How Does Marine Air Conditioning Work?

How Does Marine Air Conditioning Work?

We are often asked how does ac work on a boat and how those differ from the AC systems in houses. We are going to focus specifically on different types of marine water cooled air conditioning systems.

Unlike most home air conditioning systems, marine ac units have access to an unlimited supply of cool 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 marine AC units are usually more energy efficient than air-cooled heat pumps and ac units.

[note: this is usually NOT the case with portable, home-style air conditioners, which use air for cooling.]

If you’re wondering how does boat ac work, we detail the mechanics of marine air ac below. Let’s start with some basics.  

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

On some deep-keeled sailboats, instead of running seawater through the ac unit, the unit runs liquid coolant on a loop through the keel where heat (or cool) is then transferred to the water around it. This is how sailboat ac differs in some cases.

As a side note, higher humidity levels in the air increase how much heat the air can hold and how quickly that heat is transferred to what it touches - which is why humidity makes hot air feel hotter.

In an ac marine unit, there is refrigerant with a very high specific heat capacitance and relatively low boiling point in a closed loop inside the ac unit and water flowing steadily from outside the boat. The water comes from a water intake through the hull under the waterline and leaves through an above-waterline discharge. The salt pump (aka circulation pump, raw pump or seawater pump) provides the energy to move this seawater through the system.

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) in the evaporator coils.

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 the piston in the pump with the wall of the cylinder. 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 high pressure 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 or evaporation process to provide cooling inside the environment. [Note: With coolant, the AC compressor not only compresses 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 home air conditioning units) to dump heat.

The compression takes a lot of energy, which is why most systems use ~30 amp shore power or require alternator upgrades. Some battery-powered marine AC (like 12v marine ac) units have battery banks of Lithium Ion or AGM batteries to provide hours or even days of whisper quiet climate control.

In a marine a/c 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 A/C 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 water, like a shallow bay, your efficiency will be less than in deep, cool ocean water.

[The same process is true of almost all marine ac unit - from Dometic Marine AC Units to Marine Air Split Systems to Frigomar engine-room Chillers.]

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).

The fan (air handler) in your air conditioner unit then sucks cabin air from inside your boat through the return air grills and over condenser coils (which looks a lot like a car's radiator) and into the supply 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 to pump away the condensate. We typically install separate quieter, more efficient dedicated condensate pumps or special Venturi T's that use the flowing water in your salt pump discharge to suck out the condensate.

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?

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 air conditions 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 and center consoles to have marine air conditioning systems. Either way, you may have to upgrade your boat batteries and electrical box in order to power the marine air conditioning unit.

How Does Reverse Cycle Air Conditioning Produce Heat on a Boat?

Many marine ac systems come with both cooling capabilities and heating capabilities. With a heat pump reverse cycle, you are able to heat the air coming through the unit instead of cooling it. The refrigerant compression process generates hot compressed refrigerant (heat source) and cool gas that comes from the evaporation of the refrigerant (cold to be removed).

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.

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.

If you'd like help with a marine air condition installation or need marine ac repair near me, give us a call. We are happy to help install ac on your boat or talk you through your available options.