The difference between direct and indirect cylinders

Direct, indirect, unvented…the list goes on. There are many different of types of hot water cylinder, but what is the difference between direct and indirect cylinders? In this article, we’ll discuss exactly what those terms mean.

The difference between direct and indirect cylinders

The answer to this question relates 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 electric elements known as immersion heaters.

Indirect cylinders

In a household with a cold water storage cistern and a feed and expansion cistern in the loft, and a regular gas or oil 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, hot water from the boiler is pumped through the coil. This heats the rest of the water in the cylinder. The thermostat should be set to 55-60°C. 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.)

However, the heat exchanger is not always a coil. In plumbing systems with a primatic hot water cylinder, the cold storage cistern supplies the water for both the cylinder and the central heating. The two bodies of water are separated by an air lock inside the heat exchanger. The air lock forms while the cylinder fills with water. This is known as priming the cylinder – hence the name.

Most indirect cylinders are still fitted with an immersion heater. This means that household still won’t be without hot water even if the boiler isn’t working. However, this should only be used as a temporary measure. Depending on it will have a big impact on your energy bills.

Direct cylinders

In a direct cylinder, there is no heat exchanger. Instead, the water is heated directly by two immersion heaters. Direct cylinders go hand in hand with Economy 7 electricity tariffs. These tariffs offer 7 hours of cheaper electricity at night. The lower immersion heater heats the whole cylinder on the cheaper night tariff, and the top one keeps the water up to temperature as it is drawn off throughout the day.

The difference between direct and indirect cylinders

So, to sum up the difference between direct and indirect cylinders:

  • Direct: the water is heated directly by immersion heaters.
  • Indirect: the water is heated indirectly by the boiler.

Fortic cylinders

Fortic cylinders have their own built-in cold water cistern. As a result, there’s no need for a separate cistern in the loft. They 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. In direct plumbing systems, water outlets in the home are supplied by the mains. In indirect systems, they’re supplied by the cold water tank. For more on that topic, see this page: Direct & Indirect plumbing systems: what’s the difference?