A boater’s guide to DC-powered marine air conditioning.
Today’s compact DC-powered units mean even smaller boats or those without a generator can enjoy the comforts of air conditioning. Photo, Frank Lanier
When it comes to boats and air conditioning, past options have been primarily limited to 120-volt AC units available for use when connected to shore power or powered by an onboard generator. The development of high-efficiency DC-powered marine air conditioning units has changed the equation, allowing boaters to run air conditioning directly from their battery banks. Here’s a look at DC-powered air conditioning and the cool options it brings to the table.
The Benefits Of Direct DC Power
DC air conditioning units provide cool goodness without the need for an AC power source. This, in turn, opens up possibilities for air conditioner installation on smaller and midsized vessels, as well as larger boats that have no generator installed. A DC-powered unit also eliminates the need for a DC-to-AC inverter (an option sometimes used to power AC air conditioning units) along with the energy conversion loss that occurs when inverting to AC from your batteries. Another benefit is that DC units are better at maintaining a steady temperature. Traditional AC-powered air conditioning units are all or nothing, meaning they blast cold air until the thermostat hits the target setting, then shut off.
DC air conditioners rely on an inverter-driven (variable speed) compressor, which allows them to throttle down to a low power mode, managing speed and optimizing energy consumption. “Inverter driven” in this case doesn’t mean it converts AC to DC, but rather it converts the DC voltage into a three-phase signal that controls the motor speed for efficiency. This more precise temperature control delivers superior temperature stability and improved humidity control for greater comfort. Other benefits include higher energy efficiency (30% to 50%), quieter operation, and extended equipment lifespan due to reduced mechanical stress.
A variable speed compressor also delivers a softer start due to lower starting surge. This reduced electrical load when starting up is easier on your boat’s electrical system and reduces nuisance breaker tripping. The three types of variable speed compressor motors typically found in marine systems are piston (reciprocating), rotary, and scroll.
- Reciprocating motors are less efficient and draw more power. They also have a lot of moving parts (e.g., piston, rings, crankshaft) that equate to increased maintenance costs and a higher chance of mechanical failure.
- Rotary motors offer higher efficiency, less noise, and less vibration than reciprocating motors. They are also more budget-friendly for smaller applications compared to scroll motors.
- Scroll motors are quieter and more efficient than piston or rotary motors, although rotary compressors can be more efficient in small-capacity systems. Scroll motors also feature continuous compression, meaning that the refrigerant is sucked in, compressed, and discharged in a smooth, simultaneous flow. As a result, scroll motor compressors are superior in terms of low vibration and noise, making them the preferred option for medium to large systems.
Disadvantages of DC air conditioning include higher initial purchase prices (typically more than traditional AC-powered units) and power consumption, along with any necessary investments in additional batteries and charging capability (such as wind, solar, or high-output engine alternators).
The data tag provides crucial information on the unit and will assist with planning the installation. Photo, Frank Lanier
Adequate ventilation and filter use is critical for the proper operation of any AC unit. Photo, Frank Lanier
What’s Your Type?
DC-powered air conditioning units can be broken down into three groups based on the power they use: 12-, 24- or 48 volts. Each voltage has its own set of pros and cons with regard to installation and operation.
- 12-volt DC system. The primary benefit of a 12V air conditioning unit is compatibility with the standard 12V marine electrical system found aboard most boats. This simplifies things considerably because it eliminates the need for complex electrical system overhauls to facilitate the installation. As the unit is based on 12V, it also means components (for installation or future repair) will likely be more readily available.
As to cons, the biggest disadvantage of a 12V system is high amperage draw. Amp draw for a 10,000 BTU unit can exceed 50 amps, which generally limits the use of 12V to smaller BTU units. This higher amperage means heavier gauge wire is required during installation, adding to overall installation costs.
- 24-volt DC system. The big advantage of a 24V system is lower amperage draw, theoretically half that of a 12V system unit. This lower amp draw allows use of smaller diameter wiring, reducing wiring costs. Twenty-four-volt systems are very efficient for running larger 12,000-plus BTU air conditioning units.
The primary downside to a 24V unit is that it requires a 24V battery bank or a 12V to 24V DC-DC converter, both of which increase installation costs and complicate the electrical side of the installation.
- 48-volt DC system. Forty-eight volt systems have the highest energy efficiency and lowest amperage draw of the three voltages. Ideal for larger BTU installations, 48V systems reduce power consumption by minimizing energy loss in the form of heat, which is particularly beneficial for high-demand cooling. This lower energy loss means lower installation costs, as smaller gauge wire can be used. Regarding cons, initial outlay is a big one.
Components for 48V systems (batteries, charge controllers) cost significantly more than their 12V or 24V counterparts, often requiring specialized installation knowledge and costlier equipment. Most marine components are not 48V rated, and few appliances currently work on it, with those that do being more expensive.
Selecting The Right Unit For Your Boat
The DC voltage you choose will depend on factors such as your boat’s currently installed DC systems, the size of your house battery bank, and your willingness to rewire. Selecting the correct DC powered air conditioning unit involves balancing cooling capacity (BTU) with your battery bank capacity and power-generating capability. Here are some key selection factors to consider when shopping for a DC-powered air conditioner:
- System type. The two primary types of DC powered air-conditioning units are self-contained and split systems. Self-contained units combine all components into a single, compact unit. They offer an easier, space-saving installation and are an attractive option for smaller boats.
Suitable for larger vessels, a split-system unit features a remote compressor connected by refrigerant lines to an air handler inside the cabin. This type of configuration reduces noise by allowing the compressor to be located in a non-living space, such as a lazarette or large, well-ventilated engine room.
- Cooling capacity (BTUs). Air conditioning systems are rated in BTU (British thermal units), a universal measure of heating and cooling. A general cooling rule of thumb calls for a minimum of 100 BTU per cubic foot of space. While there will be additional factors at play, from your boat’s location (and the temperatures you expect to encounter) to how well the space is insulated and the number of occupants, 100 BTU is a good place to start.
To calculate the cubic feet of the cabin or space you want to cool, multiply the length, width, and height (length × width × height = cubic feet). If the cabin has sloping sides, use the highest point for this calculation (6 feet of headroom is typical), and don’t worry about subtracting volume taken up by things such as furniture or equipment. Doing this lets you err on the side of more, rather than fewer, BTUs.
For example, a cabin 5 feet long, 5 feet wide, and 6 feet high is 150 cubic feet. Multiply the cubic feet of the space by 100 to find the recommended BTU. In our example, multiplying 150 x 100 gives us 15,000 BTU. Again, this is a baseline average – you can always adjust the BTU based on extenuating circumstances. For example, if you’re cruising in the tropics, increase the number of BTU by rounding up to a larger air conditioner.
- Power consumption. When shopping for a unit, you’ll need to confirm that your battery bank capacity is sufficient to power it properly. If not, you’ll have to increase battery capacity along with any required charging system upgrades. The best way to figure out your energy budget is to look at the power draw of your unit, then add to any additional power requirements plus an appropriate amount of reserve capacity.
- Reverse-cycle option. Many DC units are “reverse-cycle,” meaning they offer both cooling and heating. This is something good to consider if you boat is in cooler climates, as it reduces the need for a separate heater.