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Do I Need A Direct or Indirect Unvented Cylinder?

The type of unvented cylinder will be determined on what boiler or heat source you have available to heat your water.

There are essentially two types of unvented cylinder, a direct unvented cylinder and an indirect unvented cylinder.


Electric Only (Direct Cylinder)

Direct Unvented Cylinders typically have dual electric immersions and suitable if you have no other way to heat your water

The term direct refers to the heating element, or immersion heater, that sits directly into the heated water.

Direct Hot Water Cylinder, Megaflo Cylinder

Gas & Oil Boilers (Indirect Cylinder)

If you have a gas boiler or oil boiler the unvented cylinder that is compatible with these boilers have a coil inside the cylinder so the gas or oil boiler can be used to heat the water “indirectly”.

They will nearly always have an immersion as a back-up as well.

The term indirect means the boiler water flowing around the coil from the boiler (shown below) is sealed separately from the stored water, the process is that the water temperature flowing around the coil from the boiler is of a higher temperature, it therefore transfers its heat into the hot water cylinder. Once the hot water cylinder reaches its target temperature (typically around 55°C) it will shut down the flow from the boiler to the coil so as not to overheat the water in the cylinder.

Indirect Hot Water Cylinder

Cylinder Selection Flow Chart

We have put together a simple flow chart underneath to help you determine whether you need a direct or indirect unvented hot water cylinder

The difference between a direct and indirect cylinder

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