How To Winterize Boat Plumbing

While most of the BoatUS Marine Insurance freeze claims involve the engine or exhaust manifolds, that's not the only place where freezing water can cause serious damage. Plastic plumbing fittings, pumps, and marine heads can all be cracked by ice. Pipes, valves, and pumps in potable water systems can freeze and split open. As with engines, winterizing the plumbing systems aboard consists of replacing the water with antifreeze.

Fresh Water System

Simply draining the freshwater system isn't enough, as water will almost certainly have pooled somewhere, and will freeze over winter. Running antifreeze through the system will ensure that there is no water in the system to freeze. Keep in mind that engine coolant (ethylene glycol) should NEVER be used in a freshwater system, as it is very toxic and cannot be reliably purged from the system in the spring. Here's how to winterize a freshwater system:

1. The dockside freshwater hookup, if you have one, should be shut off on shore and the hose drained and stowed.

2. Go below and open all water outlet spigots to drain the onboard freshwater tank(s). When water sputters from the outlets, close them and pour two or three gallons of nontoxic propylene glycol antifreeze (more if you have a hot water heater) into the tank. Note: If you drain the water heater and rig a bypass from the inlet to the outlet, you'll save a lot of antifreeze.

3. Open the outlets farthest from the tank and run until the antifreeze flows out. Close, and work backward toward the tank, repeating the procedure at each outlet — you may need more antifreeze than you think. Do this with both the hot and cold outlets.

If the boat is in the water, pour anti-freeze into the sink drains and close the seacocks. If the boat is on the hard, open all seacocks to make sure they've drained completely, then close all but the cockpit seacock. If there is water in the shower sump, drain it too.

Marine Heads

Heads without holding tanks:

  • Pour disinfectant into the bowl, and pump throughout the system.
  • Close the intake seacock, disconnect the hose, and put it in a bucket of nontoxic antifreeze.
  • Pump the antifreeze through the head, reconnect the hose, and close the remaining seacock.

Note: Manufacturers of some heads, such as the Raritan PH II, advise against using nontoxic antifreeze because it may soften the gaskets. If you use toxic antifreeze, do not pump it overboard. Detach the outlet hose for the head and pump it into a bucket, then recycle it at your marina.

Heads with holding tanks:

  • Empty the holding tank and pump disinfectant and then antifreeze through the bowl and into the tank (and through the “Y” valve if you have one). Close all seacocks.

Marine sanitation systems:

  • Consult owner's manual.

Air Conditioners

There are two ways to winterize an air conditioner. You can drain the system, including the raw-water pump and strainer, if that's possible. But as with freshwater systems, enough water often remains in low points of the system to cause damage. The better alternative is to remove the raw-water intake, and place the hose in a bucket of propylene glycol antifreeze and run the pump until you're certain antifreeze comes out of the discharge line. No need to run the air conditioning, but clean out the air filter and raw-water filters first so that the whole system is ready to go come spring.

Bilge and Other Pumps

If your boat is in the water, you don't need to (or want to) winterize your bilge pump(s), but they need to be checked so you know they're working. If your boat is being stored ashore, run antifreeze through them or blow out the discharge lines if you can. Most centrifugal bilge pumps are self-draining and don't need any special care. Make sure that anchor washdown pumps, live well pumps, and any other raw water pumps don't have water left in them. Don't forget to sponge out live wells, fish boxes, lockers, and other places where water might have accumulated.

Photo of digging out of the snow and icePhoto: Jack Hornor

No one wants to end up like the poor guy in the photo above! So don't plan on staying away too long. Visit your boat every few weeks to make sure lines are secure, drains haven't become clogged, bilges are dry, etc. Checking the boat is especially important after heavy storms or extended cold spells. If you have friends at the marina, arrange to check each other's boats whenever possible. 

— Published: September 2014

Seaworthy, the damage-avoidance newsletter, is brought to you by the BoatUS Marine Insurance Program. For an insurance quote, please call 1-800-283-2883 or apply online at


Photo of a bottle of marine antifreeze

Antifreeze is antifreeze, right? Wrong. Not only are there different kinds, the temperature at which they freeze is different, too. And that's important because if they do freeze, your boat is likely to be damaged. Antifreeze for winterizing engines and freshwater systems is not the same as the coolant you put in your engine's heat exchanger (the boat equivalent of a radiator). Ethylene glycol, the chemical used in coolant, is highly toxic, and winterizing antifreeze must be nontoxic because it goes in drinking water systems and may eventually go overboard. The antifreeze you use in your engine and plumbing systems aboard must also have the proper freeze protection to keep your systems safe at the lowest possible temperatures your boat may face.

Choose antifreeze that contains propylene glycol, which is safe and tasteless, and one that has rust inhibitors for engine protection. Most importantly, check the freeze rating, but be aware that the numbers used don't correspond to what you may think. A typical antifreeze labeled for minus 50 degrees Fahrenheit will only protect PVC water pipes from bursting down to about minus 10 degrees and will actually begin freezing at about 15 degrees above. At 50 below, it's a solid chunk of ice, capable of cracking an engine block. Play it safe and use an antifreeze that is rated well below any temperature you're likely to experience.

Most winterizing antifreeze must be used full strength — don't dilute. When pumping antifreeze into your engine or freshwater systems, don't shut down as soon as you see the antifreeze come out of the faucet or engine exhaust. The antifreeze first mixes with the water already in the system and has to displace all of it before the antifreeze can protect fully. It's better to invest in a couple of extra gallons than end up with a cracked block at the beginning of next season.