2. Water Heating - indirect system.

In the illustration at right, two new components have been added to convert what was a 'direct' system to an 'indirect' system. The calorifier is a coil of copper tube inside the hot water storage tank, and the new feed/expansion tank keeps the calorifier (and also the boiler in the stove) filled with water.
XXWe now have two completely separate circuits, known technically as 'primary' and 'secondary.' The 'primary' is the closed heating circuit containing a trapped volume of water topped up as necessary by the feed/expansion tank. The 'secondary' supplies the hot-water taps.
XXIt is essential to adopt an indirect system when central heating is planned since panel radiators are made of mild steel and are subject to corrosion. In a direct system, rust particles would appear in the hot tap water. An indirect system eliminates this problem and also allows the water circulating through the primary circuit to be dosed with anti-rust and anti-freeze compounds. With this in place we can also now fabricate the boiler itself in mild steel. The cost-saving over copper or stainless is substantial particularly on the much larger size of boiler needed to drive radiators.