How do I winterise a boat heater to prevent freeze damage?
My yacht has a diesel cabin heater and a hot-water tank heated by the engine and shore power. She'll be ashore through winter with the electricity disconnected; what needs draining or antifreeze before the first frost?
Answer
To prevent freeze damage, drain and bypass the domestic hot-water tank and check the antifreeze protection in any liquid-filled heating circuit. A diesel warm-air heater normally needs no antifreeze because it contains no water, so identify your heater type first.
Identify the heater circuits
Check the model plate and trace the hoses before disconnecting anything. Boat owners use the word heater for three different installations, each with its own winter preparation:
- Diesel warm-air heater: units such as a Webasto Air Top or Eberspächer Airtronic burn diesel and blow heated air through ducts. There is no water circuit to drain.
- Hydronic heating: a Webasto Thermo Top or Eberspächer Hydronic circulates heated coolant through fan matrices, radiators or a calorifier coil. This circuit needs suitable antifreeze.
- Calorifier: an insulated domestic hot-water tank, commonly from SureCal or Isotemp, usually heated by an engine-coolant coil and an electric immersion element. Its drinking-water side needs draining or a winterising procedure expressly approved for it.
A calorifier's engine coil and domestic water are separate circuits. Draining the taps does not drain the engine coolant, and engine antifreeze does not protect the domestic water inside the tank.
Switch off and isolate the immersion supply, let the water cool and relieve pressure before opening the tank or disconnecting hoses.
Drain and bypass the calorifier
Start with the calorifier. A full tank can split even when the surrounding pipework has been treated. Switching off the pump or opening a tap alone does not reliably empty it.
- Switch off the freshwater pump and open a hot tap to release pressure.
- Open the calorifier's designated drain and admit air through the connection or valve specified by its manufacturer. Collect the discharge in a bucket where practical.
- If the installation requires hose removal, disconnect the domestic hot and cold hoses and let the tank empty completely. Check whether a non-return valve prevents drainage.
- Join the supply and delivery hoses with a suitable bypass connector and secure the connections. Some Beneteau installations specify a short length of 1/2-inch pipe; use the size that matches your hoses.
- Winterise the remaining freshwater plumbing with non-toxic antifreeze labelled for potable boat or caravan water systems. Open hot and cold outlets until treated liquid reaches each one.
Follow the label's concentration instructions. Some products are ready mixed and must not be diluted. A product's burst-protection rating is not necessarily its freeze point. Never put automotive antifreeze into drinking-water plumbing.