The heat conductivity of the insulation material is 0.045 W m−1 K−1. The overall heat loss coefficient of the tank in winter is measured to be 2.04 W m−2 K−1. An antifreeze-water heat exchanger is positioned in the water tank with a double helix structure. The exchanger is an immersion coil type and the material is DN32 PEX pipe with a total length of 56.52 m.