Boat Air Conditioning Systems and Maintenance

Boat AC has gotten complicated with all the systems and brands flying around. As someone who’s sweated through enough summers aboard, I learned everything there is to know about keeping cool on the water. Today, I will share it all with you.

Quick Answer: Marine AC systems are self-contained units that pull seawater through the condenser to cool things down. You’ll want to stay on top of your raw water strainers, clean those filters monthly, and winterize properly so you don’t end up with freeze damage. Nine times out of ten, when cooling drops off, it’s restricted water flow or a dirty air filter.

Understanding Marine Air Conditioning

System Types

So let’s break this down. Self-contained marine air conditioners pack everything — compressor, evaporator, condenser — into one unit. They usually sit under a bunk or tucked away in a locker. I’ve installed a couple of these myself, and honestly, they’re about as straightforward as marine projects get. That’s saying something, right?

Then you’ve got split systems, which separate the evaporator (the air handler part) from the condensing unit. The benefit? Way quieter in sleeping areas because the compressor isn’t sitting right there next to your pillow. The tradeoff is a more involved installation since you’re running refrigerant lines between components. Worth it if you’re a light sleeper, though.

Chilled water systems are the big-yacht solution. They push cooled water through air handlers all over the vessel. Think of it like a central HVAC setup for your home, but on a boat. They’re efficient for multi-zone cooling, but I won’t sugarcoat it — there’s more maintenance involved compared to the self-contained units.

Raw Water Cooling

Probably should have led with this section, honestly. This is the fundamental thing that makes marine AC different from your house unit. Instead of blowing air over the condenser like a regular air conditioner does, marine systems pump seawater through a heat exchanger. That’s what makes raw water cooling endearing to us boaters — it lets you pack serious cooling into a surprisingly compact package.

Here’s the flow path: seawater comes in through a through-hull fitting, runs through a strainer that catches all the debris and crud, moves through the condenser, and then gets kicked back overboard. Pretty elegant, actually. But if anything restricts that flow? You’re looking at reduced cooling capacity and, worst case, compressor damage from overheating. Don’t let it get to that point.

Regular Maintenance

Strainer Cleaning

I can’t stress this enough — check and clean your raw water strainers before every use. During the season, I check mine regularly because marine growth, shells, and random debris build up faster than you’d think. One weekend you’re fine; the next, you’ve got a strainer that’s half clogged. I always keep spare strainer baskets aboard so I can swap one in fast if cleaning the current one isn’t practical at the moment.

Air Filter Maintenance

Clean or swap out your return air filters every month if you’re running the AC hard. And believe me, in July and August, you’re running it hard. Clogged filters choke the airflow, which tanks your cooling capacity and cranks up the humidity. Even worse, dirty filters become a breeding ground for mold and mildew. Nobody wants to step aboard to that musty smell — it gets into everything.

If you’ve got washable filters, just rinse them with fresh water and a little mild soap, then let them dry completely before putting them back. Disposable ones? Grab the type your manufacturer specifies and toss the old ones. Don’t try to squeeze extra life out of disposable filters; it’s not worth it.

Condensate Drain

Your AC pulls moisture out of the air — that’s literally part of its job. All that condensate has to go somewhere, and if your drain lines are clogged or your pump isn’t working right, you’ve got a problem. Standing water in the drain pan is basically a mold factory, and it can overflow into your living space. I’ve seen it ruin cushions and carpet more than once. Just peek at those drain lines regularly and test your pumps.

Impeller Replacement

Raw water pump impellers don’t last forever. Plan on replacing yours annually or around every 2,000 hours, whichever comes first. What happens is they slowly degrade, and you’ll notice your cooling getting a bit weaker before the thing finally gives up entirely. By then you’re sitting in a hot boat wondering what went wrong.

Keep a spare impeller and the right tools on board. Seriously. Many units let you swap the impeller without pulling the whole pump out, so it’s actually a pretty quick fix if you’re prepared.

