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Old 16-08-2017, 01:47   #16
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Re: Sizing the right air conditioner for your boat

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Originally Posted by CaptainJohn49 View Post
Great topic. I have a boat and an RV. The boat has no air but I'm interested. The RV I just removed the Coleman AC and replaced it with an AirCommand made in Australia and rated at 15,000 BTU's. I found both the Coleman 13,500 BTU unit and the AirCommand 15,000 BTU unit just reduced the temperature by a certain amount on the inside from what it was outside. The drop seems to be 12-15 degrees difference. So the point is, if it's 80 outside I can get it 70 inside no problem, if it's 90 outside it's 76 inside, if it's 100 outside then it's 84 inside. So what?

Well my point is that 15,000 BTU's is the max my generator can handle. I think you should be looking at the power you are willing to devote to the AC then figure if you are happy with a certain temperature drop from that setup. I know that I'm happy with 12-15 degree difference. Sure sometimes it's a little hotter than I like but it's still a big improvement. A larger AC unit means a larger generator, more fuel and larger wiring.

Maybe next year I'll look at AC for my boat.

Hope this helps
John
If I can get 80 when its 95 outside it would be very good, certainly more than good enough, I suspect a boat will not be as well insulated as an RV, but yeah, I will size it according to generator and see how it went.
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Old 16-08-2017, 02:37   #17
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Re: Sizing the right air conditioner for your boat

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Take the cubic footage of the area you want to cool and multiply it by a factor of 14 if below decks and use 16 for above decks. Then add 200 BTU for every square foot of non vertical windows like a catamaran. If you have shades or tinted windows you may not have to account for the windows.
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.

How to Size HVAC
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Old 16-08-2017, 04:43   #18
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Re: Sizing the right air conditioner for your boat

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Not really.

Variables include:
  • location
  • deck and topsides core construction
  • liner
  • window area
  • use of awnings and window covers to reduce solar gain
  • how quickly you want to cool
The down sides of over-size include:
  • additional electrical installation cost
  • power problems in some marinas
  • noise
  • poor humidification performance (cycles are too short)
Also...

Cat or Mono?

The big deck houses of cats can be a challenge to keep cool. Degree of challenge also depends on deckhouse window design. Similar challenge for pilot house/deck salon mono I expect.

Another consideration on cats is two hulls and widely separated cabin spaces. Getting air to all these spaces can be a big challenge. Depending on boat details, it may easier and cheaper to run coolant lines to radiators in each cabin rather than trying to route air around the constricted spaces of the hulls. Some opt to just put an AC unit in each cabin to avoid this problem.
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Old 16-08-2017, 04:53   #19
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Re: Sizing the right air conditioner for your boat

Just did this exercise and not pleased with results!

Caliber 40 Sailboat
Interior space to cool is 25' long and an average of 11' wide with a ceiling height of 6' 7" (275 sq feet). There is one bulkhead that separates the galley/main salon from the forward sleeping area/head.

Boat is in SW Florida where water temperature is currently 88 degrees F. The daily temperature varies from 77 degrees F at 6 AM to 93 F from noon till 4 PM.

Prior to installing the A/C the boat interior was always above 90 F from mid-morning till late in the evening.

Installed a Haier 14,000 BTU portable A/C with a dual hose intake/exhaust system. A pair of 12V 6" fans run 100% of the time in the nav station and in the passage way to the forward compartment. There is also a Nicro solar powered 5" exhaust fan in the forward shower. There is great airflow thru the boat.

The boat interior is 87 F all day long with the A/C running continuously (9.5 Amps/120V) but the relative humidity is down to 55% rather than 80% without the A/C.

I calculated that there was 43,000 BTU/Hour heat gain thru the ceiling due to the direct sunlight on the boat deck. The main salon ceiling was always warmer than 98 F between 11 AM and 5 PM.

I mounted our full boat Sunbrella awning to eliminate 90% of the sun ondeck. The A/C can now keep the boat interior at 81 degrees F at all times but has to run 100% of the time from 10 AM until 6 PM. The runtime drops to 60% between midnight and 7 AM.

The boat interior is pleasant at 81 degrees F, probably because of the significant decrease in relative humidity.

I did not expect to have to run the A/C 20 hours a day in order to have a 81 degree F boat interior. Nor did I expect to run two big fans 100% of the time.

But, I am a Seattle native where 70 degrees is HOT and 80 degrees is noteworthy.
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Old 16-08-2017, 05:52   #20
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Re: Sizing the right air conditioner for your boat

One item often overlooked when sizing ac/hat for a boat is the length and location of duct runs. If you have a 12k btu unit, you will not actually see all 12k in cooling, as there are losses, one of which is in the ducting. Simply put, you want to make the ducting runs as short, and as un-complicated as possible. If you very short (We have one with seven inches) you end up with significantly more noise, but high efficiency. You also run the risk of unbalanced air pressures, which can effect cooling ability negatively.

