What is an indirect unvented cylinder?
What is an indirect unvented cylinder? An indirect unvented cylinder is a sealed domestic hot water storage tank heater fed directly by the mains water supply. It is “indirect” because it uses an internal heat exchanger coil connected to an external boiler or heat pump to heat the water, rather than heating it directly via an internal immersion element.
We split the term “indirect unvented cylinder” into its two separate parts.
Indirect Cylinder
The term “indirect” refers to how the water is heated. Instead of using an electrical immersion heater submerged directly in the water, the primary heat comes from an external source, typically a gas boiler, oil boiler, or even a heat pump. This external heat source warms a separate, closed loop of water (the primary circuit). This hot primary water is pumped through an internal heat exchanger, usually a coiled pipe near the bottom of the cylinder. The heat transfers through the metal coil (normally stainless steel) into the stored domestic hot water without the two water supplies ever mixing.
Most indirect unvented cylinders also feature a secondary electrical immersion heater as a backup. If your boiler breaks down, this direct immersion heater element can kick in to keep you supplied with hot water.
Unvented cylinder
Unlike traditional vented systems that rely on a cold-water storage tank in the loft to feed the cylinder, an unvented cylinder is connected directly to the incoming mains water supply. This delivers hot water at the same high pressure as your cold taps, providing excellent flow rates ideal for power showers and multiple bathrooms. Because the system is sealed, it requires specialised safety controls to manage the expansion of water as it heats up, including an expansion vessel and temperature/pressure relief valves.
Our duplex indirect stainless steel cylinders up to 500 litres in volume are detailed here: Indirect unvented cylinders
Or, our commercial and industrial indirect unvented cylinders (which can also be supplied in an open-vented configuration) are detailed here: Commercial indirect hot water cylinders
