Marine Air Conditioning Guide: Systems, Sizing and Power
Twenty minutes of reading and two calculators. By the end you will know which type of system to shortlist, roughly what size, and whether your batteries can carry it through the night.
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How marine air conditioning works
A home split system dumps heat into the air outside. A boat dumps it into the sea. A small pump draws seawater through a strainer, pushes it across the unit's condenser and sends it overboard a few degrees warmer. That is why marine units are compact, why they work in a locker with no airflow — and why the seawater side is where most problems start.
Nearly every unit sold in Australia is reverse-cycle: flip the valve and it pulls heat from the seawater instead, warming the cabin. It works well down to about 7 °C water — fine for Hobart in winter, marginal in the Southern Ocean.
- 1Seacock and skin fitting
- 2Strainer
- 3Pump
- 4Condenser
- 5Evaporator and blower
- 6Overboard discharge
Six components, and every one of them is a place a fault can start. The fault-finding pages name which part each symptom points at.
Through-hull below the waterline, seacock, then a clear-bowl strainer you can see into.
A centrifugal pump, mounted below the waterline so it self-primes. Quiet, and the part that fails first.
Refrigerant sheds its heat to the water. Warm tropical seawater means less capacity — plan for it.
A trickle of warm water from a hull fitting above the waterline. No trickle, no cooling — it's your health check.
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Marine Air Conditioning System Types
Every marine air conditioner is one of these. The difference is where the compressor lives and what carries the cold to the cabin.
Compressor, evaporator and fan in one box under a bunk or settee, ducted to grilles. One unit per zone.
For: simplest install, cheapest, easy to service, one pump can feed two units.
Against: compressor noise is inside the cabin; each unit eats a locker.
Typical: 6,000–16,000 BTU · $4k–7k per unit installed
Compressor lives in the engine room or a lazarette; a small air handler with the fan sits in each cabin, joined by refrigerant lines.
For: much quieter where you sleep, air handlers fit in small spaces.
Against: refrigerant lines need a licensed tech to run and charge; more to go wrong.
Typical: 9,000–24,000 BTU · $6k–10k installed
One or more chillers cool a loop of fresh water; small fan-coil units in every cabin draw from the loop. Each cabin gets its own thermostat.
For: scales to any boat, quietest cabins, staged chillers match the load.
Against: cost, weight, and a plumbing loop through the whole boat.
Typical: 24,000 BTU and up · $15k–40k+ installed
A fourth path for cruisers: 12 V / 48 V DC self-contained units with variable-speed compressors. They run straight off a lithium bank with no inverter and throttle down once the cabin is cold. Dearer per BTU, but the only realistic way to sleep cool at anchor without a generator. Not sure? Take the two-minute quiz
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Boat Air Conditioning Sizing Calculator
Cooling is sold in BTU per hour (12,000 BTU ≈ 3.5 kW). The honest rule of thumb is cabin volume times a climate factor, then a margin for a dark hull and lots of glass. This gives you a shortlist number — an installer will refine it.
The same arithmetic is worked through in full on the sizing pages — by boat length, by capacity, by cabin count and by climate — each with the installed price and the shore-power budget that follow from it.
04
Marine Air Conditioning Power Calculator
A rough guide: a modern marine unit draws about 1.1 kW for every 12,000 BTU, plus 80–100 W for the seawater pump. Once the cabin is down to temperature a fixed-speed compressor cycles roughly 60 % of the time; a variable-speed one just slows down.
A 400 Ah 12 V lithium bank ≈ 4.8 kWh; 600 Ah ≈ 7.2 kWh. Lead-acid: halve it.
1.6 kW
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Marine Air Conditioning Installation Requirements
A self-contained installation may look compact, but the through-hull, 240 V, refrigerant and cabinetry boundaries still need the right trades. Splits and chilled water add more interfaces. Whatever you fit, these are the parts that decide whether it works in five years. Use the full installation planning guide before requesting quotes.
| Part | What to look for | Why it matters |
|---|---|---|
| Through-hull & seacock | Dedicated, bronze or composite, well below the heeled waterline | Sharing the engine intake starves one or the other under way |
| Strainer | Clear bowl, reachable without unpacking a locker | Weed and jellyfish are the number-one cause of a warm cabin |
| Seawater pump | Centrifugal, mounted below the waterline, sized for all units it feeds | Above the waterline it loses prime; too small and capacity drops in the tropics |
| Condensate drain | Gravity to a sump or a shower drain, never the bilge | A 16k unit makes litres of water a day in Queensland humidity |
| Ducting & grilles | Short, insulated, large — return air bigger than supply | Long crushed ducting is the usual reason a unit "isn't big enough" |
| Electrical | Dedicated 240 V circuit, RCD, soft-start if you plan to run on an inverter | Compressor start current is 3–5× running current |
Go deeper on duct and grille placement, condensate drainage, then take the installer and handover checklist to the boat.
06
Marine Air Conditioner Noise and Placement
Quiet marine air conditioning starts with a suitable system layout, correctly sized air paths and vibration isolation. A product decibel figure alone cannot predict the sound beside your bunk.
Compare sound measurements at the same distance, fan speed and operating mode. Cabin noise also depends on compressor location, duct airflow and vibration through the furniture. We do not have comparable cabin measurements for the products listed here.
Request an onboard demonstration or an installation-specific sound specification. Use the marine air conditioning noise checklist to record where noise is transmitted.
Cheap wins: mount the unit on rubber feet, line the locker lid, use flexible insulated ducting with one gentle bend, and oversize the return-air grille so the fan can run on low.
07
Reverse-Cycle Heating and Refrigerant Choices
Reverse cycle heating is close to free with a marine unit and it is efficient — roughly 3 kW of heat for 1 kW of electricity in 12 °C water. Below about 7 °C seawater the output falls away and a diesel heater is the better tool. For Sydney, Melbourne and Hobart winters aboard, a reverse-cycle unit is genuinely useful; for Darwin it is a feature you will never switch on.
Almost every current marine unit uses R410A. Newer designs are moving to R32, which is more efficient and has a third of the global-warming potential but is mildly flammable, so installers need the right ticket. Older self-contained units on R134a are still serviceable but losing capacity as the gas gets dearer. In Australia, anyone who charges or recovers refrigerant needs an ARCtick licence — ask to see it.
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