Marine Air Conditioning Installation: Sizing, Cost, and Layout

The industry has settled on a rule of thumb that has held up well over decades: roughly 14 BTU per hour for each cubic foot of conditioned space. That is the baseline, and the adjustments around it are where judgment enters.

A salon with large windows, heavy midday use, and southern exposure sits closer to 16 to 18 BTU per cubic foot — South Florida boats live at the upper end of that band. A below-deck cabin used mainly at night, on a vessel that is shaded or covered before anyone goes aboard, can often be served at 10 to 12. Applying a single flat figure to an entire vessel is how a boat ends up with a comfortable salon and a forward cabin that never quite cools.

Read that curve as a floor rather than a specification. It tells you the smallest unit that can possibly work in a shaded, night-use space. It does not account for glass, exposure, or a galley running at dinner time — and it is the reason we measure and calculate each zone rather than quoting from a length-overall table.

Once you know total capacity and zone count, architecture largely selects itself. There are only three real choices, and vessel size is the dominant variable in choosing between them.

This is the part of the project that consumes the most engineering time and appears nowhere in a brochure. A boat offers a fixed and awkward set of locations, and every one of them carries a constraint.

A marine air conditioning system running in a South Florida summer pulls a genuinely surprising volume of water out of the air — gallons per day across a multi-zone boat. That water has to go somewhere deliberate. When it does not, it finds the cabin sole, the bulkhead core, and eventually the joinery.

Manufacturers are specific about this, and their requirements exist for good reason. Following manufacturer installation requirements for the condensate drain, the line must not terminate within three feet of any engine or generator exhaust outlet, must not discharge into a compartment housing an engine or generator, and must not simply drain into a bilge unless it is properly routed to a sealed condensate or shower sump.

In practice we look for three things on every survey of an existing system: continuous fall along the entire drain run with no sagging low spots in flexible hose, a sealed and pumped sump rather than an open bilge discharge, and a pan with enough slope that it actually empties instead of holding standing water. Standing water in a drain pan is where corrosion and odour both begin.

Those ranges are wide because the equipment is rarely the largest line on the invoice. On a retrofit, labour and access dominate: pulling joinery to route ducting, cutting and bonding new through-hulls, running dedicated circuits, and reassembling interior finishes to a standard the owner will accept. A new zone in an open engine room is a fraction of the cost of the same zone behind a fitted stateroom. Cooling and canvas decisions tend to arrive together on a flybridge refit, because enclosing a bridge changes the cooling load you have just paid to size, and our guide to marine canvas and enclosures covers the fabric, fit and lead-time side of that same job.

The honest guidance we give owners is to spend on routing and access, and to be unsentimental about equipment brand. A well-routed system from a mainstream manufacturer will outlast a premium unit installed with a sagging drain line and a strainer nobody can reach.

Marine air conditioning touches three systems that can each sink or burn a boat: seawater through the hull, refrigerant under pressure, and AC electrical in a wet environment. That is why ABYC A-6, the consensus standard for marine refrigeration and air conditioning equipment, exists alongside the council’s electrical standards, and why we build to it rather than to whatever the previous installer left behind. Air conditioning is one line in a much longer refit conversation, and our 2026 marine service guide sets out how the trades sequence across a yard period.

The practical consequences are concrete. Every below-waterline penetration gets a proper seacock, not a ball valve on a nipple. Bonding and grounding follow the electrical standard so the seawater pump and unit chassis do not become part of a galvanic problem. Circuits are dedicated and correctly sized for compressor inrush. None of this shows up in the cooling performance on day one — all of it shows up in year five, and in the survey when the boat is sold.