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Old 16-08-2017, 06:15   #21
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Re: Sizing the right air conditioner for your boat

The post on ductwork brings up another can of worms that goes well beyond sizing the unit. Static air pressure in ducting is a whole other science. Bad ducting can cause inefficiency and on the other end of the spectrum freeze ups.
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Old 16-08-2017, 06:36   #22
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Re: Sizing the right air conditioner for your boat

Additionally, ducting can consume a lot of valuable storage space.

For any given size, 6' of duct =~ size of AC unit.

For a <35' mono, 2 x 5K BTU/Hr units can fit the bill. For <40', 2x8K, for <45', 3 x 8K, for < 50', 3x 12K.

This also allows flexibility for temp vs power available. If it's warm and only 30A available, run 12K or less. If hot and lots of power available, run everything flat out.

If one installs a 16K unit, they pretty much need a dedicated 30A connection just for Air Con.

Multiple units also allows redundancy when a unit fails.
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Old 16-08-2017, 06:43   #23
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Re: Sizing the right air conditioner for your boat

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Originally Posted by ramblinrod View Post

If one installs a 16K unit, they pretty much need a dedicated 30A connection just for Air Con.
Amen that was mentioned.....around here the sheer number of boats with a 16k unit, water heater, charger, outlets, etc...on a single 30A cord is amazing. They'll load these cords right up to 30 amps and wonder why they end up with melted/burnt ends!!

a 30A cord with a common twistlock connector is good for roughly 20A continous.
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Old 16-08-2017, 07:13   #24
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Re: Sizing the right air conditioner for your boat

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Additionally, ducting can consume a lot of valuable storage space.

For any given size, 6' of duct =~ size of AC unit.

For a <35' mono, 2 x 5K BTU/Hr units can fit the bill. For <40', 2x8K, for <45', 3 x 8K, for < 50', 3x 12K.

This also allows flexibility for temp vs power available. If it's warm and only 30A available, run 12K or less. If hot and lots of power available, run everything flat out.

If one installs a 16K unit, they pretty much need a dedicated 30A connection just for Air Con.

Multiple units also allows redundancy when a unit fails.
Good suggestion, or I can run a 6k for cabin and 10k for salon.
Easier to control usage too.
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Old 16-08-2017, 08:02   #25
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Re: Sizing the right air conditioner for your boat

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The figure are still plenty high and use a lot more energy than I like.
I think I would have it configure in a way that is cold for the stateroom and merely enough for the saloon.

Remember that the load is MUCH lower at night. You also don't use the staterooms during the day. I think you will find that if you cool the salon during the day, cooling the staterooms once the sun goes down will be easy.
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Old 17-08-2017, 07:50   #26
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Re: Sizing the right air conditioner for your boat

Forgot to mention earlier that I am sitting in a 36' 5th wheel RV about an hour north of Tampa. Been living here for 2 weeks now, since selling the house, while the contract on our new to us boat goes through all the steps.

Not a direct comparison as the RV has good insulation, not great but good, all the way around and double paned windows. It is 10:45AM and already 91, projected to go to 94 degrees today. Been pretty much the same all week.

The RV has the typical roof top A/C units but they are ducted. The front unit is the smaller and is 13K and ducted 1/2 way toward the back of the unit. The rear unit is larger at 16K and ducted running 1/2 way toward the front of the unit. Suffice to say the A/C units are sized by the factory and fairly well installed (duct work, not on every RV!).

I have them both set for approximately 84 degrees, best I can tell with the older, mercury bulb style slider thermostats. From about 10AM to 5:30 or 6PM They never shut off!
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Old 17-08-2017, 10:25   #27
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Re: Sizing the right air conditioner for your boat

I can only hope I can have as much power and good insulation in my boat.
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Old 17-08-2017, 17:55   #28
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Re: Sizing the right air conditioner for your boat

I installed two AC's in my 38' IP. A 16K in the Salon and a 6K in the fwd Stateroom, I had no shade tent.
It takes all of both AC's running now in S Fl to keep the boat cool.
Both together pull 21 AC amps. You cannot run both AC's and the 1500 W water heater, but if it's Africa hot, we don't need hot water.
You do have to power manage things with a 30 amp boat.
Boat came with one 16K AC, and that just didn't cool it.

I say oversize a little, you don't have to run them on high, ideal is to have them work with the fans on low speed as they make a lot less noise that way.
Running them on high doesn't seem to really add much cooling, it only speeds up the fan not the compressor
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