Keep Cool
Many boaters in the temperate climates of the Northeast and Northwest coasts or the Great Lakes basin consider the best air conditioning as a nice cool sea breeze through an open hatch. Given the typically cold waters of these regions, relief from summer’s heat is usually no further away than the boat itself. Lately, however, it seems these areas have been getting really hot summers and being on the boat may not always provide refuge from the humidity and heat. This is a reality which Gulf coast and Florida boaters have always had to deal with. Ask any southern boater and he or she will tell you that air conditioning is not a luxury but a necessity. It can make the difference between being able to use the boat and not, particularly if your family's enthusiasm for weekends aboard declines as the mercury rises. How Does A/C Work? Air conditioners (A/C) are similar to refrigeration units and use the same physical principles. They are designed to pump a specially tailored refrigerant (usually Freon 22) through a sealed heat exchange circuit. The fluid picks up heat from the cabin air and transfers it overboard. In more detail, at the beginning of the heat exchange cycle the refrigerant first travels as a cool liquid to a fluid-to-air heat exchanger known as the evaporator (it looks a lot like a car's radiator). Here the refrigerant expands into a low-pressure gas that, by the normal laws of physics, causes the refrigerant temperature to plunge. This cold gas passes through the evaporator heat exchanger at the same time the cabin air is being drawn through by a blower. The warm humid cabin air cools, usually below the dew point, on contact with the evaporator and the humidity condenses out and drips into a catch tray under the unit to be drained away. Meanwhile, the blower circulates the cool dry air back into the cabin through ducts. At the same time, the refrigerant, now a warm gas, continues to circulate back to the compressor (usually driven by an electric motor) where it's compressed into a hot gas under high pressure. The hot Freon then flows to another heat exchanger, called the condenser, which usually looks like a coil of pipe wrapped around the compressor and in this case, is cooled by seawater. Here it transfers its heat to the seawater and condenses back to a cool liquid to continue the cycle. The seawater, now warmed by the heat removed from the cabin air, is pumped over the side. It's worth noting that a large part of the A/C's contribution to comfort comes from removing humidity. When the unit is first turned on, a great deal of the initial work goes into condensing humidity which holds a lot of latent heat from the air. Once the humidity is lowered then temperatures will start to come down and even if the A/C is undersized for the job, just the removal of the humidity, even without much lowering of the air temperature, will make a big difference in comfort. What is Reverse Air? A/C units with reverse air capability can run the cycle backward to pull heat from sea water for warming the cabin. This is a bonus to boaters in more northern climes but there is a catch, particularly for liveaboards. It will not work if the seawater temperature gets to 40°F (4.4°C) or lower. At temperatures approaching freezing there isn't enough heat left in the water to be useful before it freezes and causes the heat exchanger to ice up internally. This cuts off the water flow and any further heat exchange. Types of A/C Systems Here's a quick review of what's available in the marketplace. All air conditioners are AC powered; there are no DC-powered units. Domestic Window Units: Just cut a hole in the wall, er, bulkhead and prop into place with 2x4s. Perfect for that houseboat permanently tucked away off the waterway or tied to a mangrove in the back bayou. RV Units: RV units are just a more streamlined version of the domestic units and are designed for rooftop mounting on a motor home. They are quite tidy, relatively economical and simple to install. Capacity is from 5,000 to 15,000 British Thermal Units (BTU), and they look at home on the pilothouse of a small tug or work boat. I question how secure they would be in a big sea and you would not want to be tripping over one on a crowded deck; they can be a viable option nonetheless. Marine Portables: The two popular marine portable A/C units are Cruisair's Carry-On and KoolKart by Komfort Industries. Like a domestic window A/C, the 4,800 BTU Carry-On has an air-cooled condenser and mounts outside the living quarters, blowing cool air in through a shrouded opening — in this case, any hatch 12" (30.5cm) square or larger. No installation is required other than placing it over the hatch, attaching the fabric hood and plugging it into an extension cord. The KoolKart is a suitcase unit designed to roll on and off the boat as needed. Similar to built-in marine A/C units, it uses water cooling but from portable hoses with a submersible pump on the end. The hoses are simply lowered over the side. It's a 6,500 BTU unit, about 35% more capacity than the Carry-On, but it's also proportionately more expensive. Water cooling means that its efficiency will remain unaffected as the ambient temperature rises. Both of these units are perfect for dockside use in boats from 20' to 30' (6m to 9m) but must be stowed while underway. Built-in Air: These are the true marine A/C units installed permanently and water cooled. Once regarded as equipment practical only for larger yachts, prices have dropped and improvements made in the efficiency and packaging, particularly with the smaller self-contained