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Old 28-08-2013, 18:50   #1
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Sizing of Air Conditioning

Ok so now that we are working again and living aboard at a marina in the BVI the Admiral and I are considering adding AC to the boat. When we lived aboard in Annapolis before cruising we used a pair of hatch units which were ok but thinking more about a proper system.

So reading a bit and measuring a bit has led me to an approximate interior volume of 2,900 cubic feet. Broken down by 2,000 cubic feet of salon, galley, and fwd cabin and 900 cubic feet of aft cabin and walk thu cabin. Suggestions on sizing recommend 14BTU/cubic foot.

I'm considering a 24k BTU unit for the salon,galley, fwd cabin, and a 12k BTU unit for the aft and walk thru cabins.

Questions:

1. Separate sea water pumps or a single relay controlled pump that will service both units?

2. Am I over sizing too much? We are a 43' wauquiez Amphitrite ketch in the tropics with little shade on deck.

3. Would a single dedicated 120volt 30amp shore power connection be ideal for the AC? We currently have a single 30amp for the boat and one thought was to upgrade the shore power service to 50 amp but I'm not sure that would be adequate. Thus the thought to add a second 30amp service just for AC.

4. Looking for suggestions on equipment to use/avoid. Had a mermaid air system on a prior boat that seemed ok? I'm only interested in cool only units no need for reverse cycle heat.

5. What else should I be asking?
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Old 28-08-2013, 19:07   #2
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Re: Sizing of Air Conditioning

Your sizing sounds about right. I would install one seawater pump controlled by relays.

The 24,000 btu unit is going to draw about 18 amps and the 12,000 btu unit will draw 12 amps on a 120V circuit. Both on a 30 amp circuit will blow the breaker. So how about a new 30 amp circuit dedicated to the 24,000 btu unit and put the 12,000 btu unit on your existing 30 amp service.

You could upgrade your 30 amp to 50 amp 120V but that isn't standard in the US at least. US marinas have 30 amp 120V or 50 amp 240V that can be split to give two 30 amp 120V circuits. I think you are better off with a second 30 amp 120V circuit dedicated to the big A/C.

Dometic Cruisaire is one of the best.

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Old 28-08-2013, 19:08   #3
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Quote:
Originally Posted by SV Demeter View Post

I'm considering a 24k BTU unit for the salon,galley, fwd cabin, and a 12k BTU unit for the aft and walk thru cabins.

Questions:

1. Separate sea water pumps or a single relay controlled pump that will service both units?

2. Am I over sizing too much? We are a 43' wauquiez Amphitrite ketch in the tropics with little shade on deck.

3. Would a single dedicated 120volt 30amp shore power connection be ideal for the AC? We currently have a single 30amp for the boat and one thought was to upgrade the shore power service to 50 amp but I'm not sure that would be adequate. Thus the thought to add a second 30amp service just for AC.

4. Looking for suggestions on equipment to use/avoid. Had a mermaid air system on a prior boat that seemed ok? I'm only interested in cool only units no need for reverse cycle heat.

5. What else should I be asking?
1. Single seawater pump with tee in the output to feed both AC units. Don't put the water paths in series. Use relays from both units to start pump when either system starts its compressor. This is very common.

2. 16KBTU is enough for the common area and 12KBTU is enough for the sleeping area. But you need to shade the boat. Without shade it will never be cool during daylight hours once you close up the hatches no matter how much AC you have. An extra 8K won't help in my experience.

3. A single 30A circuit will run a 16K and 12K. Maybe a 24K and 12K but check carefully. I don't like running 25A on a 30A twist lock style connector. this should be dedicated to AC and nothing else.

4. Dometic is the big name in boat AC. They make good units under various brands.

5. Why don't you shade the boat in a tasteful way? It's way cheaper than wasting electricity. How will you get rid of condensate?
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Old 28-08-2013, 19:42   #4
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Sea weed in the strainer is my biggest problem and cause at least one shut down a year when I don,t keep up cleaning routine. Two pumps, two strainers will give you a little redundancy.
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Old 28-08-2013, 19:43   #5
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Shading of the the deck will come eventually but the budget for canvas is geared more to redoin the cockpit Bimini/dodger and fixing or replacing stackpacks first . Condensate removal is a one I had forgot. My boat is deck stepped so the bilges are dry storage areas. Will likely plumb boxes with float switches. Wonder if one discharge could T into the AC raw water cooling water discharge with the appropriate check valve? Would like to limit the number of holes I drill!

Curious on the 16k btu suggestion. My prior boat, an Ericson 38 had a 16k btu mermaid unit that did the whole boat just fine even with no shade and dog days of Chesapeake bay summer. The wauquiez is bigger inside for sure but maybe I can get by with an 18k BTU unit?

