Marine Air Conditioning installation

Marine Air Conditioning Installation follows on from our previous Article “Marine Air Conditioning” where we looked at the types of AC available we now turn to Installation. 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 110Volt & 12Volt; produces dry cool air
• Evaporative types 12Volt; produce cool air like an ocean breeze.

Diag Courtesy Dometic
Diag Courtesy Dometic

Refrigerant Air Conditioners uses sea water and below we will go through a typical marine Installation. Evaporative AC does not need a water inlet.

Marine Air Conditioning Installation

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

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 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 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.

Sizing an Air Conditioning System

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 sizing the system will leave you hot and frustrated

Over 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 engine 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 from BoatACguy 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.