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Old 02-25-2021, 09:19 PM   #1
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Marine Air Conditioning Configuration

Hi folks,

I have a 42' Bristol. Forward v-birth, main solon, and an aft cabin with queen bed. My current cooling equipment consists of a 16k dometic unit that serves the main salon (single evaporator "air handler") and a 12k dometic unit that serves the forward and aft berths (an evaporator in each stateroom).

These units are >20 years old and the 12k unit appears to be tossing in the towel.

I'm researching vessel cooling solutions and attempting to approach this problem from a first principles point of view, rather than simply shopping for drop-in replacements for my existing units. So here's where I'm at. My required cooling capacity hasn't changed. 28k BTUs sufficiently cooled my boat in the middle of the summer here in MD. One of my two 30A shore power connections is dedicated to running only the two compressor units (and nothing else). I need to research the draw on the compressors but I'd imagine they draw near 30A at 230V in total, so perhaps 5-6,000W in total. What I'm getting at is that my design criteria is: >=28k BTU cooling required, 6,900W max power output.

Is there a reason why I would NOT just buy a 28-32k BTU compressor unit and run three separate evaporators to each room? Why was my boat set-up with a 16 and 12k unit in the first place?
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Old 02-25-2021, 09:24 PM   #2
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Thinking out loud here.. maybe there are two units instead of one so that large starting current of each doesn't trip the breakers? I suppose a 30k BTU unit startup current might be too much for a single 30A 230V breaker...?
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Old 02-25-2021, 10:58 PM   #3
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Thinking out loud here.. maybe there are two units instead of one so that large starting current of each doesn't trip the breakers? I suppose a 30k BTU unit startup current might be too much for a single 30A 230V breaker...?
That is most likely true. You don’t see much more than 18K BTU in 120 volts, at least last time I looked and it has been a while. We have a 41’ and have 2 16K units. They each do a cabin and share the salon.
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Old 02-26-2021, 02:19 AM   #4
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You confused me. You have two 120v AC machines that together should pull 25 amps. This is easy to run off of 1 30a shore power leaving the second 30a shore power to run the house. Now if you go to a single 230v system you will be required to find two out of phase 30a shore feeds to get the AC to work. On top of that you will now need half of each leg dedicated to AC which means you will be splitting up the house needs. This means you will always need two 30a shore powers to operate the house. What is the advantage?
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Old 02-26-2021, 07:33 AM   #5
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I need to research the draw on the compressors but I'd imagine they draw near 30A at 230V in total, so perhaps 5-6,000W in total.
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I suppose a 30k BTU unit startup current might be too much for a single 30A 230V breaker...?

How does 230V enter into your picture?

Are your 30A breakers not 125V?

Possible reasons to NOT change:
- ease of new drop-in
- new drop-ins likely more efficient than your older units
- if one craps out, you still have some marginal heating/cooling

Not suggesting replacing with new self-contained units is your best answer... but our experience with direct replacements was positive, for what that's worth...

-Chris
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Old 02-26-2021, 08:19 AM   #6
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If you can find space for the outside unit , perhaps a modern "mini split" would have advantages over the ancient sea water units?

These can provide off season heat very efficiently , have no seawater connections and are very quiet and easy to install, as they can be pre charged.

At this point there are dozens of brands made in countries you may have never heard of.

Purchase from a company that has been in business a decade or more.
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Old 02-26-2021, 08:39 AM   #7
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One reason to have two units instead of one might be machinery space. There might not be enough room for a large unit. Even if it fits service accessibility can be an issue.

Also, on the hook, its easier to power one smaller unit with the generator. Consumes less fuel too.
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Old 02-26-2021, 08:43 AM   #8
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I would avoid one big unit for the whole boat. For a few reasons:

1. If unit poops, you have no AC.

2. That big unit will be 240V, and require reconfiguring your boat from 2-30a/120v cords to at least one 50a/240v cord, and changes to the panel.

3. Starting a big unit may stomp on your generator.

4. Refrigerant flow continues to all three evaporators whether there is cooling demand there or not (not real sure about that, depends on if there are shutoff valves on each).
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Old 02-26-2021, 08:50 AM   #9
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Dashash Until you get your electrical information pinned down the advice you are getting here so far is well intended but is all over the place. Its kind of like "I saw the man with binoculars" Did you see him through binoculars or did you see him and he had binoculars.
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Old 02-26-2021, 10:15 AM   #10
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I mistakenly identified my current system as 230V. Apologies. I have a 120V 30amp service for both AC units.

