You really don't want to wait until the first hard freeze to start winterizing boat ac system components, because by then, the damage might already be done. If you've ever had to replace a cracked heat exchanger or a split water line, you know exactly how expensive a little bit of leftover water can be. It's one of those chores that isn't exactly "fun," but it beats the heck out of writing a four-figure check to a marine mechanic come springtime.
Most people think about the engine and the plumbing first, but the air conditioning unit often gets overlooked. Since these systems use raw water—that's the stuff you're floating in—to cool the refrigerant, they are incredibly susceptible to freezing. When that water turns to ice, it expands. Since your AC's plumbing is made of rigid copper or plastic, something has to give. Usually, it's the most expensive part of the unit.
Why the Raw Water Loop is the Real Problem
The main reason we focus so much on winterizing boat ac system parts is the raw water loop. Unlike your AC at home, which uses air to pull heat away, a boat's AC pulls in water from outside. This water runs through a pump, a strainer, and then through a condensing coil.
Even if you drain your boat's main tanks, there's almost always a "belly" in the AC hoses or a pocket of water sitting inside the pump head. If that water stays there and the temperature drops below 32°F, you're looking at a recipe for disaster. The goal here is simple: get the water out and get the "pink stuff" (non-toxic marine antifreeze) in.
Getting Your Supplies Ready
Before you crawl into the bilge or start pulling hoses, make sure you have everything you need within arm's reach. There's nothing worse than being halfway through a messy job and realizing your screwdriver is on the dock.
You'll want a few gallons of non-toxic propylene glycol antifreeze. Make sure it's the pink stuff rated for plumbing and engines, not the toxic green stuff you put in a car. You'll also need a bucket, a short length of garden hose or reinforced PVC hose that fits your sea strainer, and some basic hand tools like a screwdriver or a nut driver for those hose clamps.
It's also a good idea to have some towels or an oil-absorbent pad handy. Even if you're careful, you're going to spill a little bit of water or antifreeze, and keeping the bilge dry is just good practice.
Cleaning the Strainer First
One mistake I see people make when winterizing boat ac system setups is skipping the strainer cleaning. You're already there, and you're about to run antifreeze through the whole line. If the strainer is full of grass, mud, or those tiny little barnacles, you're just going to push that junk further into the system or restrict the flow of your antifreeze.
Pop the top off the strainer and dump the basket. Give it a good rinse. While you're at it, take a look at the O-ring on the strainer lid. If it looks cracked or flat, replace it now. It's a five-dollar part that will save you a massive headache next season when the pump won't prime because it's sucking air.
The Antifreeze Flush Method
There are a couple of ways to do this, but the "gravity feed" or "bucket method" is usually the most reliable for DIYers.
First, you need to close the seacock (the valve that lets water into the boat). If you don't do this, you're just going to pump antifreeze out into the lake or ocean, or worse, you'll just be fight against the water pressure. Once the valve is shut, disconnect the hose from the output side of the strainer or the input side of the pump.
Connect your "winterizing hose" to the pump intake and put the other end in your bucket of pink antifreeze.
Running the Pump
Now comes the part where you need a little coordination. Turn on the AC unit and set the thermostat as low as it goes so the cooling cycle kicks in. The AC pump should start humming and begin pulling the antifreeze out of the bucket.
Pro tip: Watch the thru-hull fitting on the side of the boat where the AC water usually discharges. You're looking for a solid stream of bright pink liquid. As soon as you see that pink color coming out of the side of the hull, you know the entire line is protected. Turn off the AC immediately. You don't want to run the pump dry once the bucket is empty, as that can damage the impeller.
The Pour-In Method
If your strainer is located above the waterline and is easy to get to, you can sometimes just pour the antifreeze directly into the strainer housing while the pump is running. You'll need a helper for this. One person stays at the electrical panel to flip the switch, and the other pours the pink stuff in as fast as the pump sucks it down. It's faster, but it can be a bit more splashy, so keep those towels ready.
Don't Forget the Condensate Pan
While most of the focus of winterizing boat ac system routines is on the water lines, the condensate pan needs love too. This is the tray underneath the evaporator coils that catches the drips when the AC is running in the summer.
Usually, this pan drains into the bilge or a sump box. However, if the boat isn't perfectly level on its winter blocks, water can pool in a corner of that pan. If it freezes, it can crack the plastic tray or the drain fitting.
I usually like to pour a cup or two of antifreeze directly into the pan. This ensures that any standing water is displaced and that the drain line itself is filled with antifreeze. It also helps kill any funky smells or slime that might have built up over the humid summer months.
Checking Your Hoses and Clamps
Since you're already poking around the AC unit, this is the perfect time to do a quick safety inspection. Check every hose connection. Are the hose clamps rusty? Most boat builders use stainless steel clamps, but even those can corrode over time if they're constantly exposed to salt air.
If you see a "trail" of dried salt or crusty green stuff near a fitting, that's a sign of a slow leak. Tighten the clamp or replace the hose if it feels brittle. It's much easier to fix this in November when the boat is high and dry than in July when you're out on the water and the bilge alarm starts screaming.
Dealing with Multiple Units
If you're lucky enough to have a boat with multiple AC units (say, one for the salon and one for the master stateroom), you need to make sure you've flushed all of them. Often, these units share a single large pump and a manifold.
When winterizing boat ac system setups with manifolds, you might need to run each AC unit individually to ensure the antifreeze makes it through every branch of the plumbing. Don't assume that because you see pink coming out of one thru-hull, the other one is safe. If there's an air pocket in one line, the water might just stay there, waiting to freeze and cause trouble.
Wrapping it Up
Once you've got pink slush or liquid coming out of all the discharges and you've treated the condensate pans, you're pretty much set. I usually like to finish by closing the seacock tightly and hanging a little tag on the AC control panel that says "WINTERIZED." It sounds silly, but come spring, you might forget you closed that valve. Trying to run the AC with a closed seacock is a great way to fry a pump in about thirty seconds.
Taking the time for winterizing boat ac system components properly is just one of those "responsible boat owner" things. It doesn't take more than an hour or so once you get the hang of it, and it provides a lot of peace of mind when the snow starts flying. You'll sleep a lot better knowing your heat exchanger isn't turning into a block of ice while you're warm and toasty at home.
Now, go grab a heater and start on the rest of the boat—there's always something else to do, right?