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Image courtesy of West Marine
Air conditioning for boats has come a long way in recent years. In earlier days there were typically large compressor and condenser units in one section of the boat, (frequently the engine room), with air handlers and the evaporator in cabinetry within the living areas. Freon or other "coolant" was transported to the evaporator via copper tubing covered with insulation. (Freon is no longer used because of environmental concerns and there are several products now being used. For the sake of discussion and brevity we’ll use the term "coolant.")This type of air conditioning is still used, principally in larger yachts, but developing technology has led to the option of buying a self contained unit that's small enough to install within a cabinet in a living area. On larger boats, one or more self contained units are sometimes installed. There are various benefits to self contained units, including the fact that it's usually easier and more convenient to install by the knowledgeable "do it yourselfer" and professionals, one doesn't have the long run of coolant and, depending on the unit, it may consume less overall space and typically the unit will cost less. Also many of these are essentially plug and play when it comes to replacing some of the parts and other service issues. Following manufacturer instructions as to all details is critical. Here are a few general considerations that may be helpful as you consider this type of air conditioning.
Before you buy a unit, pay close attention to what the manufacturer of the unit you're considering has to say. You'll need to know the cubic feet that you want to cool, and measure it according to the manufacturer's recommendations. For example, your cabin from bulkhead to bulkhead and/or side to side and overhead to deck is a certain number of cubic feet. But if this space contains large cabinets or other structures the interior of which aren't going to be air conditioned, your actual cubic footage may be considerably less. You'll need to buy a unit that the manufacturer says is adequate for the space you wish to cool, using the method of measurement and calculation recommended by the manufacturer. Usually you will find manufacturers and dealers quite helpful with this. Don't try to save money and undersize the unit. You may end up finding that you’ve wasted the money you spent. Also, don't try to grossly oversize the unit. This can not only waste money initially but also result in less than ideal performance. Do remember that your boat probably won't be as well insulated from the outside as is your house.
Placement is critical. You'll need to take the following into consideration as you decide the best location for your circumstances. You'll need a cabinet or other space to contain the unit. If you don't have one you may need to do (or have done) some carpentry.
The unit should be in a dry, easily-accessible and ventilated location and not in the bilge, engine room, or near combustible vapor source. The space should be in a location favored by the installation instructions. For example, you wouldn't want to place it where only your feet are cooled. Remember, that cold air settles, hot air rises. Wiring will be important. It must be of recommended gauge. Probably you'll have to install a wire run for the project. This may play a role in where you can mount the unit.
Also plumbing for the raw water cooling will play a role. These units are not cooled by air, resulting in a quite significant hot air discharge, as is your home unit. They have a heat exchanger (coil) where water from overboard takes the heat from the coolant. Therefore there must be a pump located at some point in the boat, pumping water from a thru-hull, through a hose, to the unit. (In many instances one pump can service more than one air conditioning unit.) There must also be a discharge thru-hull well above the water line at maximum heel. All fittings must be double hose clamped, and the hose should be routed slightly upward from the intake through-hull through the strainer and to the inlet side of the pump and then to the unit. Loops in the hose can result in air locks. The intake thru-hull must be low enough so that the pump won't be sucking air if the boat heels (as with a sailboat) or planes. All fittings must be double hose clamped with quality clamps and the thru-hull valve should be certified for marine use and properly installed according to ABYC standards which should be followed throughout.
There will be one more small hose run. The unit will produce condensate water on the fins. This is normally drained from a tray under the fins, either overboard or into a sump. Overboard discharge of this cold fresh water is preferable to diminish the development of mold and smell, but overboard discharge can lead to other complications, particularly in the plumbing run. This small condensate hose (more like a tube) should be run in a manner so that there are no kinks and you should be able to easily clean debris (such as accumulated dust) from its top end. Otherwise, over time this will clog the tube causing condensate water to run into your cabinetry and elsewhere. (Many will clear this, if it occurs, simply by blowing on one end of the hose.)
The cold air discharge ducts should be as short as possible. On some installations the air discharge is self contained at the unit avoiding the necessity of ducts. Also, the control containing the thermostat should not be in direct sunlight or other location which will inadequately reflect the cooling needs of the cabin space.
These units, if properly installed and operated, are usually quite efficient and they can greatly add to your enjoyment of your boat.