One thing I have heard is that greatly over sizing AC systems is not good as the units don't run long enough between cycles.
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Old 28-08-2013, 19:47   #6
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Re: Sizing of Air Conditioning

Its better to be a little undersized then a little oversized. Undersized will run more and do a much better job of de-humidifying. A larger unit will not run enough to remove the moisture in the air.
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Old 28-08-2013, 21:58   #7
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Re: Sizing of Air Conditioning

SC is right that over sizing is not the best thing. It seems counter-intuitive but an AC unit that runs almost all the time dehumidifies the air and is more efficient than one that cycles a lot. The lower humidity is a huge help on the comfort level almost as much as the air temp. The water can be removed via a venturi device in the exit water stream. Or your idea of a small collection box with a switched pump can work.

On really hot days you can run both units and use a small fan to stir the air between the sleeping area and the common area.

You can do a good bit of shading for not much money. Think of cheap white tarps. Even if they only last a few months before succumbing to UV damage they can save more than their cost in electricity bills.

The choice of AC size depends on what you can be comfortable with. If you have to have the cabin 68F during the day then you won't be happy. If you can live with 78F and 40% humidity then 16K + 12K should be fine.
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Old 29-08-2013, 05:18   #8
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I recall reading something about these Venturi devices for removing the condensate. Maybe that's a better way to go than boxes with pumps and float switches. Just don't want it draining to the bilge.

So a 12k and 16k BTU will probably do the job the question now is where to source the units and who's equipment.
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Old 29-08-2013, 10:48   #9
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Re: Sizing of Air Conditioning

We hate the venturi devices. If we forgot to clean it, it would stop up and flood. Do you have a wet bilge? If yes, just plumb it to the bilge by gravity and let the bilge pump take care of it along with the packing glands drips.

You want to get the "right" size. Undersize is pointless as it will still be hot. The problem with using cubic feet is it depends on how well insulated the boat is. If you have thick cored deck/hull with minimal windows, you can go on the light side. If you have thin solid decks with lots of window space, you need lots of btu's regardless of volume.

Your two unit system actually addresses this nicely. Oversize and if it's cycling too much, just run one unit to get the dehumidifier benefit. If it's not keeping up turn on the second.

The 24k unit is on the border with a 30amp dedicated outlet. Will it work...it depends. While it may be 18 amps, it needs 2 to 3 times that for a second in order to start. The circuit breaker will allow more than 30 amps for a very short time but as they get older, the breakers are more suseptable to tripping, so what works in one slip may not work in the next slip over. Some units offer a capacitor to reduce the startup amperage but it's still more than the rated amperage.
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Old 31-08-2013, 08:07   #10
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Good to hear about the Venturi devices. Our boat does have thick decks and not a lot of big windows. Her decks are real as well. We are deck stepped masts so the bilges never have any moisture in them. I will likely use the shower sump boxes for the condensate unless I hear more favorable comments about the Venturi. I like the idea of running the bigger unit up forward on its own 30 amp dedicated power cord and the 12kBtu aft unit on the existing 30 amp cord. Now I just need to work some more to get the $ together. Suggestions on manufacturers to go with?
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Old 31-08-2013, 08:45   #11
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Re: Sizing of Air Conditioning

We are sailing a 42' yacht in southwest Florida. Our AC is a 16,000 BTU unit made by KingAir that is mounted in the salon/main cabin. It is not ducted to the fore or aft cabins. Instead, small fans move the air around quite nicely and keep the boat very comfortable quite easily (sometimes too cold at night). The cooling water is picked up under the gally sink and passes through a Perko raw water filter that is easily reached/cleaned. Condensate is drained to our bilge sump. One thing we did that is a departure from the routine is to require two drains in the condensate pan, one each on the port and starboard sides to ensure that, no matter how the yacht heels, the condensate that remains in the pan even after the system is shut down can drain rather than overflowing onto the decks. We tried one of the venturi devices, a "Condensator", for a year or two but found it very problemsom, requiring constant maintenance and replacment check-valves (it got to the point that I would buy 10 at a time from US Plastics at a buck each). We finally did away with the thing entirely.

Our pump is a March MDX. These work well and are designed for dry operation. Maintnenance involves injecting a little 3-in-1 Oil in the oiling galleries every few months. The pump activates whenever the compressor comes on, wither for cooling or heating (in the winter). While there might be only a single water pick-up, I would suggest independant pumps for each unit if you elect to go with two, although I don't think that's really necessary (different ships different long-splices however).

It is wise to set your system up with access ports in the cooling water lines so that you can periodically backflow the heat exchanger to clear our debris and run a descaler solution through the system from time to time.

Good Luck...
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Old 31-08-2013, 08:58   #12
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Re: Sizing of Air Conditioning

Shading is going to make a huge difference. If not able shade the whole cabin make sure that all your hatches are completely shaded. Same for any windows if you have pilothouse or similar. Direct solar gains are most often you biggest heat gains and need you attention.
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Old 08-10-2013, 17:02   #13
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Anyone have experience with Flagship Marine air cnditioners? Looking at their site the units look very well built. Considering webasto FCF series and cruise air.
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