Selecting Air Conditioning
for Your Boat
By Tom Burden, Last updated: 6/25/2018
Your boat’s heating, ventilation and air conditioning systems provide climate control, keeping you cool in tropical summer weather, dry in muggy August humidity, and toasty-warm during the last October cruise of the season. They guard against odors associated with mold and mildew, extend your cruising season, and enhance the value of your investment in your boat. What follows will help you design the systems for ventilating and air conditioning your boat.
Proper ventilation is important for the maintenance of your boat and is a necessity for the comfort of you and your crew. A good ventilation system will:
- Reduce moisture and humidity below deck when the boat is left unattended
- Reduce the chances of musty air development caused by trapped moisture and humidity in the boat
- Help electronics and fabrics last longer by maintaining a drier climate
- Reduce the likelihood of osmotic blisters caused by moist air saturation of the hull
Active and Passive Ventilation
Vents fall into two basic categories: passive or active. Passive ventilators, such as cowl vents, clamshell vents, louvers, grilles, ventilating sails, ports and hatches simply provide an access path for air to enter or leave the interior of the boat. As long as either the boat or the air itself are moving, they work just fine. Of course on those hot, still days when the only things moving are the mosquitoes, they’re not much help.
Active ventilators incorporate a fan to keep air moving even when the boat or breeze is still. Solar energy or ship’s power is used to power the fan, depending on the ventilator. These vents come with both intake and exhaust fan blades for flexibility in creating your ventilation system.
Solar cell and batteries allow 24-hour fan operation with no electrical hookups, and it works for either intake or exhaust directions.
Designing a ventilation system that works
A properly designed ventilation system provides adequate air circulation throughout the boat without allowing water from waves, spray or rain to come aboard. Experts recommend that your ventilation system should provide at least one air change every hour. This means provisions must be made for both the intake of new air and the exhaust of old air. Set up your system to provide a “cross flow” of intake and exhaust ventilation wherever possible. If you only add one active ventilator, use it for exhaust.
A typical 30-footer has about 800 cu.ft. of below-deck interior volume. Unfortunately, this space is often broken up into distinct cabins or compartments that may restrict the free movement of air throughout the boat. Therefore, simply installing a pair of vents rated for 800cu.ft. per hour of airflow may not be adequate to get the total ventilating job done.
Each cabin and head should have some kind of ventilation, especially if the space can be closed off from the rest of the boat. Louvered doors, or vent grilles in solid doors help air circulate into lockers, forepeaks and other isolated areas of the boat.
During wet or rough weather, you’ll need to be able to shut off or remove vents to prevent water from finding its way below. Racing boats may need to use vents that can be removed while the boat is in use in order to keep the decks free of possible snags.
Do you need an air conditioner?
Air conditioning systems will be one of the largest electrical amperage draws when you are under way, but use modest AC current when connected to shore power. At the dock, a 5,000-16,000Btu AC-powered system draws between 4 and 13 amps. Under way in a powerboat, these amperages are manageable.
Under sail or at anchor, you’ll need a sizable inverter and the battery banks to support those loads or a genset. What makes these electrical investments worth considering? Summer cruising locations that combine high heat, humidities in the 80 to 90 percent range, and the presence of mosquitoes or other insects, requiring screened cabin areas that restrict ventilation. Air conditioning opens these locations to comfortable boating.
Hatch mounted portables: Your alternatives start with a portable unit like the Carry On, a good choice for cooling the small cabin of a cuddy cabin powerboat or overnighter sailboat. Similar to window-mounted home portables.
Self-contained air conditioning units like the FCF Series air conditioners are typically the best choice for boats up to 40'. All of the major components are mounted on a single chassis, which is installed in the living area under a bunk or settee, or in a locker. They require thru-hull connections to draw cooling water into the unit. These units use a new type of R-417A or R-410A refrigerant, environmentally-safe products that won’t destroy the world’s high-altitude ozone layer.
Split (remote) air conditioning: Central air conditioning systems, also known as split systems, are usually found on boats up to 80' in length. A split system has the air conditioning components split between two separate units. The two units are installed in different locations and connected by insulated copper tubing through which the refrigerant travels. The condensing unit, consisting of a compressor, seawater condenser, and electrical components, is mounted in the engine room or other mechanical space. The evaporating unit includes the evaporator coil and a blower and is installed in the living area.
Chilled water air conditioning: A chilled water system can air condition many cabins using a water loop instead of refrigerant between the chiller and the air handlers. It consists of a chiller, located in the engine room, that cools (or heats) fresh water, which is pumped through an insulated piping loop out to air handlers located in the living spaces, where the cabin air is cooled (or heated). Chiller systems can be configured to provide capacities of 16,000 to 2.88 million Btu. These systems offer flexible load management and reduced peak electrical load. Used on large boats and super yachts.
This 10,000 BTU Air Conditioner, EnviroComfort Reverse-Cycle Retrofit Kit lets you upgrade an older system.
Central systems and some self-contained units, like the FCF Series air conditioners, heat as well as cool. They pull heat out of the water to warm the interior of your boat. Though affected by water temperature, they can cool your boat in 90°F waters and heat your boat in waters as low as 40°F.
Retrofit kits from Dometic (EnviroComfort Reverse Cycle Retrofit/Compressor Kits) allow you to replace your old unit and re-use the old plumbing and ducts. They fit into a similar footprint and are charged with environmentally-safe refrigerant.