units. All the major manufacturers have new entrants in the economy 5,000 BTU class, targeting the smaller boat market and packaged for do-it-yourself installation. There are three basic configurations for built-in marine A/C systems: self-contained, remote condensing and chilled water. They are all water cooled and require a pump, plumbing, wiring and thru-hulls. Self-contained units mount all of the major components, including the entire sealed refrigerant system, together as a single assembly and are most often found on boats under 40' (12m). They range in capacity from 5,000 to 6,000 BTU; larger if 230-volt power is available. They are required to be installed within the boat's living spaces (typically under a berth or seat) and for safety reasons must not share any air circulation with the engine compartment. Well suited for DIY installation, the installer is not required to open and recharge the refrigerant circuit. Remote condensing or "split" systems, found in boats up to 75' (22.5m) also range in size from 6,000 to 16,000 BTU with 230-volt units to 60,000 BTU. They have the compressor and condenser unit mounted in the engine room, only the air-handling equipment and evaporator are in the living space. Insulated pipes convey the refrigerant between the two units. This requires vacuum collection and recharging of the refrigerant during installation to prevent the release of ozone damaging Freon to the atmosphere. Environment regulations in both the U.S. and Canada dictate a licensed technician must handle this work. Chilled- or tempered-water systems are used when a large central air system is called for. In large yachts or ships where many cabins need to be cooled it's impractical to run refrigerant ducting. In contrast to the direct expansion systems previously described, a large central refrigeration or "chiller" unit (which includes the evaporator) cools water instead of air and circulates the cold water throughout the boat to individual heat exchangers or "air handlers." These are equipped with blowers to circulate and cool the cabin air. Although the actual refrigerant system does not usually have to be opened during installation, these chilled water systems, which range incapacity from 24,000 to 60,000 BTU, are unlikely to be DIY installed. Another alternative is Technicold engine-driven A/C systems by Rich Beers Marine. These operate similar to an automotive unit. Its target market seems to be workboats with a requirement for pilothouse A/C.
Purchasing Guidelines 1. Dealer versus Catalog: You can purchase a unit from a full service dealer or buy direct from a discount catalog where you may save up to 20% off the list price but you're on your own with the installation. A dealer may be willing to negotiate on price to stay competitive and will certainly be more inclined to assist with installation if they sold you the unit. Many dealers also may be willing to handle the most difficult parts of the installation and leave the simpler tasks to you. Catalogs are limited to the available pre-packaged kits; a dealer can assist you with working up a kit from scratch, tailored to your requirements. 2. Heat and Cooling: Once you have decided to undertake the installation you first need to decide if you require the reverse air heating option. If your boat remains year-round in a tropical location the answer is no, but remember that even Florida gets some frosty winter nights. 3. Cooling Capacity: The next step is to determine what cooling capacity you need. Take some measurements to determine the volume you need to cool, choose the BTU/cu.ft. "K" factor that best matches your plans ( Table 1 ) and multiply the "K" factor by the cu.ft. volume to determine your BTU requirements. 4. Dockside Power Requirements: You may need to install a second 30-amp shorepower service, assuming you'll want to have other 115-volt loads running at the same time (i.e. water heater, microwave). Check the manufacturer's data for the unit's power consumption. Generally, an A/C unit will draw from 1 to 1.5 amps at 115 volts per 1,000 BTU of capacity while running; starting and cycling loads can be two to three times this amount. This means that a 16,000 BTU unit — the maximum that a single 30-amp 115-volt circuit can handle — that draws 15 amps may pull 45 amps momentarily at start up; a slow trip 30-amp breaker is usually used. This is why single A/C units larger than 16,000 BTU require 230 volts. 5. Power Requirements Underway: Using your A/C away from shorepower usually requires an auxiliary 115-volt generator with enough capacity to handle most, but not necessarily all, the startup loads. Gensets can usually handle momentary surges 1.5 times above rated capacity (check with the generator manufacturer). A 4.5 kW genset should be able to handle a single 16,000 BTU A/C unit, maybe more. It's technically feasible to run some smaller A/C units off an inverter. This would only be practical while motoring and assumes the boat is equipped with an alternator big enough to carry the constant power requirements of the A/C plus 10% for inverter losses and an additional reserve for other DC loads. Most inverters have a generous surge capacity and can handle the startup load. The drawback to inverters carrying any high load appliances is the big DC amperages involved ( Table 2 ). Even with proper fusing these large currents can lead to sudden meltdowns, usually at a connector, caused by a tiny bit too much resistance that leads to a runaway local heat build up. Ocean Marine offers an automatic load managing inverter system designed to set AC priorities and avoid meltdowns. Another approach to 115-volt power while underway is a specialized high-output alternator system designed to provide 110-volt AC power in the lower kilowatt range without using an inverter. 