Keep Cool

How to add air conditioning to your boat

27th October 2001.
By David Brown

Air conditioning is one improvement that can add more value to the boat than the money it costs. The resale value of a boat with air conditioning usually increases more than the price of the equipment! Although complicated, installing a reverse-cycle air conditioning and heating unit can be done by the average boat owner. The work breaks down into four distinct jobs: physically mounting the unit; wiring the electrical power; plumbing the cooling water; and installing air ducts.

Choosing the Right Size Air Conditioner

Sizes of the air-conditioning units vary. This size is most often expressed in British Thermal Units, or BTUs. Small units put out from 5,000 to 7,500 BTUs. At the other end of the scale are 16,000 to 20,000 BTU giants for larger yachts. Factors for determining the size of the air conditioner you’ll need include the type of space to be cooled and the generally required BTUs per cubic foot. In general, sleeping cabins require 12 BTUs per cubic foot, main saloons and open living areas require 15 BTUs per cubic foot, and galleys and heads required 18 BTUs per cubic foot. (These numbers are based on tropical conditions: 95 degree outside air temperature and sea water temperature of 90 degrees. Northern areas will need less cooling capacity.)

While one large unit may be large enough to do the job, it is often easier and better to install two smaller ones. Routing cool air ducts from one central unit is not easy. This is especially true on triple cabin boats. Two units, one in the bow and the other in the stern, make the job much easier. There is the added benefit that if one unit should break down, the other will keep at least a portion of the boat cool.

Avoid buying too much air conditioning. A big unit will cool the cabin so quickly that it won’t have an opportunity to get all of the moisture out of the air. The result will be a cold, damp atmosphere more like a cave than a boat. A smaller unit takes longer to cool the area, but it wrings the maximum humidity out of the air. As a general rule, an air conditioner is properly sized if it runs about 75 percent of the time on a hot day.

Mounting the Unit

Most units are small enough to tuck into an odd corner of the cabin or into a seat locker. Make a cardboard mockup of the unit to see where it will fit. The best location is above the water line to keep the unit out of potentially corrosive bilge water. David Butler, national sales manager of Aqua-Air warns that the unit should never be located in an engine room. “Don’t even think about installing the unit in the engine room or generator space,” he says. “Carbon monoxide or other exhaust gasses could be introduced into the living spaces of your boat resulting in sickness or death.”

The compressor, chiller coil and cooling coils are all installed on a single base plate. This plate is bolted into position to install the entire mechanism. It sounds simple, but the nature of fiberglass boats gets in the way. Few of them have any convenient floor timbers on which to mount the system. It may be necessary to fabricate a small shelf out of white oak and marine plywood. Install this shelf between stringers and mount the air conditioning unit on it.

Cooling Water

Raw cooling water comes into the boat via a standard through- hull fitting and sea cock. A small electric pump then circulates it through the air conditioning unit. From there it goes overboard via a second through-hull fitting located above the waterline. Connecting the plumbing system requires installing the two through-hull fittings, a seacock and the appropriate hoses.

The intake fitting should be located where it can always draw water even when the boat is operating at planing speeds. This allows the air conditioner to function under way as well as at the dock. Choose a fitting, seacock and tail piece based on the size of the cooling water hoses needed for the particular air condi- tioning unit.

Although not absolutely necessary, prudence demands that an in-line water filter be installed between the seacock and the electric pump. This prevents the pump from being damaged by eel grass, zebra mussels or other gunk sucked out of the water.

Wire reinforced, non-collapsing rubber hose is suggested for pump intakes. Most installations get by with high quality automotive heater hose available through auto parts stores. Go to one that services marine customers and tell them how the hose will be used. They’ll know the right one to sell. At the same time be sure to get enough stainless-steel hose clamps to put double clamps on every connection.

Electric pumps used to circulate the cooling water are usually of the non-priming type. This means cooling water pumps must be located below the water line to work properly. Hoses from the sea cock to the pump and then to the unit should always run “uphill.” The cooling water outlet is traditionally set into the side of the hull just above the boot stripe. A nylon hose barb fitting is sufficient here, although bronze is the belt-and-suspenders way to go. A seacock is not considered necessary on a powerboat.

There is always some condensation on the air conditioner unit. This can be allowed to run down into the boat for removal by an automatic bilge pump. A better solution is to collect this condensation in a drip pan and let it drain overboard via a second through-hull fitting above the boot stripe.

Reverse Cycle Operation

Boat air conditioners are cooled with raw sea water which circulates through a simple heat exchanger. This design combined with the laws of physics allows the unit to perform an interesting dual function. In hot weather, heat is taken from the air inside the boat and dissipated into the cooling water. In spring or fall when the air is cool, the air conditioner takes heat out of the water and dissipates it into the boat. This switch from cooling to heating is known as reverse cycle operation.

Reverse cycle heating works well until the sea water temperature reaches approximately 38 degrees. Below that temperature there isn’t enough heat in the water for the unit to work efficiently. Still, reverse cycle heating can help extend the cruising season during cool spring and fall months.

Electrical Connections

Marine air conditioners operate on 110-volt A.C. This allows them to run directly from dockside current or from the output of an onboard generator. Use the appropriate gauge UL-approved boat cable to bring power from the boat’s A.C. switch panel to the unit. Air conditioners should always have a separate circuit breaker for each unit. Consult the manufacturer of the unit for the size breaker to install. A separate cable takes power to the cooling water circulation pump from the control unit on the air condi- tioner. All wiring should be supported every 18 inches with nylon cable clips.

A thermostat must be located in the cabin served by the air conditioner. This allows the occupants to adjust the temperature of the space to their comfort. A marine thermostats is actually a miniature control center. For best results, it should be mounted on an inside bulkhead out of the direct blast from a cooling duct.

Cool Air Ducts

The hardest part of the job is installing the ducts which take cool air from the unit to the discharge vents in the cabins. Unless the boat was prepared for air conditioning during construc- tion, routing ducts will require a lot of crawling in tight quarters. Ducts must run well above the bilge and never through an engine room or generator space. They are typically routed along the hull in the back of seat lockers and vertically in closets or other cabinetry.

Special 4-inch insulated air conditioning duct should always be used. This is a flexible hose insulated to prevent loss of the cool air and to avoid condensation on the ducting. Because the ducts are inside lockers, any condensation can cause mildew on clothing or other items. Cheap dryer vent ducting causes condensation problems and has a limited lifespan before it begins to crack.

“Y” or “T” connections are available to branch off cold air to two or more discharge vents. If off-the-shelf hardware won’t do the job, talk to a local company that custom installs residential air conditioners. They may be able to build a special plenum out of light gauge aluminum (don’t let them use sheet tin as it will rust).

When designing the system, remember that bends and long runs create resistance to air movement. The more resistance in the ducting, the less cool air in the cabin. Runs over 10 foot long should be avoided. Pull the ducting tight to reduce internal friction.

A variety of discharge vents with associated plenums are available. Those made of brushed aluminum are quite attractive, but the metal tends to collect condensation. Wood or plastic grilles are less prone to this problem. Wood grilles are available in raw teak to match many boat interiors. Unfinished pine grilles can be painted or covered with wallpaper to match the surrounding decorative scheme.

The most forgotten aspect of ducting is the return air system. Warm air from the cabin must be ducted to the air conditioning unit for chilling. If the unit is installed into a seat locker or near an interior bulkhead, a simple grille in front of the air intake may be the entire return air system. A unit hidden deep in the boat will require ductwork. Keep in mind that friction also cuts down the flow of air in the return system. Aluminum grills are acceptable on the return system as condensation is unlikely.


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