Marine Air Conditioner: How does that Thing Work?

Air conditioning is an essential piece of equipment in modern life, and that includes on a boat or yacht. To most people, air conditioners are mysterious boxes that magically produce cold air. They are typically taken for granted until they break and you begin to sweat. Before discussing how to repair marine AC units, we thought it prudent to first publish an article about how they work. This background knowledge is intended to help you in the AC unit purchasing process, in conversations with marine AC technicians, and to help you troubleshoot and DIY repair if you are so brave.

In this post, we review the types of marine AC systems and the basics of their inner workings. This post is meant as an introduction to marine AC units with high level descriptions of functionality and basic components. If you’d like to see more information about common problems and troubleshooting, then you can read our post Marine Air Conditioning Maintenance and Repair Tips.

Cooler than Being Cool: Types of Marine AC Systems

There are several types of marine air conditioning systems, and most today work with seawater or freshwater. Raw water is the term used for natural water that hasn’t been conditioned with chemicals or anything else.

Direct expansion (DX)– This is the most common type of air conditioning system found on boats up to approximately 40-feet long. On boats up this size, the typical unit is self-contained, meaning the condenser, evaporator, and fan are one unit. The AC system pumps refrigerant directly to the air handler. The units generally range from 5,000-30,000 BTUs, though smaller ones are available for small cabins.

Chilled water – This system works by cooling water that is continuously circulated through the air handlers installed in living spaces. It is normally found on yachts or larger boats over 80-feet long. You can install multiple chillers.

Split system – In this setup there are two units. One is the evaporating system inside the boat that consists of the fan and evaporator. The second unit is the condensing unit outside the boat that contains the condenser and compressor. Given it has two units, this type of system is mostly found on very large boats. For example, the split-gas DX system has two units connected by insulated, copper refrigerant tubing. The system is generally applicable to boats up to 80-feet long.

Reverse cycle – The reverse cycle unit is an air conditioner that can change the refrigerant flow and convert the system into a heat pump. You can buy a direct expansion or chilled water AC that has a reverse cycle. The compressor pumps pressurized hot refrigerant into an air handler. The air handler transfers the heat to the cabin by blowing the cooler cabin air over the coils.

It should be mentioned there are marine evaporative air coolers (that don’t work well in high humidity) and portable air conditioners for small boats.

Basic Components of the AC System

All air conditioners work by using heat exchange, but there is a difference between the home and marine units. The marine AC system uses water to cool air, unlike a home AC system that uses air only. Most avid boaters have noticed one or two streams of water coming out of a through-hull fitting which is the water discharge from the air conditioning unit(s).

The modern units are managed via digital controls in the cockpit area. There is usually a display with a keypad, and offers a variety of functions like automatic fan-speed control and a dehumidification program. The simpler system has mechanical controls with rotary knobs that control the variable fan speed, thermostat, and mode of operation.

Generically speaking, there is a through-hull fitting located underwater through which water is pumped. The water is pumped to the air conditioner unit, and cool air flows through ductwork into areas on the boat. The water is then discharged through a through-hull fitting located above water.

Air conditioners remove heat, rather than produce cold. Every marine AC unit has these basic components:

  • Evaporator – has a low pressure (evaporate) and high pressure (condense) side to manage refrigerant
  • Compressor – increases temperature of the refrigerant gas on the high pressure side and keeps air circulating through the air conditioner
  • Condenser – cools and liquifies the refrigerant and sends it back to the evaporator on the low pressure side
  • Pump – maintains steady flow of cooling water in the AC system
  • An “air handler” is the system composed of the evaporator, fan and ductwork. The fan blows the warm cabin air across the evaporator and discharges the cooler air into the cabin. The warm air pulled in from the cabin is called the “return air,” and the cold air coming out is called the “supply air.”

Basically, water is pumped from the lake, river or ocean through the hull to the air conditioning unit and is then discharged, carrying the heat from the condensers. A heat exchanger is the component responsible for transferring the heat absorbed from the boat’s interior to the water. A unit can use either fresh water or sea water.

There are other parts to the system, of course. They include a water strainer, sea cock (valve that controls water in and out of the through-hull fitting), air vents, wiring, circuit breaker, thermostat and controls.

Marine air conditioning systems can run when on an outing or while docked and connected to shore power. Having said that, each year new concepts in AC are appearing. Larger boats have battery banks and inverters that convert DC to AC with the batteries recharged with a high output alternator fitted to the engine. A boat air conditioner is one the main consumers of electricity. When docked and plugged into shore power, a 5,000-16000 BTU air conditioner will draw 4-13 amps. When cruising or moored, the inverter and battery banks support the loads.

But there are now air conditioning units available even for small boats, though installation can get tricky due to space limitations.