The boiler is made of GL-180M silicone injected, gray cast iron giving it excellent corrosion resistance, flexibility and high thermal conductivity. The double wall sectional construction allows for a high efficiency three pass heat exchanger. Flue ways are sealed gas-tight with tongue and groove designed sections and elastic high temperature sealing rope. The unique water flow pattern eliminates thermal shock by using supply water to preheat the return water before it enters the main heat exchanger. Under normal conditions, there is no minimum return water temperature and no minimum flow requirements.