Choosing the Right Boat AC System Size: A Marina Professional’s Guide to Keeping Your Vessel Comfortable

When the summer heat hits Springs Marina and the surrounding waters of Long Island, having the right-sized air conditioning system can make the difference between a comfortable cruise and a sweltering ordeal. The general rule of thumb for sizing a pleasure boat is 14BTU’s/cubic foot, but selecting the perfect marine HVAC system requires understanding your vessel’s unique characteristics and intended use patterns.

Understanding BTU Requirements by Vessel Type

Different boat types and layouts require varying cooling capacities. General BTU sizing guidelines are provided for reference only and are not a substitute for a professional load calculation: Pleasure boats (average use): ~14 BTU per cubic foot. However, specific areas demand different calculations.

Use the formula of 16 to 19 BTU’s/cubic foot for areas that are used during the heat of the day that have a lot of sunlight coming in, such as a pilothouse, especially if the roof is not shaded or well insulated. For areas below deck that are primarily used after sunset such as a stateroom, you can use the formula of 10 – 12 BTU’s/cubic foot.

Self-Contained vs. Split Systems: Size Considerations

Self Contained DX systems are typically the best choice for boats up to 40 feet, due to their lower cost and ease of installation. A single unit can cool one or multiple areas with ducting and wyes to save space and cost. For larger vessels, Split Gas are often seen on boats up to 80 feet. The condensing unit of a remote system is generally installed in an engine room while the air handlers are mounted in the areas needing cooling.

Popular sizing options include MarinAire self-contained AC systems range from 6,000 to 24,000 BTU, with options for 110-120V, 208–230V, and inverter-powered setups. As a very general rule, a 16,000 Btu unit can adequately cool a vessel up to 35′ -38′, dependent on type and configuration.

Layout-Specific Installation Considerations

Proper placement significantly affects system efficiency. Typical spots are under the V-berth, in hanging lockers, under dinette seats, in cabinets or in outside dry lazarettes and the bottom of lockers. The equipment and peripheral components must fit in and/or be accessible in the selected location.

The larger sized units can supply conditioned air to multiple cabins or areas via ductwork incorporating duct splitters, while smaller units of between 4,000 Btu and 11,000 Btu are typically employed in individual cabins. This flexibility allows boat owners to customize cooling zones based on their vessel’s layout and usage patterns.

Climate and Usage Factors

Environmental conditions significantly impact sizing requirements. A boat in a temperate climate will require less BTU’s to cool than a boat in a tropical climate. We will define a Temperate Climate as: 95oF air, 85oF water, moderate humidity. Tropical Climate is defined as: 105oF Air, 95oF water, high humidity.

The water temperature also affects efficiency. The cooler the water, the more efficient the cooling process. So, if you are in a warmer water, like a shallow bay, your efficiency will be less than in deep, cool ocean water.

Professional Installation and Maintenance

When your marine AC system needs professional attention, whether for boat ac repair springs or new installation, choosing the right contractor matters. We’ve been delivering exceptional refrigeration and HVAC solutions for over 40 years. Proudly serving clients from Montauk to Manhattan, our licensed and insured team provides reliable, specialized services tailored to meet your needs.

Most HVAC contractors don’t actually understand marine systems. They’ll look at your boat’s refrigeration with residential knowledge and miss the marine-specific issues. They don’t know why your water-cooled condenser isn’t flowing properly, why your system short-cycles in a marine environment, or how to properly size a replacement for a boat’s power constraints.

Avoiding Common Sizing Mistakes

Make sure the selected capacity is not oversized for the applicable area. Over-sizing might cause damage to the electrical components and/or to the power supply due to frequent starting and stopping. Over-sized air conditioning will not effectively remove the humidity and may cause high humidity and encourage mold in the cabin.

Proper return air sizing is equally important. 6000 btuh to 9000 btuh needs 80-100 square inches (8X10″ to 10X15″) 11000 btuh to 16000 btuh needs 120-150 square inches (10X12″ to 10X15″) 20000 btuh to 24000 btuh needs 200 square inches or bigger.

Future-Proofing Your Investment

The new self-contained marine ac units are generally much quieter, more reliable and use less power than the Dometic 16000 btu marine air conditioner or Cruisair split systems that we are typically replacing. For customers with space concerns, we have been very pleased with Mabru Power Systems’ small marine air conditioners.

Modern marine AC systems offer reverse-cycle heating capabilities, making them year-round comfort solutions. They’re self-contained air conditioners and many feature reverse cycle heat pumps that can help you stay cool on warm days and warm on cold days in your boat’s cabin.

Selecting the right AC system size for your vessel involves careful consideration of cubic footage, usage patterns, climate conditions, and layout constraints. For the most accurate sizing, we strongly suggest consulting a local HVAC professional (marine or residential) or checking the boat manufacturer’s original specifications. With proper sizing and professional installation, your marine AC system will provide years of reliable comfort on the water.