Marine Airconditioning  Heating, Ventilation  and Refrigeration design and repair by Greg Moore 619-218-1018 
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Marine airconditioning repair San Diego. Refrigeration repair and design for sail boats and power boats. Offshore systems for fishing, cruising and commercial comfort. 

Cruise air and marine air are leaders in equipment found on most boats. Sea frost is most common on sail boats and cold plates are found on both power and sail boats. 

Refrigerant gas is controlled by pressure compressors compressing gases then electrical controls allowing that gas to expand and compress to transfer eat to usually sea water or air. 

Ducting is a major consideration in the installation of airconditioning. The main factor is to not undersize the unit and ensure suction air is far from exhaust air. 

Piping and plumbing. The most crucial part of any installation is the piping both for gases and for cooling water. Gas lines easily develop leaks from vibration and compression type fittings. The best systems are hard piped and all seams welded with an acetalyne torch and solder. Its vital these lines be supported no less than every 10 inches. 

Electrical must take into consideration te very high loads that occur when compressors start. care and expense must be taken to ensure each item starts in a staggard trigger delay fashion. 

The biggest reason for failure in HVAC systems is electrical, then gas leaks and finally pressure and sea water clogging. 

Boats with big windows that sun hits will need most the air concentrated here. Small fans throughout boat will reduce aircondtioning needs. 

Bad installations include no drain for condensation, poor piping, too small of ducting or duct kinks. Email HVAC Engineer Greg Moore

Marine Air Conditioning Principals 

If you have adequate power, marine air conditioning has great advantages. Air conditioning not only cools the boat, but dehumidifies and if you have reverse cycle you can heat in cooler times. Dry boats not only are more comfortable, the onboard systems like it too, and you will be rewarded with less corrosion.

If you do not have adequate power for air conditioning you should see the article about boat shades and ventilation. proper ventilation and shade can keep a boat cool, even in the worst conditions. Even if you have air conditioning shade can help the air conditioner work more efficiently.

Air conditioners range from the very simple to the very complicated. The simplest are plug and play portable units that just need power. More complicated fully installed units require power, a dedicated space, ducting, sea water through hulls and discharges. However, the advent of the self contained AC system has made DIY installation easier.

There are two types of Marine air conditioners: Refrigerant types and Evaporative types. Refrigerant Air Conditioning produces dry cool air while Evaporative Coolers have cool air that feels like an ocean breeze.


What is Marine AC

Marine Air conditioning is different from most land Air conditioning because it uses water cooling rather than air cooling. The abundance of water and lack of space makes this preferable.

There are two types of Marine air conditioners: Refrigerant types and Evaporative types. Refrigeration AC produces dry cool air while Evaporative Coolers have cool air that feels like an ocean breeze.

Refrigerant air conditioning uses the principle of evaporating and condensing a refrigerant, just like a refrigeration system. Marine air conditioning uses a closed loop with a high and low pressure side. On the high pressure side, refrigerant gas temperature rises rapidly, due to the compressor, it is then cooled and turned into a liquid by the condenser. The warm liquid passes into the low pressure side and evaporates back into a gas via the evaporator. The evaporator needs energy in the form of heat to do this. The heat in question is the heat in the room you wish to cool, hence the room gets cooler. The compressor and all the parts in this closed loop system uses quite a bit of energy so a good source of power is needed.

Evaporative air conditioners use anther method, evaporative cooling, from water or ice. Cool air from the water or ice is blown via a fan, to cool down a small area.

Types of Refrigerant Air conditioning

Refrigerant Air Conditioners are powered by AC or DC current

 110Volt powered (generator, or shore power) 
Split Systems

Split systems are separate condenser units and fans connected by long runs of tube. The long runs of tube the biggest disadvantage of this system and needing a professional to charge the system with refrigerant. Advantages; quieter as compressor is not in living space, and less room is required

Self containedSelf contained units contain the condenser, evaporator and fan in one unit and can be installed in the space you are cooling. The typical self contained unit needs only a 5/8" non-insulated raw water supply for cooling the condenser. Advantages: easy for DIY installation. These self contained units are the most popular for recreational boaters. The complete units are only a few inches larger that the evaporators of the split systems and the fan coils of the chiller systems. 
Chiller systems

Commonly found on larger yachts and ships, the chiller unit which is typically placed in the engine room, and fan coils placed throughout the boat where needed. The chiller makes cold water that is constantly pumped in a closed loop throughout the numerous fan coils of the vessel.

Carry on portable unitA portable unit placed over a overhead hatch and powered by shore power. these are like the window units seen in houses. Advantages; no need to take up space below and plug into the shore. 