Sizing your air conditioning system
Air conditioning systems are rated in Btu, or British Thermal Units, a universal measure of heating and cooling. To calculate the number of Btu needed to cool or heat your space, follow the steps below, then select the unit with the capacity you need.
Step 1 - Required Capacity
Divide your boat into three basic load areas:
- Below deck: cabins where the hull slopes inward toward the keel and there are minimal port lights and hatches.
- Mid-deck: areas on the main deck with small or shaded windows.
- Above deck: areas with large glass surfaces and direct sunlight.
Measure the length and width of each room to be conditioned to determine the square footage. It is assumed that your boat has average headroom of about 6 1/2' and you have an average amount of furniture. If one end of the compartment is narrower than the other, take your measurement in the middle. Multiply the length by width to get the area of each.
Determine which load factors to use from the table below and multiply the area of each cabin by the load factor (Btu per sq. ft. per hour) to determine the required air conditioner capacity:
Select Moderate Factors where the maximum air temperature is 95°F, maximum water temperature is 85°F, and moderate humidity exists.
Select Tropical Factors where maximum air temperature is 105°F, water temperature is 95°F, and high humidity exists.
Step 2 - Number of Units
The number of air conditioners and their locations will be determined by the size and layout of your boat, and the space limitations for ducting and plumbing. The typical maximum ducting run is 15', but if there are many bends then the overall length must be reduced to ensure good airflow. Also consider which areas require independent thermostat control, and which cabins will be served by ducting or a secondary air handler (where the only temperature control is by reducing airflow with an adjustable grille or fan-speed control).
Step 3 - Location
As in step 2, the location of the units will be determined by your boat layout. Check the appropriate specification sheet for unit sizes, and make sure that there is sufficient space to service the unit and remove if necessary.
The self-contained unit or air handler must have an open return-air path, and be located such that the discharge ducting can be routed to a high point in the cabin. The return-air grille does not need to be directly in front of the unit, in fact, the system will be less noisy if there is an indirect path for the return air to follow. Most units have blowers that rotate to allow routing the discharge duct in the most direct path to reduce restrictions.
Step 4 - Seawater Components
The seawater cooling system consists of an inlet thru-hull fitting, seacock (water valve), strainer, pump, and overboard discharge fitting, all connected by hose or piping. If multiple air conditioning units are served by a single seawater pump, then a pump relay and water manifold are required. Cruisair recommends a centrifugal seawater pump for efficient, quiet operation and long life. Centrifugal pumps are not self-priming and must be mounted below the waterline.
It is important that the seawater plumbing be “self-draining”, meaning that if the boat is lifted, all water in the piping will drain out. An air conditioning system plumbed this way will have no air locks that could disrupt the flow of seawater. For shallow-draft boats where it is impossible to mount the pump below the waterline, a self-priming pump must be used.
It is normally recommended that you use one seawater circulation pump of adequate capacity for all of the air conditioning systems on board. The rule of thumb is to have 180 gallons per hour (3gpm) of water per ton of air conditioning (one ton is 12,000 Btu/hr). If more than one Marine Air System shares a common pump, you will also need a pump relay and manifold.
The table below shows recommended seawater flow rates and minimum inlet (through-hull) for a system of a given capacity. Our Product Advisors can work with our suppliers to give you more guidance.
|System Capacity (BTU/hr)||Seawater Flow (Rate gph)||Inlet Size|
Step 5 - Duct and Grille Sizing
See the table below for recommended duct and grille sizing. Our Product Advisors can help with duct transition boxes and sizing of branch ducting, as well as with the large selection of grilles available, some of which include aluminum, plastic, and many species of wood.
|Capacity (BTU/hr)||Duct Size||Return Air Grille||Discharge Grille|
|5,000||4"||60 sq. in.||30 sq. in.|
|7,000||5"||80 sq. in.||45 sq. in.|
|10,000||6"||100 sq. in.||60 sq. in.|
|12,000||6"||130 sq. in.||70 sq. in.|
|16,000||7"||160 sq. in.||80 sq. in.|
|18,000||7"||200 sq. in.||100 sq. in.|
Other Required Elements: A complete air conditioning system requires the control/switch assembly, the seawater cooling system, and the air distribution system. The Seawater Cooling System consists of an inlet through-hull fitting, seacock (water valve), strainer, pump, and overboard discharge fitting, all connected by hose or piping. If multiple air conditioning units are served by a single seawater pump, then a pump relay and water manifold are required. Cruisair recommends a centrifugal seawater pump for efficient, quiet operation and long life. Centrifugal pumps are not self-priming and must be mounted below the waterline.
It is important that the seawater plumbing be "self-draining", meaning that if the boat is lifted, all water in the piping will drain out. An air conditioning system plumbed this way will have no air locks which could disrupt the flow of seawater. For shallow-draft boats where it is impossible to mount the pump below the waterline, a self-priming pump must be used.
Air distribution systems: Cabin air is drawn into the self-contained unit or air handler through a return air grille. It is then cooled or warmed and blown back into the cabin through a ducting system. The air should be discharged high in the cabin and away from the return air grille to ensure good circulation. Plenums (transition boxes) can be installed in the ducting to allow multiple discharge grilles, in one or more cabins. An insulated duct is recommended to prevent secondary condensation. An air filter, located on the cooling unit or on the return air grille, must be cleaned regularly.