Houses have much higher ceilings than boats, much more cubic feet, but are better insulated.

I'm not sure why the focus on AC though, but I dont know where you are or your plans. Throughout Mexico and the Caribe I never used AC once on the water. One boat had it already installed too. With the boat stored on land I had an AC as it was hot/muggy there. If you are living at the dock then a house unit in the overhead hatch is overkill but will keep you cool/cold in any weather.
Glad you think it may be possible. This all theoretical of course as I don't have a boat, but I know sleeping in a tent above 25'C (77'F) is not fun, so I wish to plan for the few days a year that to be expected. keep in mind Im in Australia, and it can get pretty hot here haha.

Quote:
Originally Posted by sailjumanji View Post
We had an 8000 btu Dometic Turbo unit for the port hull of a Seawind 1000 catamaran. We would close the companionway, and air con the entire hull including the aft head. We used it in South Texas summer, which is pretty warm, and it could achieve 72 deg F setpoint during the day. Problem was that it didnt run long enough to dehumidify well.
That gives me hope, never been on a SW 1000, but the Leopard 40s master cabin we chartered I would have thought could have been sectioned off and cooled effectively with a small unit. Thanks all for your input
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Old 25-01-2021, 10:56   #10
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Re: air cond unit size for masters cabin

The water pump cycles on with the compressor.

Frankly I am a little more concerned about oversized unit vs undersized. You want it to cycle and run enough to get the humidity down.
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