How Does Marine AC Work?

How Does A Boat Air Conditioner Work?

We are often asked how does a boat air conditioner work or how does the air conditioner work on a boat and how do those differ from the AC system in my house. Unlike most home air conditioning systems, marine air conditioning systems have access to an unlimited supply of water. [note: here is a good overview of how a home AC system works.] This is why ac installation is a bit more complicated on boats than a home ac system and also why boatac units are usually more efficient.

Water holds heat very well (it has a high specific heat capacitance). In contrast, air does not hold heat particularly well (higher humidity levels in the air increase how much heat the air can hold). This is why water cooling is so helpful in marine air conditioning - the chilled water helps dissipate the heat from the compression cycle.

How Do Boat Air Conditioners Work?
Marinaire 16000 BTU marine air conditioner

In a marine A/C unit, there is refrigerant with a very high specific heat capacitance in a closed loop inside the unit and water flowing steadily from outside the boat with an intake through the hull under the waterline through an above waterline overboard discharge.

The same mechanism that is used in other air conditioning environments is still used in a marine environment. Refrigerant is compressed (into a liquid form) in the condensing unit and then the pressure is released (in a gaseous form).

The compression itself generates a lot of heat. If you've ever used a hand pump to put air in a tire or a ball, you may have noticed that the pump gets very hot. Most of the heat doesn't come from the friction of pumping. Instead, it comes from the compression of the air itself.

Conversely, if you've ever used compressed air for cleaning or in power tools, you may notice the cool air coming out is usually much cooler than surrounding air. Air conditioners take the heat from compressing refrigerant (sometimes referred to by the brand name Freon) and move it out of the environment you want to cool.

And, they then use the de-compression process to provide cooling inside the environment. [Note: With coolant, the AC compressor not only compress the gaseous refrigerant, but it actually compresses it so much that it becomes a liquid (a phase change.)]

Once the coolant is compressed back into a (hot) high pressure liquid, it goes to the condenser coils (which would be the outdoor component in a home AC system). The compression takes a lot of energy, which is why most systems use ~30 amp shore power or require alternator upgrades. 

marine circulation pump. Salt Pump on Boat.
Circulation pump for marine ac system

In a marine ac unit, this hot liquid is cooled through a heat exchange with the sea water that's being sucked from under the water line (through a strainer that removes debris) back into the AC unit and then pushed out again through the above water line ports through special hull fittings.

In other words, the marine AC unit brings in cool water from the sea and heats it with the energy from the compression process before returning it to the sea. The cooler the water, the more efficient the cooling process. So, if you are in a warmer fresh water lake, your efficiency will be less than in deep, cool ocean water.

This now cooled-off, pressurized liquid refrigerant goes through the evaporator coils where the pressure is released. This causes the liquid to become a gas and further cools the gas. Both the phase change (liquid to gas) and pressure reduction of the gas cool the evaporator and its heat exchanger (radiator).

MarineAirr marine ac repair
Clean evaporator coils on a marine a c unit

The fan (air handler) in your air conditioner unit then sucks cabin air from inside your boat through the return air grills and over the coils in the condenser coils (which looks a lot like a car's radiator) and into the ducts and grills in your living spaces. [See this video about the impact of damaged or dirty evaporator coils.]

The warm, moist air going through these now-cold coils produces 2 effects. First, the air is cooled by going through these coils. And second, moisture is pulled out of the air as the air gets colder than the dew point - the temperature at which it starts shedding its moisture.

The moisture turns to water and is then released into the condensate pan with a condensate drain where it usually will then be pumped out of your boat (some people use the bilge pump, we always install separate quieter, more efficient dedicated pumps) with an air conditioning pump.

Depending on the humidity setting on the marine AC unit, some of this moisture may be directed back into air as humidity. The volume of air flow and how many vents you'll need depend on the size of the space you want to climate control where the unit installations are - (i.e. are they in the engine room or in the state room?).

How does a split system marine air conditioner work?

Diagnose HPF code on cruiseair marine air system
Cruisair Marine Air Conditioner

In a split system, the condenser and raw water circulation system are separated from the air handler (fan). The cool, high pressure liquid refrigerant flows from the condenser into the evaporator coils and the evaporated gas flows back the other way back to the condenser unit.

Most modern boat ac systems are self-contained, not split systems. These also take up much less space, which is particularly helpful to free up valuable storage space and to make it possible for smaller boats to have marine air conditioning systems.

How Do Reverse Cycle Air Conditioners Work on a Boat?

With a heat pump reverse cycle, you are able to heat the air coming through the unit instead of cooling it. As described above, the refrigerant compression process generates hot compressed refrigerant (heat source) and cool gas that comes from the de-compression of the liquid (cold to be removed).

How to buy AC for my boat
Teak Air Return for Marinaire AC on a Sailboat

The reverse cycle runs the air over the condenser coils (hot) and runs sea water through the evaporator (cold) to push the cold out. In other words, instead of pumping the heat from your boat into the seawater, the reverse cycle pumps the heat from the seawater into your boat (and cold from your boat back out).

This is how a heat pump air conditioner in your home would work with one exception - where a home heat pump takes heat from outside air, a marine heat pump takes heat from seawater.

Why does having a properly charged refrigerant system matter?

Because the refrigerant (aka Freon) is needed for each part of the process and goes through such extreme pressure, phase and temperature changes, the system loses efficiency very quickly if there isn't enough refrigerant and/or if the refrigerant is replaced with air (which makes a terrible refrigerant).

Large vessel air conditioning system
New Marinaire installation

Because it is a closed system, low pressure almost always means that there is a leak. Unfortunately, leaks are very hard to identify in order to fix. And, refrigerant is very expensive. As a result, it often is more cost effective to replace a system (or at least major components) than to try to find and fix a leak.