How to Winterize (and Recommission) Your Boat's Freshwater System

Turn off the dockside water at the shore spigot, then drain and stow the hose. Switch off the water pump breaker. Switch off the water heater breaker and put a piece of tape on it that says "HEATER EMPTY – DO NOT TURN ON."

Turn the pump back on and open the faucets until the tank runs dry and the pump starts sputtering. Then turn the pump off right away; do not let it run dry for long. If the tank has a drain or an inspection port, use it. The pump pickup sits above the bottom of the tank, so sponge or vacuum out the last bit.

Let the heater cool if it was on. Open its drain plug or disconnect a hose and let it empty into the bilge or a bucket. Then set the bypass valves, or disconnect the inlet and outlet hoses and join them with a short bypass hose. This keeps antifreeze out of the heater tank, which would otherwise swallow gallons of it. If the heater is also warmed by the engine's cooling system, that side is protected when you winterize the engine.

Remove the pump's inlet strainer bowl, clean it and drain it. Discover Boating notes that water does not flow through the accumulator, so pumping antifreeze through the lines puts almost none into it: disconnect it and drain it, shaking it empty if needed. Take out any filter cartridge and empty the housing. Do not pump antifreeze through a carbon filter; plan to replace the cartridge in spring.

Two common setups:

• Feed the pump directly: disconnect the hose on the pump's inlet side (or open the valve of a pump winterizing kit) and put a short hose from the pump into an antifreeze jug. This keeps antifreeze out of the tank.
• Pour it into the tank: with the tank empty, pour in a few gallons of antifreeze as BoatUS describes, then let the pump draw it. You will need more antifreeze this way.

Turn the pump on. Start at the outlet farthest from the pump. Open the cold side until antifreeze comes out, then the hot side (with the heater bypassed, the hot lines still need it). Close it and move one outlet closer each time. Keep an eye on the jug and swap jugs before it runs dry so you do not suck air.

The first color you see is antifreeze mixed with leftover water. Keep the outlet open until it runs the full, strong color of the product in the jug. Xylem lists "stopping as soon as you see pink" as one of the most common winterizing mistakes.

Run antifreeze through the shower heads (hold the wand in the drain), the cockpit or transom shower, any freshwater washdown and a freshwater-flush toilet. For an ice maker or a water-using appliance, follow its manual. The dockside city-water inlet usually has a check valve and a short line that can hold water: check your system's instructions for how to clear it.

If the boat stays in the water, pour a little antifreeze into each sink drain and close the drain seacocks. If the boat is on the hard, BoatUS suggests opening the seacocks so they drain completely, then closing all but the cockpit seacock. Empty the shower sump box and its pump; a pump never leaves the sump fully dry, so sponge it out. The head and holding tank are covered in the marine head guide.

Turn the pump breaker off. Leave the heater bypass in place. Put a tag at the helm or electrical panel listing what you did: "Freshwater winterized, heater bypassed and empty, filter removed." Write the date in BoatMend so you know what to undo in the spring.

Reinstall the strainer bowl and accumulator connections. Fill the tank with clean water. Leave the heater bypassed for now and run every outlet, hot and cold, until the water comes out clear with no color or sweet taste. Propylene glycol is non-toxic, but you still want it gone. Where you can, catch the first strong pink rinse in a bucket instead of sending gallons through the sink drains at the dock, and dispose of it the way your marina and local rules require; Xylem asks owners to follow local regulations for any antifreeze disposal.

Close the heater drain, remove the bypass (or set the valves back to normal), then open a hot faucet and run the pump until the heater fills and water flows steadily with no air spurts. Only then remove the tag and switch the heater breaker back on. Powering an empty electric heater ruins the element.

Bleach is the usual choice. Use plain, unscented household bleach in the amount your boat builder or water-system maker specifies. A widely published RV and marine method is about 1/4 cup per 15 gallons of tank capacity, mixed into a gallon of water first, with about 4 hours of contact time (typical; confirm in your manual). Pour it in, top the tank up, run every outlet until you smell chlorine, then let it sit. Drain, refill and flush until the chlorine smell is gone. Install a new filter cartridge after sanitizing, not before.

With every faucet closed, the pump should run until it reaches pressure and then stay off. If it cycles on and off by itself, there is a leak or a bad check valve. Look under the pump, at the heater fittings, behind sinks and under the shower. A crack found now costs a fitting; found in July it can cost a soaked cabin.