If your yacht spends the cooler months docked or hauled out, winterizing your marine air conditioning system is one of the smartest maintenance steps you can take. Without it, stagnant water sitting in the lines can freeze, corrode, or grow bacteria. A little preparation now saves you from costly repairs and keeps your boat ready for smooth cruising when the season returns.
Why Winterize Marine Air Conditioning?
Your AC works by pulling heat from inside the boat and transferring it into seawater. That means seawater is constantly flowing through strainers, pumps, hoses, and heat exchangers. Once the system is shut down for the season, that water just sits. If temperatures dip below freezing, it can damage parts of the system. Even if you never see a freeze, leftover water creates the perfect breeding ground for algae and mold.
Winterizing clears the system of water and replaces it with propylene glycol antifreeze. This non-toxic antifreeze protects the components without harming the environment.
The Tools and Supplies You’ll Need
Here’s what we recommend gathering before you start:
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2 to 5 gallons of propylene glycol antifreeze (rated to -100°F)
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A funnel or engine winterizing kit
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Screwdriver for hose clamps
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Short length of 5/8” ID hose for connections
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Bucket or jug to catch discharge
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Rags for cleanup
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Shop vac for cleaning strainers and coils
Having these on hand makes the process smoother and prevents rushing around mid-job.
Step-by-Step: How to Winterize a Marine AC System
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Clean the sea strainer – Shut the seacock, remove the strainer basket, rinse it, and reinstall.
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Prep the AC unit – Vacuum dust off the coils, check filters, and look for mold in the ducting.
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Disconnect intake hose – Loosen the clamp on the inlet side of the strainer and attach a hose from your antifreeze container.
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Introduce antifreeze – Fill the container with antifreeze, open the valve, and let gravity feed the liquid toward the pump.
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Run the system – Turn on the AC until pink antifreeze flows from the discharge port. This confirms the entire system is protected.
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Reconnect hoses – Reattach the original intake hose and secure all clamps.
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Shut it down properly – Turn off the breaker and leave the seacock closed.
That’s it. You’ve displaced every bit of raw water with antifreeze, and your system is safe until spring.
How to Winterize a Closed Cooling System on a Boat
Closed cooling systems circulate coolant rather than raw seawater. To winterize:
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Top off coolant with the manufacturer’s recommended mix.
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Inspect hoses, clamps, and heat exchangers for leaks.
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Run the system briefly to circulate coolant.
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Protect the seawater side of the loop with antifreeze if applicable.
This keeps both the coolant side and seawater side protected during downtime.
How Many Gallons of Antifreeze Do I Need to Winterize My Boat?
Most AC systems take 2 to 5 gallons of antifreeze depending on the size of your yacht and length of hose runs. Always buy a little extra so you can flush until solid pink fluid discharges overboard. That way you know the system is completely protected.
Do you leave antifreeze in boat engine over winter?
Yes, you should leave propylene glycol antifreeze in your boat engine over the winter. Once the antifreeze is pumped through the raw-water side of the cooling system, it stays in there during storage. That’s the whole point—it replaces any seawater so nothing can freeze, expand, or corrode inside your engine or AC system.
A couple of key points:
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Use the right antifreeze: Only use non-toxic, propylene glycol antifreeze (the pink stuff), never automotive antifreeze. It’s safer for marine environments and won’t damage seals and gaskets.
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Run the engine until antifreeze comes out the exhaust: This ensures the antifreeze has fully circulated and displaced all raw water.
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Don’t drain it afterward: The antifreeze needs to stay inside the system all winter to provide freeze protection and corrosion resistance.
Come spring, you’ll flush it out with fresh water before launching or running the system again.
What is the best antifreeze for winterizing boat engines?
The best antifreeze for winterizing boat engines is propylene glycol-based antifreeze that’s specifically labeled as marine or RV antifreeze. It’s non-toxic, safe for seals and gaskets, and much better for the environment than the green or orange automotive stuff (ethylene glycol), which is highly toxic and should never be used in boats.
