Direct & indirect hot water cylinders: what’s the difference?

Direct cylinders, indirect cylinders, unvented cylinders…the list goes on. There are many different of types of hot water cylinder available, but what is the difference between direct and indirect hot water cylinders, and what do these terms actually mean?

The difference between direct and indirect hot water cylinders explained

The answer to this question relates primarily to how the water in the cylinder is heated. In an indirect cylinder, the water is heated by the boiler. In a direct cylinder, the water is heated by an electric element

Indirect cylinders

In a household with a cold water storage cistern and a feed and expansion cistern in the loft, and a gas or oil fed heat-only/regular boiler, the hot water cylinder contains a coil of copper pipe, which functions as a heat exchanger. A thermostat is strapped to the side of the cylinder.

When requested by the thermostat, the boiler fires up and hot water is pumped through the coil, heating the rest of the water inside the cylinder to around 55-60 degrees celsius. The two bodies of water – the water from the boiler and the domestic hot water – never mix. (If they do, this is the result of a broken coil.)

Note that the heat exchanger is not always a coil. In plumbing systems with a primatic hot water cylinder, the cold water storage cistern supplies the water for both the hot water cylinder and the central heating. The heat exchanger in a primatic cylinder is shaped in such a way which allows an air bubble to form. It is this air lock which prevents the two bodies of water from merging. Due to their fundamental design, primatic cylinders are always indirect.

Most indirect cylinders are fitted with one electric immersion heater so that even if the boiler isn’t working, the household still won’t be without hot water.

Direct cylinders

In a direct hot water cylinder, there is no heat exchanger – the water is heated directly by two electric immersion heaters only. Direct hot water cylinders go hand in hand with Economy 7 electricity tariffs. The lower immersion heater heats the bulk of the water on the cheaper night tariff, and on the standard rate, the top heater keeps the water up to temperature as it is drawn off throughout the day.

Fortic cylinders

Fortic cylinders are hot water cylinders which have their own integrated cold water storage cistern, instead of requiring a separate cistern that is usually in the loft. Fortic cylinders can be both direct and indirect.


Don’t confuse direct/indirect hot water cylinders with direct/indirect plumbing systems. The latter is a reference to a whole household’s plumbing system, namely as to whether the water outlets are supplied entirely by the mains, or from a cold water storage cistern. See this page for more information on that topic.

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