System Operation

Startup Procedures

First things first — open that raw water seacock before you fire up the AC. I know that sounds obvious, but you’d be amazed how often people forget. Check the overboard fitting to confirm water’s actually flowing through. Running the system dry will chew up your impeller and can overheat the compressor. Neither of those is a cheap fix.

Give the system a few minutes to settle in after you start it up. It might blow warm air initially while the refrigerant pressures balance out and the condenser gets up to temperature. Don’t panic and start jabbing at the thermostat — just let it do its thing.

Thermostat Settings

Look, I get it — when it’s 95 degrees and you step below, you want it arctic. But cranking the thermostat down to 60 just means your system never stops running. That burns more power, wears components faster, and honestly doesn’t cool things down any quicker. I keep mine between 75 and 78, which is comfortable without making the system work itself to death. Find your sweet spot somewhere in that range.

Shore Power Considerations

Here’s something that catches newer boaters off guard: marine AC units are power hungry. We’re talking 10 to 15 amps per 16,000 BTU unit. If you’ve got two or three units and you’re plugged into a 30-amp pedestal, well, you can do the math. Some marinas limit what you can draw, especially during peak afternoon hours when everyone’s running their AC. Plan your load accordingly, or you’ll be the one tripping breakers for your whole dock section.

Troubleshooting

Reduced Cooling

When the AC isn’t keeping up, most people jump straight to “I need a refrigerant charge.” Slow down. Check the basics first. How’s your strainer look? Is water actually discharging overboard? When’s the last time you cleaned the air filters? I’d say 80% of the cooling complaints I’ve dealt with over the years came down to basic maintenance stuff. Save yourself the service call and check the simple things first.

Water Leaks

Finding water inside? Before you freak out, it’s almost always a condensate drain issue, not a raw water leak. Check your drain lines for clogs, make sure the pump is kicking on, and verify you’ve got proper drainage slope. Now, if it IS raw water leaking — and you’ll know because it’ll be salty — that needs your immediate attention. Saltwater will eat through surrounding materials in no time.

System Won’t Start

Okay, so nothing’s happening when you flip the switch. Walk through it methodically: Is power actually getting to the unit? Check your breakers. Is the thermostat set below the current temperature? You’d be surprised. Also, a lot of modern units have a built-in safety lockout that prevents them from restarting right after a shutdown. Wait three to five minutes and try again before you go tearing things apart.

Winterization

Freeze Protection

If you’re in a climate where it freezes — and even some “mild” climates get surprise cold snaps — you absolutely must winterize your marine AC. Water left in the system will freeze, and frozen water cracks condensers and destroys pumps. I’ve seen the repair bills. They’re ugly.

You’ve got two options: drain the raw water system completely, or push non-toxic antifreeze through the cooling circuit. Either way, disconnect and drain pump volutes where water likes to hide. And close those seacocks so no water sneaks back in over the winter.

Mold Prevention

Before you button everything up for the season, run the fan without cooling for a while. This dries out the evaporator coils and drain pans. I also hit interior surfaces with a mold inhibitor — it makes a noticeable difference come spring. Leave your access panels cracked open too, so air can circulate while the boat’s in storage. Trust me, future you will be grateful when you’re not scrubbing mold in April.

Upgrade Considerations

If you’re thinking about upgrading, modern inverter-driven air conditioners are a real step up. They run at variable capacity, meaning they throttle down when full blast isn’t needed. The result? Lower energy consumption, less noise, and more consistent temperatures. I’ve been on boats with the newer inverter units and the difference in cabin comfort — and noise level — is honestly impressive.

One more thing when you’re shopping for a new system: don’t just go biggest-you-can-fit. A properly sized unit runs more efficiently and actually dehumidifies better than an oversized one that keeps cycling on and off. Talk to someone who knows your boat’s layout and insulation situation before you buy. It’ll pay off in comfort and energy savings for years.