Hi Chris, I see from the directory you have a 44 Performance. The setup is likely to be very similar to our 45e. The hot water tank contains an immersion heater protected by a large integral anode, this, of course, gradually dissolves and when it has gone the heating element corrodes. The discolouration is rust. It is easily replaced. The Dufour 45e does not have an “anti-freeze line”
Andrew
The membership association for owners of Dufour Yachts