Keeping Your Cool on the Water: How does a Boat Air Conditioner Work?

Just because you’re on the water doesn’t mean that you won’t get uncomfortable, especially if you’re anchoring overnight or staying at the dock. It’s not an issue if you’re moving. The wind rushing past you will keep you cool. Staying stationary gums up the works. You may gravitate toward the obvious. That may leave you wondering, how does a boat air conditioner work?

The limiting factor with boats is the size. Unless you have a massive yacht, space is at a premium whether you’re packing water toys or making room for an extra marine battery. Then, there is seeing to comfort. It might not be an issue if there is a breeze, and you can sleep on deck. It takes on a different spin if the mosquitoes are on the hunt, or it’s raining. That’s where your AC comes into play.

How Does A Boat Air Conditioner Work?

The first thing to understand is that a boat air conditioner removes the ambient heat from your cabin instead of cooling it, per se. That concept rests with its design. There are three primary components to the system:

  • Condenser
  • Evaporator
  • Compressor

The evaporator collects the warm cabin air via the return. The refrigerant within it removes the heat. The compressor is the business end of the unit and keeps air circulating through the air conditioner. The condenser then cools the airflow and sends it back to the evaporator. A fan pumps the cooled air back into the cabin. Collectively, the fan and vents are called air handlers.

Types of Air Conditioners

The most common AC that you’ll see on a power boat is the self-contained, direct-expansion system. As the name implies, it’s compact, making it an excellent choice in tight quarters. You can run them off of shore power or a generator. You can also get an inverter and use a dedicated battery, instead.

Another option is a chilled-water unit. This one makes use of the water your boat is sitting in to take over the role of the refrigerant, not unlike the setup for an outboard motor. These systems require some extra work to install with ductwork from the chiller to the air handlers in the cabin. You’ll usually see them on larger yachts.

The split system is similar to the central air conditioning that you have in your home. There are two units, the evaporating portion inside the boat and the condensing part outside of it. The former consists of the fan and evaporator. The latter includes the condenser and compressor. Like the chilled-water option, you’d typically find this one on larger vessels.

A more straightforward design exists with an evaporative cooler. It’s an ingenious riff on the concept of having a fan blow over a bowl of ice. In this case, the supply is inside of your cooler. Most aren’t powerful and will only cool the ambient space.

Finally, there is a portable air conditioner. You can use the same type that you would in your home. It works on a similar principle with an exhaust vent that you out outside of the boat. You’ll often see them as multi-functional models with a fan, dehumidifier, and heater too, known as reverse-cycle AC. There are also hatch-mounted units if you only need to cool a small area.

Selecting an Air Conditioner

The essential considerations for choosing an AC include the amount of space you need to cool, the room you have for a unit, and the ease of setup. Price, of course, is always a factor too. Depending on the type, you can expect to pay anywhere from a few hundred dollars to well north of $1,500. Reverse-cycle units are the most expensive of the lot.

AC Capacity

You can begin with the square footage of your cabin. However, that’s only the beginning. You also need to consider the effect of the climate and the temperature of the water. Remember that boats are only watertight and usually not insulated. Also, there are passive vents located throughout the design, bringing outside air into your living quarters.

If the weather is on the tropical side, that takes evaporative units off the table. The design and airflow will only increase the humidity and make the cabin more uncomfortable and sticky. A direct-expansion system or portable AC offer better options for dealing with these conditions.

If you have more than one cabin, you might need a second unit. Alternatively, you can have plenums or transition boxes installed to direct the airflow to where you want it to go, with additional ductwork and grilles. That is a workable solution with direct-expansion, chilled-water, and split systems. Alternatively, you can install a hatch-mounted unit into each one.


Where you set up your AC is another essential consideration. The crucial thing to remember is that the airflow to the returns must be free-flowing with no obstructions in its path. Ideally, you can install it where it won’t encroach on your living space. Then,  make sure it is discharging the cooler air toward the top of the headroom to help it create its own flow and circulate better.

If you’re using a portable air conditioner, bear in mind that it will only cool the area in the direct path of the discharge. You may be tempted to store these larger units in a corner. However, you’ll miss out on their benefits.

Troubleshooting Problems With Your Air Conditioner

Unfortunately, when you find out there’s a problem with your AC is usually the worst time. Airflow is often a culprit. That’s why you must clean the air filter of the unit at least once a month or whenever it seems less than capacity. Make sure the area around the returns is clear too. Debris and buildup on the condenser or coils can also impede airflow. Those are the simple fixes.

A quick way to pinpoint other problems involves using a digital thermometer to measure the temp differences between return and supply air handlers. The latter is the flow discharging back into the cabin. If it’s less than 20 degrees, the unit may need recharging. However, it’s not a DIY job. Federal regulations require that only an EPA-certified technician handle these materials.

Marine air conditioners, like the ones in your home, are closed systems. Any loss of the coolant is a sign of a bigger problem that you’ll need to investigate—the sooner, the better. The refrigerant not only keeps you comfortable, but it also keeps the unit from overheating. If your air conditioner is older, it likely contains chlorofluorocarbons (CFCs) that are harmful to the environment.

If you have a portable air conditioner, check the layout of the duct. You’ll get optimal airflow if it is in an S-shape before it vents outside of the boat. You can fine-tune it to see if that improves its cooling power. Also, check the angle of the supply. Changing its position may offer an easy solution.

If humidity is the problem, bear in mind the operation of a portable unit. It differs from a standard window unit in that the moisture that it collects stays inside to cool the motor. It doesn’t drain out of it. The motor and condenser would fry out without it.

Tips for Using an Air Conditioner on a Boat

You can get the most out of your AC by helping it with its job. While you can’t dodge the sun in the open water, you can reduce the workload on the unit with shading. A properly placed bimini is a godsend if the sun is brutal, but there is still a refreshing breeze. You can use a boom or shade tree awning for some welcome respite on your sailboat. A sun-fly strut can cover the back or stern of your boat.

You can also install sun-blocking curtains or blinds inside the cabin to keep the inside comfortable during those blazing hot summer afternoons. They will also help keep it warm if you boat later in the year to take advantage of your reverse-cycle system.

Also, take a close look at the floor plan of your boat and its ventilation. The chances are that there are plenty of passive vents to ensure proper airflow and to help prevent carbon monoxide build-up within the cabin. You might consider adding some active ventilators or fans to improve the cross circulation. The movement of air may override the need to run your AC all the time, a boon if you use a 12-volt battery.

Regular maintenance remains the single best thing you can do to get the most out of your marine conditioner. It’s sound advice for any craft whether it’s a yacht or a personal watercraft. For your AC, that means checking the unit at the start of the boating season before it stops working when the temperature is 90 degrees outside and climbing.

Clean the air filter and check the condensate pan to make sure the AC is draining all right into the bilge. You should also inspect the sea strainer of chilled-water systems. If you have a reverse-cycle unit, it’s also smart to run it too to make sure everything is working correctly. You should have the AC flushed out every few years if it’s a built-in model.

Sometimes, having an AC on your boat is the only way to get the most out of being on the water. It can make the difference between a pleasant day to a miserable day in hot, humid conditions. Fortunately, there is a range of solutions that answers the question, how does a boat air conditioner work? With regular maintenance, you’ll always stay cool when the weather takes a turn.