Winterizing Marine Air Conditioning

By Brian Gordon, Last updated:10/2/2020

Multi Stage 1

Preparing your boat for winter storage is something of a somber event, but it is necessary to protect and prolong the life of your investment. Along with preparing marine engines, potable water, and sanitation systems to withstand freezing conditions, marine air conditioning systems also require protection, since water in the system can freeze, resulting in damage to air conditioner heat exchangers and lines.

Marine Air Conditioners—The Basics Explained

As opposed to household air conditioners, which absorb heat from interior spaces and then radiate it to the outside air, marine air conditioners absorb heat from a boat’s interior and then transfer it by means of a heat exchanger to seawater, which gets pumped overboard. Typical system components include: a thru hull intake strainer, a seacock, in-line sea strainer, seawater pump, heat exchanger and an above-the-waterline thru hull discharge port. Water enters through the first thru hull, passes through the in-line sea strainer, and is pushed by the pump through the heat exchanger (where it absorbs the heat) and then gets discharged to the outside. When the system is not operating, water remains in the heat exchanger and lines. In cold weather, this water can freeze and damage the system if it has not been removed.

Seawater circulation and air conditioning components.

Antifreeze vs Compressed Air

Some mechanics and yards blow the water out of marine A/C units with compressed air. This process is quick and easy for them, but it might not be for you, especially if you do not have an air compressor or the appropriate fittings. You also run the risk of over pressurizing the system and causing damage. Compressed air can also leave some water behind which might still freeze and cause damage. For these reasons, we recommend that you displace the water with propylene glycol antifreeze.

About Antifreeze

There are two types of antifreeze. The first is common automotive antifreeze, such as Prestone, which is ethylene glycol. This type of antifreeze is highly toxic and should not be used for any winterizing application on a boat. In place of ethylene glycol, use non-toxic propylene glycol, like our West Marine brand. For winterizing applications, we recommend antifreeze with a freeze rating of at least -100°F. Why use -100°F antifreeze when the temperature never goes below -50°F? The answer is that what goes into the system as -100°F antifreeze, does not come out as -100°F antifreeze. There is always some residual water that lowers the concentration of antifreeze, so the resulting freeze point may be much higher than the rated temperature. The small cost difference between the -50°F, -60°F and -100°F antifreeze is a bargain for the peace of mind you get knowing that your systems will be safe, no matter what the weather.

West Marine propylene glycol antifreeze is recommended for winterizing potable water systems, air conditioning systems and all engines.

Even though Pure Oceans propylene glycol antifreeze is non-toxic, we do not recommend that you discharge it on to land, or into the water. After use, dispose of all used antifreeze in a manner consistent with federal, state and local regulations. For more information about the antifreeze we sell, read Antifreeze 101.pdf File

What You Will Need

Marine air conditioners can be winterized with the boat in or out of the water. In either case, you will need a friend to assist. Necessary tools and supplies include a shop vac, a screwdriver, a funnel, two gallons of propylene glycol antifreeze, and a length of 5/8” ID hose. In place of the hose and funnel, a much easier way to get the job done, is to use our Engine Winterizing Kit, which consists of a 5-gallon tank with a valve, 5/8” ID hose, fittings and instructions for winterizing engines. In addition to winterizing engines, you can also use this set up for winterizing marine air conditioners. It is also a smart idea to have some rags on hand to sop up any antifreeze that happens to spill.

Some Preliminary Tasks

Before beginning the actual winterization process, it is a good idea to clean the in-line sea strainer. If the boat is in the water, close the seacock, remove the top, take out the strainer basket, rinse it out, replace it, and screw the top back on. Go over the entire A/C unit with a brush and a shop vac to remove any dust or dirt that may have collected. If possible, use the shop vac to suck any collected dust out of the fins on the evaporator. You should also inspect the ducting and look for evidence of mold. Mold remediation is beyond the scope of this article, but you should be aware of any mold that exists, since it can lead to health problems. Now is the time to remove, clean and reinstall the original slide-in mesh air filter, or better yet, install a Breathe Easy Air Filter. These filters work much better than the factory originals, and improve air quality by attracting and retaining micro particles from diesel smoke, dust, lint, bilge odors and pet dander.

The Winterization Process

Step 1:

If your boat is in the water, start by removing the hose from the discharge thru hull port and run it into an empty one-gallon bleach or antifreeze bottle. Once you have done this, put the hose and bottle into the hands of your friend. You should also place an additional empty one-gallon jug within your friend’s reach in case it is needed. If your boat is out of the water, you can run the antifreeze solution completely through the system and out the outlet port, so in this case, position your helper on the outside of the boat, next to the outlet port with a bucket.

Before describing the next step, it is necessary to understand that marine A/C water pumps are not self-priming. This means that the antifreeze must be introduced into the system from a height above the highest point in the seawater circulation system. Using the Engine Winterizing Kit (mentioned above) makes this a lot easier, since you can place it on top of your boat’s cabinetry, which leaves your hands free. For the rest of this article, we will assume that you are using this kit in place of a funnel and a hose.

Step 2:

Fill the Engine Winterizing Kit tank with two gallons of antifreeze solution. The actual amount you need depends on the distance from the seawater strainer to the seawater exit port, but two gallons should be plenty. With the screwdriver, loosen the hose clamp on the inlet side of the sea strainer. Pull this hose off, and in its place push on the hose from the Engine Winterization Kit tank and secure it with the hose clamp. (If the hose from the tank has a fitting on it, you will need to remove it first. Also, if the hose from the Winterizing Kit does not fit, you will need to purchase a short length of vinyl hose that does fit, along with the appropriate nylon or plastic barbed hose reducer.)

Step 3:

Open the valve on the Engine Winterizing Kit tank to allow antifreeze to flow to the inlet side of the in-line strainer and further down line into the pump. Start the air conditioning unit. If the boat is out of the water, your friend (who is hopefully still standing outside by the outlet port!) will very soon be catching the antifreeze in the bucket, or if the boat is in the water, feel it surging into the bottle he or she is holding. As soon as there is an uninterrupted flow, the job is nearly done.

Step 4:

Turn off the air conditioner, remove the hose from the Engine Winterizing Kit, and then clamp the hose from the seacock back on to the inlet of the in-line sea strainer. If the boat is in the water, you will need to clamp the outlet hose back on to the outlet thru hull fitting. The job is now done!

When spring arrives, you can follow the procedures described above to replace the antifreeze in the system with freshwater before you launch. Dispose of used antifreeze properly to help keep our waterways clean!