Marine Air Conditioning: How Boat AC Works, How to Size It, and How to Keep It Cold
Ask any South Florida boater what breaks their summer and you'll hear the same answer: the AC. Not the engines. Not the electronics. The air conditioner. At Marea Marine we install, repair and supply marine air conditioning out of our Miami shop, and most of the "dead" units we see are perfectly healthy machines starved of seawater or power.
This guide walks through how a boat AC system actually works, how many BTUs your cabin needs, what it costs to run, and the fixes you can handle yourself before calling a tech.
How Marine Air Conditioning Works (and Why It Isn't Home AC)
Your house dumps heat into outside air. Your boat dumps it into the ocean.
Every marine air conditioner runs the same refrigerant loop as a home unit: a compressor squeezes refrigerant, an evaporator coil pulls heat out of cabin air, and a condenser rejects that heat. The difference is the condenser. On a boat, a seawater pump pulls raw water through a thru-hull and strainer, pushes it across a coil-in-tube condenser, and sends the warm water back overboard. Water carries heat away far better than air does, which is why a compact marine unit can cool a cabin that would need a much bigger air-cooled box ashore.
That design gives you two lifelines. Lose either one and the cabin warms up fast:
- Steady AC power from shore power, a marine generator, or a properly sized inverter bank.
- Uninterrupted raw water flow through the strainer, pump and condenser.
Types of Marine AC Systems
Self-Contained Units
Compressor, condenser, blower and controls share one chassis that tucks under a berth or settee. Ducting carries cold air to one or two vents. These run from about 5,000 to 24,000 BTU and suit most boats under 45 feet. Our marine air conditioning systems collection includes units like the Webasto BlueCool S6 (6,000 BTU) for a single stateroom and the BlueCool S20 (20,000 BTU) for a larger salon.
Split Systems
The condensing unit lives in the engine room while a separate air handler sits in the living space. You get a quieter cabin and more flexible placement, at the price of refrigerant lines run between the two.
Chilled Water Systems
Yachts in the 50-foot-plus range usually go chilled water. One central chiller cools a water-glycol loop that feeds an air handler in each zone, so every stateroom sets its own temperature. Installation is bigger, but so is the payoff. The Webasto BlueCool C-Series chiller covers 16,000 to 108,000 BTU; browse more in chiller systems.
Reverse-Cycle Heat Pumps
Most modern units reverse the loop to pull heat from seawater on cool mornings. It works well while water stays above roughly 50°F (10°C). Below that, you'll want an auxiliary heater.
How Many BTUs Does Your Boat Need?
Undersize and the unit never catches up on a July afternoon. Oversize and it short-cycles, leaving the cabin cold but clammy. Start with cabin volume (length × width × headroom, in cubic feet), then apply a climate factor. Large glass areas, dark hulls and a full crew push you toward the high end.
| Space (approx. volume) | Temperate waters | Florida, Gulf & Caribbean |
|---|---|---|
| V-berth or small stateroom (≈250 cu ft) | 5,000 BTU | 6,000 BTU |
| Mid stateroom (≈400 cu ft) | 6,000–7,000 BTU | 8,000–10,000 BTU |
| Salon on a 35–40 ft boat (≈700 cu ft) | 10,000–12,000 BTU | 12,000–16,000 BTU |
| Salon with large windows on a 45–55 ft boat (≈1,100 cu ft) | 16,000 BTU | 18,000–24,000 BTU |
Treat these as starting points. We run a full heat-load calculation before any install, and we're happy to run one for you.
Power Draw: Shore Power, Generators and Inverters
Air conditioning is usually the single biggest AC load aboard. Plan the electrical side first and the rest falls into place.
- Running load: a 16,000 BTU unit typically pulls around 1,500–1,800 watts once it settles. Check the nameplate; it rules.
- Startup surge: compressors can spike to 3–5× running amps for a split second. Soft-start kits tame this and let a smaller generator carry the load.
- Shore power: a single 30-amp/120V cord (3,600 W) handles one mid-size unit plus light loads. Two or more units, a watermaker and battery charging usually call for 50-amp service.
- Generator sizing: budget 1.5–2 kW of generator per 12,000 BTU of AC running at once. A boat running 32,000 BTU of air plus normal galley loads needs a 7–9 kW marine diesel generator, while a single 16,000 BTU unit runs happily on a compact set like the Phasor K3 6.5 kW. Our marine generator buyer's guide covers the math.
- Inverters: running AC from batteries works for short stretches with a big lithium bank and a heavy-duty marine inverter. Overnight on batteries alone rarely pencils out.
What Marine AC Costs
A self-contained unit runs roughly $1,500–$4,500 in hardware depending on BTU and brand. Add a seawater pump, strainer, thru-hull, ducting, grilles and a digital control, then installation labor. A single-zone retrofit typically lands between $3,500 and $8,000 all in. Chilled water systems on larger yachts start well into five figures. Prices vary by boat, so ask us for a quote with your make, model and cabin layout.
Maintenance That Keeps the Cold Coming
Most of our summer service calls trace back to water flow. Keep this routine and you'll skip most of them:
- Clean the raw water strainer weekly in season. Grass, jellyfish and barnacle larvae clog it fast. Spare baskets live in our strainers and baskets section.
- Swap the pump impeller once a year if your pump uses one. A tired impeller moves half the water and nobody notices until the unit trips. Grab a spare from replacement impellers.
- Check zinc anodes on the condenser circuit every haul-out. Replacements are in marine anodes.
- Clean the evaporator coil and return-air filter every season. Salt and mildew choke airflow and make the cabin smell like a locker.
- Flush the condenser with a descaling solution every one to two years to clear marine growth inside the tubes.
Troubleshooting Common Boat AC Problems
Blowing Air but Not Cooling
Look overboard first. No steady stream from the discharge? You have a water problem: clogged strainer, air-locked pump or worn impeller. Strong flow but warm air usually means a dirty filter, iced coil or low refrigerant. Refrigerant work needs a certified technician.
High-Pressure or High-Temp Fault
The control shut the compressor down to protect it. Almost always a flow issue. Clear the strainer, bleed air from the pump, and restart. If it trips again within minutes, the pump itself may be failing; a run-dry tolerant magnetic-drive pump like the KoolAir PM1000 is a common upgrade.
Unit Won't Start
Check the dedicated breaker, then the control display for a fault code. Low shore-power voltage at a busy marina can also stop a compressor from starting.
Water Dripping in the Cabin
The condensate drain is blocked or disconnected. Clear it and confirm it drains overboard or to a sump with a working pump, not straight into the bilge.
Marine Air Conditioning FAQs
How long does a marine air conditioner last?
With clean water flow and yearly service, 10–15 years is typical. Units that run hot because of starved seawater flow fail much sooner.
Can I run boat AC at anchor without a generator?
Only briefly. A large lithium bank and inverter can carry a small unit for a few hours, but overnight cooling at anchor almost always needs a generator.
Should I repair or replace an old unit?
If the compressor is healthy and parts exist, repair. Units over 12–15 years old that use phased-out refrigerant are usually better replaced with a modern, variable-speed model that draws less power.
Do you install marine AC in Miami?
Yes. Our technicians install, repair and maintain marine air conditioning across South Florida. See our marine services or call 305-964-8270.
Stay Cool This Season
A good marine AC system is only as reliable as the water and power feeding it. Keep the strainer clean, size the unit honestly, and match it to a generator that can carry the surge. Shop marine air conditioners, AC seawater pumps and service parts at Marea Marine, or book our team to handle the install.