 12volt marine air conditioning
12v refrigerant systemsThese systems have all the basic parts of the 110Volt systems.  The self contained 12V unit and then require; thru hulls and discharges, power, installation and ducting. They come into their own on small boats without the space for a generator. 
12v Evaporative coolersEvaporative coolers are completely self contained and do not need any sea water for cooling. these are the simplest systems. 

Marine & Boat Air Conditioners

There are two distinct types of Refrigerant Air Conditioning System for marine use. First is the Direct Expansion Systems and second is the Tempered Water Systems or Chiller systems. Direct Expansion systems are available as Self-Contained or Split Systems and are more common on mid range boats. While the Chillers are designed for larger yachts

Self Contained refrigerant 110V AC units. These self contained systems are the most popular for recreational vessels and can run on AC power, shore power or generator. These units range from 5,000 BTU up to 30,000 BTU

Mermaid M5 5200 BTU air conditioner


Mermaid M5 5200

For small boats the M5 5,200 BTU air conditioner from Mermaid packs all the features of a large unit into a small tapered footprint. The taper lets you install the unit under a V berth and with the rotatable blower you can easily attach ducting to circulate air around the cabin.

At 15 inches by 11.5 by 11.5 the M5 is one of the smallest on the market.

This unit will run off the  Honda 1001i portable  1,000 watt generator, while in cool mode. Another alternative if you have the batteries is to use an inverter. According to Mermaid a 1,800 watt inverter can handle the unit with 450 amp hours of battery. That is a lot of weight, but you will be cool and it will be quiet.

The Cruisair MTD series is designed for installation flexibility, reliability, maximum performance, and easy accessibility for maintenance. Chiller units are generally housed in the machinery space and pipes run to each of the air handlers.

Cold water is made in the chiller and run through the pipes and through the air handlers. This is a more complicated system to install, but for larger vessels makes sense.


Carry On 7000 air conditioner
Portable Air Conditioner

Living on a small boat for even a weekend in the summer is going to be warm. For small boats a carry on air conditioner that fits over your cabins deck hatch can make life much easier. The unit runs off normal dock marina shore power. The hatch needs to be larger than 12 by 12 inches for the unit to fit.
12 Volt DC Air Conditioners

The advantage of 12v DC is that you can use your battery system and not need shore power or a generator, great if you are lying at anchor in a small boat. Many small boats do not possess AC power, plus they have small spaces, perfect for DC air con. Still the problem is draining batteries so monitoring  is a must. It would be best to have a battery bank just for the air conditioning separate from starter batteries. 110Volt refrigerant air con requires large start up loads, DC systems do not have this high start up load.

12 V air conditioning is limited to small units, due to the power required.

For a small cuddy cabin of around 200cu ft a 3,000 BTU air conditioner will suffice (200 * 15). Power to this device for 8 hours you will need 3 group 27 batteries (or similar combination) totaling 270-315 amp hours. If you run the engine and have a large alternator you can boost power to the batteries. If you are running the AC and your batteries are getting down to 60% discharge, you can run the engine.

A 12Volt system like the ones below can cost in the $2,300 - $2,900 range, including the unit and pump. You need to ad water inlet and discharge, plus DC power to the inverter.


Dometic DC cuddyDC Cuddy


120v with dc module

This small A/C 12V powered unit can fit under a v berth. Ideal for small cabin cruiser or sailboat for the occasional stay overnight. This small unit can make small boats comfortable where a traditional AC unit would be too big. 12V power and produces 3,500BTU. The unit is powered by 3- 100 amp AGM batteries and run time is approx. 8 hours between battery charges.

The 3500 BTUs, 29-pound unit can run for 8 hours off 3 Group 27 batteries, or run on 115- or 230-volt 50 and 60 HZ AC from a genset or off shore power via a battery charger.

If you have a small powerboat say 20-25 ft with a small interior a DC air conditioner works. The engine is operating when you are away from shore and so the alternator (check the alternator output) is beefing up the batteries, so for a night at anchor the DC air con can do its work. Check your battery bank for the required capacity. 

12 Volt Evaporative Coolers

An evaporative cooler (wet air cooler) is a device that cools air through the simple evaporation of water. Evaporative coolers are simple much cheaper and do not need thru hulls or any complicated installation.

MightyKool M400 Evaporative Coolers

mightykool 12v air conditioner

MightyKool, is a 12v evaporative air conditioner made by Swampy Cooling Systems who have been making 12 volt portable cooling systems since 1989.

MightKool M400 is a Self-Contained, Cooling System integrated into the lid of an Igloo ice chest. You can add enough ice to the ice container of the M400 to have air conditioning up to five hours. If you are using water the M400 will operate much longer. The two vents can be moved to direct cool air in different directions and there is hosing you can also add to give it more reach.

