How do you properly winterise a boat's water heater and freshwater system?
I'm laying up my 38ft cruiser for the winter in a yard where temperatures regularly drop below freezing. I've winterised the engine, but I'm unsure about the best way to tackle the domestic freshwater system, especially the calorifier. How do I make sure nothing cracks over the winter?
Answer
To properly winterise your freshwater system, you must physically drain the water heater (calorifier) and bypass it, then pump non-toxic marine antifreeze through the remaining pipework. Leaving water in the calorifier or using the wrong antifreeze will split the copper tank or poison your water supply, resulting in a £500 to £800 replacement bill come spring.
Bypassing and draining the calorifier
Start by turning off the domestic water pump and switching off the 230V AC mains breaker to the immersion heater. Draining the tank with the power on instantly burns out the element. That is a £40 mistake. Locate the cold water inlet and hot water outlet on your calorifier. Disconnect both hoses. Modern European boats typically use 15mm Whale Quick Connect or Hep2O push-fit fittings. Older vessels might use reinforced hose and jubilee clips.
Connect the hot and cold hoses together using a 15mm double-ended bypass fitting. This costs about £5. Open the pressure relief valve (PRV) on the calorifier and the drain cock at the base. Let the tank drain completely into the bilge or a bucket. Surejust or Quick calorifiers usually feature a small knurled brass knob for the drain valve. Leave the drain open all winter.
Emptying the main water tanks
Do not pour antifreeze directly into your primary 200-litre water tanks. Achieving the correct freeze-protection concentration requires gallons of fluid. This wastes money and leaves a persistent chemical taste the following season. Empty the main tanks entirely instead.
- Open all the taps on board.
- Let the main water pump run until the tanks are empty and the taps start spitting air.
- Turn the pump off immediately to prevent the rubber impeller or diaphragm from tearing.
Pumping antifreeze through the system
Buy 5 to 10 litres of non-toxic marine antifreeze. This is propylene glycol. Manufacturers like Starbrite typically dye it pink. A 4-litre jug costs around £15. Never use automotive engine antifreeze (ethylene glycol). It is highly toxic and will poison your drinking water.
Disconnect the inlet hose from the tank side of your freshwater pump. Attach a short length of 15mm clear PVC hose to the pump inlet. Drop the other end directly into your jug of pink antifreeze. Turn the water pump back on. It will draw neat antifreeze straight from the bottle.