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How To Winterize Boat Plumbing

Updated: Dec 3, 2018

While most of the 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

This would be a good time to see if ice and freeze coverage is included in your boat insurance policy. Even the most competent do-it-yourselfer can make a mistake, and in many policies, ice and freezing damage is excluded unless you purchase special coverage (for a modest cost).

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 beca