MightyKool will not cool an entire room, but is designed to be a personal cooler, making the immediate surrounding area around comfortable. MightyKool is an evaporative air conditioner, so it is not so effective in very humid areas. .

The M400 costs $949 inc shipping, less if ordered online. Plus you can keep your drinks cool.


Boat Misters

Boat MistersWhen its hot outside any way of cooling down is worth looking at. Misting is based on the theory of Evaporative Cooling which is the scientific principal of Thermal Dynamics. Water is forced through the specially designed misting nozzles, and a fog of ultra fine water droplets with an average size of 50 microns or less. 

The new Comfort Zone Boat Misting System attaches to the boats Bimini or T-Top and emits a cooling mist that reduces on-deck temperatures by up to 30 degrees. The unique mid-pressure pump delivers 150 psi of misting power at the rate of about one gallon of fresh water per hour while only drawing one amp. The system can be used on existing Bimini and T-Tops or designed-in by the boat manufacturer as an integrated system.

KoolerAire evapoative cooler

The koolerAire

Fill up your cooler with ice and turn it into an air conditioner for your boat and boating activities. KoolerAire can be used below deck, or on deck. The koolerAire fits a 25 qt Igloo or a 28 qt Coleman. The power comes from a car type cigarette adapter. While the ice is cooling your drinks it can keep you cool at the same time. $40 retail 16 1/4 X 9 X 1 1/4 . The fan sucks the cool air from the cooler and spreads it around.

Turn your cooler into a 12 volt air conditioner!

With KoolerAires space saving design air conditioning has become more portable than ever before!

Its attractive compact profile, whisper quiet fan, and low power consumption makes KoolerAire the economical alternative to 12 volt, portable air conditioners costing hundreds of dollars!



Installation for 110volt water cooled Air Conditioner: Most Marine air conditioning systems are water cooled rather than air cooled.

Dometic air conditioner installation
1.Calculate cabin volume and size system, see size below

2. locate a suitable space for the AC, e.g. under berth, in a hanging locker 

3.Installing the thru hull, seacock and strainer 

4.Installing the water pump unit, and its accessories,

5.Install AC unit and connect to power supply, 

6.Install ducts and grills

7. Install discharge

The diagram right (pinched from Dometic) shows a typical installation. The thru hull is placed at a lowest point in the boat. It is important to get the lowest point, to reduce air getting sucked in with the water. If the deepest point in the boat is not close to the unit, its OK to run the hose a long distance. It is also recommended a scoop for sea water intake facing forward. This will help direct sea water into the pump, and prevent air coming in.

With the thru hull you should have a seacock and strainer. Not only can air get in the system so can seaweed and debris. Anything that clogs the water flow will cut off the AC unit. 

The Water pump pump and filter must be placed below water level, to provide positive pressure for pump priming,

The discharged water fitting is normally above the waterline, which helps you know if water is moving through the system. If water is not coming out it means there is a clog. Check the filter and look for air traps. When running the hose make sure there are no kinks and use elbows if necessary.


Here we are looking at the size of the AC unit you need in BTU (British Thermal Units). 

The general rule of thumb for sizing a pleasure boat is using the formula of 14-19 BTU's/cubic foot of air conditioned space. Cubic dimensions (LxWxH)  Use the higher number for areas that are used during the heat of the day that have a lot of sunlight coming in. For areas below deck that are primarily used after sunset such as a stateroom, use lower number. Catamarans and boats with large open areas surrounded by glass: a higher number may be needed.

Under sizingUnder sizing the system will leave you hot and frustrated
Over sizingOver sizing the system significantly, will result in system "short cycling"  not adequate run time.


One thing you must imagine is the heat coming into a boat via overhead hatches. Sun beating through glass hatches dramatically heats up an interior. Therefore to keep the boat cool ad awnings and hatch shades. Even with Air conditioning it is worth keeping the deck in shade. This way the air conditioner will not be working so hard and you will eliminate hot spots. 

See this article about deck shade and boat ventilation and this article on shade, showing many options for shading your deck.

Maintenance & Troubleshooting

 There are several visual checks you can make. 

Check the water flow coming out of the discharge hull fitting. This is like checking water is coming out of your engineac book ac guy exhaust. If water flow has stopped or is weak you probably have barnacles in the thru hull, or weed or debris in the filter.

It is also essential to check the tubes for leaks. A small hole in a hose or leak in a connection will reduce water flow.

Have a look at the air con unit and feel the coils. the coils on to should be hot and the coils on the bottom should be cold. This indicates the system is operating correctly.

Take a look at the condensate pan. Check that there is no debris blocking the drain outlet.

If you would like to diagnose your own AC systems then this book fromBoatACguy will be a help.





When air conditioning works properly spending time onboard is very comfortable even in searing heat. To keep the system running properly do the necessary maintenance and keep the deck shaded.