If your water is heated by a boiler and stored in a cylinder, the chances are that the water is heated indirectly. This means that you have two hot water systems, the primary circuit and the secondary circuit. In the primary circuit, a pipe runs from the boiler to a heat exchanger (usually a coil of copper pipe) inside the hot water cylinder and then back to the boiler. When the boiler is running, hot water flows continuously around the primary circuit. As the pipes of the heat exchanger warm up, so too does the water in the cylinder, which can then be drawn off from the top of the cylinder to supply the hot taps around the house (the secondary circuit). A Wandsworth Plumber is familiar with both systems.


The hot water cylinder, usually found in an upstairs airing cupboard, is kept topped up with water from the cold-water storage systems in the roof. The primary circuit as its own top-up supply of water from a small tank in the roof which is known as the feed-and-expansion cistern. Wandsworth Plumbers work on cylinders. The cylinder’s hot water draw-off pipe continues to the roof and terminates at the cold-water storage cistern, the boiler’s hot feed pipe to the heat exchanger terminates over the feed-and-expansion cistern in the roof. This allows any air or steam in either circuit to escape safely. One of the major advantages of the indirect system is that the water that passes through the boiler does not mix with the water in the cylinder and, effectively, is recycled time and time again. As a consequence there will be less buildup of scale in the boiler and means also that a corrosion inhibitor can be added to the water, which leads to lower maintenance costs.

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