Thanks for the advice. For ease of replacement, it probably is best to just replace my current split configuration with new. Would love more cooling capacity, however. I need to look up combined current draw for two 16k units and see if my 30amp breaker can support.

The all-in-one units are appealing... Need to do more research.
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Old 02-26-2021, 10:25 AM   #11
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Compare running amp draws. The units you currently have are almost guaranteed to be piston compressors. Rotary compressors have been the standard for the last 10+ years. There is a substantial power consumption reduction going from piston style to rotary style compressors. I replaced my 3 units in 2015 and saw a substantial reduction in amperage draw. My numbers won't be quite the same as yours as I also changed sizes.

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Old 02-26-2021, 10:40 AM   #12
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I mistakenly identified my current system as 230V. Apologies. I have a 120V 30amp service for both AC units.

Thanks for the advice. For ease of replacement, it probably is best to just replace my current split configuration with new. Would love more cooling capacity, however. I need to look up combined current draw for two 16k units and see if my 30amp breaker can support.

The all-in-one units are appealing... Need to do more research.

Maybe just a matter of setting them so they don't both try to start at the same time.

We replaced our original marine 16K BTU Airrrr Vector Compact units with Dometic Turbos. (At one point, the new ones were named Vector Turbos, but things changes as/after Dometic acquired Marine Airrrr.)

Anyway, our results were pretty good: quieter compressor, more air flow (with a bit more wind noise, controllable by varying fan speeds), composite drain pan, I think lower power draw maybe...

You might find a 16K Turbo fits into approx the same space (close, anyway) as your older 16K units...

-Chris
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Old 02-26-2021, 11:27 AM   #13
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Fantastic information all around. Much appreciated.
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Old 02-27-2021, 08:01 AM   #14
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"I need to look up combined current draw for two 16k units and see if my 30amp breaker can support.'


Your local RV dealer will have a device that only allows one AC to start at a time .
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Old 02-27-2021, 08:55 AM   #15
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In normal boat life, it is quite rare for all AC's to start at the same time, takes a while to mover around the boat to turn them on, thermos cycle independently, etc. Even if you power up boat from black with all on (should not do that!!), most have a built in delay that is not exactly the same.
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Old 02-27-2021, 10:21 AM   #16
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If you are considering a major change like combining compressors, I would consider changing from water condensed to air condensed and eliminate the through hulls, clogs, fouling, and all the headaches that go with them.

I would also look at DC compressor. Split cycle systems have come a long way recently.
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Old 02-27-2021, 12:10 PM   #17
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I recently replaced a 12K BTU Marine Air compressor with a Dometic unit (Dometic bought Maine Air). The old unit only pulled 11 amps on start up. The new Dometic draws 21 amps on start up! Generator said NO WAY!! when I fired it up.

It will require you to also get a "Smart Start" unit from Dometic. They are around an additional $500.
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Old 02-27-2021, 01:23 PM   #18
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Seems backwards, for amp draw.

We must have already had a "smart start" thing in our original Marine Airrrr installation; didn't have to do that again when we replaced individual units with the Dometic Turbos.

-Chris
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Old 02-27-2021, 02:01 PM   #19
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If you are considering a major change like combining compressors, I would consider changing from water condensed to air condensed and eliminate the through hulls, clogs, fouling, and all the headaches that go with them.

I would also look at DC compressor. Split cycle systems have come a long way recently.
Yes, seems like that is going in the wrong technological direction. Same BTUs?
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Old 02-27-2021, 04:27 PM   #20
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I just replaced a 16k unit in my main cabin. I elected an all-in-one unit made by CTM Marine. It fit in the same space as the original evaporator/air handler and is much quieter even though it is both a compressor and turbo blower. It produces gobs of heat when the water temp is 40 degrees or higher. It's a safe assumption that it will cool well also. Highly satisfied. Plus, I recaptured space in the engine room with the removal of the compressor. I also removed the 38yo, won't-hold-freon-charge v-berth compressor and captured that space as well. We can do without v-berth heating and cooling quite well.
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