Here’s what to look for:
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Non-toxic, propylene glycol – This is the “pink” antifreeze you see sold for marine and RV use.
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-50°F vs -100°F protection – Many boaters grab -50°F, but I recommend -100°F for engines and AC systems. When mixed with any residual water, the protection level drops, so the extra margin is worth it.
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Corrosion inhibitors included – Quality marine antifreeze not only protects from freezing but also coats internal metal surfaces to fight corrosion during storage.
A few popular choices boat owners trust are:
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Star brite -100°F Marine/RV Antifreeze
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Camco -100°F Marine Antifreeze
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West Marine Pure Oceans -100°F
The main thing is to make sure it’s propylene glycol, non-toxic, and rated for at least -100°F if you want the safest coverage.
Should you cover an air conditioner compressor in winter?
For a home AC compressor, people often ask if it should be covered in winter. On a boat, though, your marine air conditioning compressor is indoors, usually tucked in the engine room or another protected space, so you don’t need to wrap or cover it like you would with a backyard unit.
That said, there are still smart steps you can take to protect it during the off-season:
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Winterize the raw water side with antifreeze so no water is left in the pump, hoses, or heat exchanger.
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Turn off the breaker so the unit isn’t accidentally powered up while it’s laid up.
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Inspect and clean coils, filters, and ducting so you’re starting fresh in spring.
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If you haul out or store in freezing climates, some boaters also remove the seawater pump impeller and store it in a dry place to prevent it from deforming.
So no, you don’t need a “cover” for your marine AC compressor. Winterizing properly does the protecting for you.
Do bilge pumps need to be winterized?
Yes, bilge pumps do need a little winter prep, especially if your boat is stored in a climate where freezing temps are possible. The pump itself doesn’t usually need antifreeze, but the hoses and low spots where water can sit do. If that trapped water freezes, it can crack the hose or fittings and leave you with a nasty surprise in the spring.
Here’s what to do:
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Run the pump dry before storage so there’s no standing water left inside.
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Pour a little marine antifreeze (propylene glycol, same as you’d use for your AC or water system) into the bilge. Then cycle the pump briefly so the antifreeze runs through the hoses. This coats the lines and prevents freezing.
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Check the float switch while you’re at it. Make sure it’s clean and moves freely so you know it’ll work when needed next season.
It’s a quick step that protects against cracked hoses and gives you peace of mind.
Do I Need to Do Anything to Winterize My AC Unit?
Yes. Beyond running antifreeze through the raw water side, it’s smart to:
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Clean or replace air filters.
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Vacuum dust and debris off coils.
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Inspect ducting for mold.
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Lubricate or remove the seawater pump impeller for storage, as recommended by manufacturers like Dometic.
This attention to detail keeps your system clean and prevents surprises in the spring.
What Is the Most Popular Method of Winterization for Boats?
The most common and effective method is the antifreeze flush method. Pumping pink, non-toxic propylene glycol antifreeze through the raw water side until it flows out the discharge line ensures all water is displaced. While some boaters use compressed air, antifreeze is safer, more thorough, and recommended by manufacturers.
Extra Pro Tips
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Label seacocks and breakers so you don’t forget what’s been shut off.
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Use -100°F antifreeze to account for dilution with any leftover water.
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If your boat has multiple AC units, winterize them all individually.
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Schedule coil cleaning, impeller inspection, or professional servicing during the off-season for maximum system longevity.
Winterizing your marine AC system doesn’t take long, but it’s one of the most important things you can do to avoid costly repairs. At Coastal Air Systems Marine, we take care of the full process for yacht owners across Palm Beach, Jupiter, Boca Raton, and Fort Lauderdale. Although Florida doesn’t get temperatures cold enough to require winterization, many boat owners live part of the year up in Northern states, where winterization is definitely needed!
Whether you prefer us to handle it or want guidance to do it yourself, we’re here to make sure your comfort and investment are protected.