6. Unit Location: Once you have determined what capacity unit is practical, check what is available and the unit's dimensions. The current size versus BTU champion is probably Marine Air's Vector Compact. The next step is to find a location on board that will accommodate it and meet the following criterion: it should be in a dry location with good access for installation and service, as close to the floor as possible. This is usually under a berth, seat or in the bottom of a hanging locker. Make sure you have room for the recommended size for supply ducting and return air grill, otherwise the unit will never perform properly. It's critical that the location has no access to air or vapors from the engine compartment. Self-contained units are not ignition protected and exhaust leaks can be a source of carbon monoxide. This restriction also applies to drip tray drain hoses. 7. Control Options: Electronic digital controls are not necessarily more expensive than mechanical controls and offer greater flexibility in tailoring the system's operation to your needs. A reverse air system, in particular, can be programmed to provide full climate control and will automatically switch from cool to heat as required. Most of the more sophisticated controls also offer an automatic dehumidifying cycle that can operate the unit just enough to keep the dampness and mildew under control while you are away. Cruisair offers a remote control option for their SMX II digital climate control. With the SMX Modem (US$410), you dial the appropriate code, probably from your wireless phone while on route to the boat, and it remote starts your A/C unit so the boat is nice and cool when you arrive. It requires a dockside phone line onto the boat and has optional outputs to switch on other devices on board as well.
DIY Installation Don't believe the catalog ad that says "Can be owner installed in the morning and cooling the cabin in the afternoon." A simple installation done properly will take 16 to 24 hours. A complex installation can take 40 hours or more ( Table 3 ) and that doesn't include hauling the boat to install the thru-hull. The manufacturers supply very complete instructions, which should be followed religiously. The unit must be securely fastened with stainless-steel screws or bolts to a flat sturdy surface and reinforced if necessary to take the usual bouncing, lurching and heeling that boats are subject to ( Figure 1 ). The seawater pump location ( Figure 2 ) is critical especially in sailboats — it must be below the waterline at all times. The quiet centrifugal pumps with magnetically driven impellers that are supplied are not self-priming. Therefore, the hose routing must also be at a constant slope upwards to the pump with no loops or traps. It must incorporate a seawater strainer, a shutoff valve (use a 1/4-turn ball valve) and the thru-hull must be a scoop strainer facing forward, otherwise the boat's motion through the water will suck the pump dry. Don't attempt to tee into another thru-hull already in use for some other purpose; the pump will have priming problems. Use reinforced marine-grade hose throughout and secure all fittings with double hose clamps. Most A/C water pumps are ignition protected — check the label on the pump — and can go in the engine compartment. It may be impossible in some sailboats to keep the pump below the waterline. In this case, you can incorporate a quick priming rig or use an impeller pump and expect to replace the impeller frequently. The water outlet thru-hull should be just above the waterline so the water flow can be checked visually but not so high it makes an annoying splashing sound. Attach a hose to the condensate drain and lead it into the bilge but not to the engine room unless a separate sealed sump is provided. It's also possible to connect it to a thru-hull above the waterline provided it's not near any exhaust outlets. Locate the return air duct to provide unobstructed air flow to the evaporator, usually right at floor level. The cool air supply grills should be as high on the bulkhead as possible (Figure 3) . Don't install the supply and return grills too close together, otherwise the airflow will "short circuit" and lead to inadequate cooling. Avoid bends and kinks in the supply ducting as much as possible, they cost in efficiency. Strictly follow the manufacturer's duct and grill size requirements. Aluminum grills may sweat so avoid mounting on wood bulkheads or use wood or plastic grills. Follow the manufacturer's recommendations when installing controls and wiring. Stick to the manufacturer's color code for connections and refer to MRT Series "Marine Electrical Systems CD-ROM" for proper wiring techniques. If you're using a digital control that incorporates a thermostat into the control panel make sure you don't mount it in a sunny location, otherwise your A/C will run constantly. Mechanical controls usually don't have their own thermostat but use one mounted on the A/C unit. If all else fails, remote thermostat kits are available. With your new A/C unit up and running, you'll find that just like a new backyard pool everyone you know will want to drop by for a visit on a hot summer day, so you had better make sure the fridge is well stocked. About the Author: Nick Bailey is service manager of Bristol Marine in Mississauga, Ont.
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