Marine Air Conditioning Configuration

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Dashash

Dashash

Veteran Member
Joined
Nov 13, 2020
Messages
70
Vessel Make
Bristol 42
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?
 
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...?
 
Dashash said:
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...?
Click to expand...

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.
 
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?
 
Dashash said:
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.
Click to expand...

Dashash said:
I suppose a 30k BTU unit startup current might be too much for a single 30A 230V breaker...?
Click to expand...


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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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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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.
 
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).
 
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.
 
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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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.

Ted
 
Dashash said:
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.
Click to expand...


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
 
Fantastic information all around. Much appreciated.
 
"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 .
 
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.
 
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.
 
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.
 
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
 
HeadedToTexas said:
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.
Click to expand...

Yes, seems like that is going in the wrong technological direction. Same BTUs?
 
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.
 
Although you are not considering it I thought i would add something that is high on my "Most Hated" list.

I read an extensive article, complete with professional woodworking, wiring and ducting about a boater who concealed a home window air conditioner under his flybridge deck. It was a good job, not really cheap but it still looked exactly like what it was. A cheap window air unit adapted to marine use, it looked lousy, home made and cheap.

NEVER use ANYTHING you buy at Home Depot on your boat!!

Rant done..

pete
 
You are probably going to end up with two 120VAC 16kBTU drop-in units because that's what WORKS in your system. If you want peace of mind about your starting amps, order your Dometic DTUs with thier soft/smart starts already installed. With a Micro-Air soft starter installed in both my 10KBtu Dometic DTU and my Dometic 13KBtu rooftop AC, I can run them both with my small 3.5KW NextGen generator because the starting amps are down about 70% from before the the Micro-Airs microprocessors were installed.
 
12 BTU Dometic Died .in the Aft Cabin . .

Just went through a similar situation . . . March 2020 the salon/V-Berth HVAC 14,000 BTU Dometic saw it’s last trip and we went with a 16,000 BTU Dometic Turbo . . . And in January 2021 the aft cabin/Salon unit 12,000 BTU Dometic unit lost the compressor . . . Researched marine air, Mabu Marine, Dometic Turbo and Flagship Marine . . . Went with Flagship, 16,000 BTU unit with heating element for heat (no raw water pump running in that mode). Steve (the sales guy) was extremely helpful, all units built to order, heavy duty components, no proprietary circuit boards (parts available if needed) . . . Had to increase return air vent volume, very pleased with unit(s).

We have an 8kw Phasor Ginnie and she handles them well.

Nice to have options in the world of boating!

MV.CLASSEA
1988 DeFever 41
 
With the issue that I have with my 16,000 BTU CruisAir I found out today that a drop in unit may not be as easy as it seems. My unit has a split system meaning the air handler is separate from the compressor apparatus. They are on different sides of the vessel and at different levels. I was told today that the drop in units are compressor, condenser, and air handler in one “package.” For me to install a new drop in, I would be by-passing the existing air handler and will have to run new ductwork from the new air handler to the outlets for the air. Because that would require running ducts up a level and to the other side of the boat, it may be labor prohibitive.
 
Steve Bedford said:
With the issue that I have with my 16,000 BTU CruisAir I found out today that a drop in unit may not be as easy as it seems. My unit has a split system meaning the air handler is separate from the compressor apparatus. They are on different sides of the vessel and at different levels. I was told today that the drop in units are compressor, condenser, and air handler in one “package.” For me to install a new drop in, I would be by-passing the existing air handler and will have to run new ductwork from the new air handler to the outlets for the air. Because that would require running ducts up a level and to the other side of the boat, it may be labor prohibitive.
Click to expand...

Pretty much the identical situation I am in. Both my compressors are in the engine room amidships. The 12k compressor services evaporators/blowers located in the aft and forward staterooms. The 16k compressor services a single evaporator/blower situated in the salon. An all-in-one unit makes sense to replace my salon 16k split unit, but not sure it makes sense to replace the 12k unit. I could, say, install a smaller self-contained unit in both forward and aft berths, but I really don't want to install a second (or third) raw water pump. And I'm not sure the output of the current one would be enough to push raw water the length of the vessel.
 
JimW said:
Just went through a similar situation . . . March 2020 the salon/V-Berth HVAC 14,000 BTU Dometic saw it’s last trip and we went with a 16,000 BTU Dometic Turbo . . . And in January 2021 the aft cabin/Salon unit 12,000 BTU Dometic unit lost the compressor . . . Researched marine air, Mabu Marine, Dometic Turbo and Flagship Marine . . . Went with Flagship, 16,000 BTU unit with heating element for heat (no raw water pump running in that mode). Steve (the sales guy) was extremely helpful, all units built to order, heavy duty components, no proprietary circuit boards (parts available if needed) . . . Had to increase return air vent volume, very pleased with unit(s).

We have an 8kw Phasor Ginnie and she handles them well.

Nice to have options in the world of boating!

MV.CLASSEA
1988 DeFever 41
Click to expand...

Thank you very much for this recommendation. I will do some research into Flagship.
 
Dashash said:
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?
Click to expand...


I had your "dual" hook-up on my 40' trawler for 24 years. started balking and i "converted" to a larger, single compact unit with an adjustable diverter for aft cabin sharing and let the fwd, lower cabin rely on convection. regretted it from the gitgo. sold the boat soon after but if i were you, i'd replace components and maintain the system as is. good luck
 
The Department of Redundancy Department suggests maintaining separate systems. Just added 3rd stand alone 12k unit for forward stateroom.

It's nice to be able to shift load if one system goes down. If you have an issue with the central condenser on a multi evaporator system you loose everything at once.
 
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