Using a boiler to heat your domestic water and storing it in a vessel called a cylinder is a clue that your hot water is provided by an indirect heating process. This also indicates that you have two main hot water systems consisting of a primary circuit and a secondary circuit. With the primary circuit, there is a pipe running from the boiler to a coil or heat exchanger (usually made from copper pipe) that sits inside the hot water cylinder and then returns to the boiler. This means that when the boiler runs, hot water flows continuously through the primary circuit. When the pipes that form coil of the heat exchanger become warm, the water stored in the cylinder also heats, and is able to be drawn off from the top of the cylinder so it can supply the hot taps around the house (this is the secondary circuit). A Kensington Plumber is trained to work on both systems. This hot water cylinder, normally located in an upstairs airing cupboard, is kept full of water from the cold-water storage systems in the loft space.

A primary circuit will have its own top-up supply of water that comes from a small tank in the loft and is known as a feed-and-expansion tank. Kensington Plumbers work on water storage tanks. The hot water draw off pipe runs up to the loft and ends at the cold-water storage cistern. The feed pipe for the heat exchanger from the boiler finishes over the top of the feed-and-expansion tank in the loft. This design means that any air or steam in either of the circuits will escape with no affect. The big advantage of an indirect system is that water passing through the boiler will not mix with the water in the cylinder and recycles time and time again.

The result is less buildup of scale in the boiler and also a corrosion inhibitor can be introduced which is beneficial for maintenance costs.
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