The bottom coil is normally connected to the renewable heat source, which in most cases will be solar thermal. By doing so, the heat can transfer into the cooler water at the bottom and make the best of the available solar energy. The upper coil then gets connected to the backup heat source, which can be any number of things, like a gas boiler, an oil boiler, an electric boiler or even a biomass boiler. If the solar energy can’t meet the hot water demand, the boiler can top the cylinder up to the required temperature.

Twin coil cylinders are available in both vented and unvented versions (both are detailed on the following webpage – Twin coil cylinders unvented or vented. If you go with an unvented twin-coil cylinder, you get mains-pressure hot water to all your taps, showers and baths. This makes it a very popular choice for both residential and large commercial properties, provided they have a decent incoming mains pressure and flow rate.

The main advantage of a twin coil hot water cylinder is the flexibility it offers. You can use renewable energy when it’s available and still have a reliable backup heat source. This should all help reduce the amount of time the boiler is on, lower your fuel usage and overall improve the efficiency of the system.

Twin coil cylinders are a great way to future-proof your hot water system too. For example, one coil can be used with the old boiler, and the second coil can be left free for solar thermal or another renewable heat source when the time comes.

So, to put it simply, a twin coil cylinder is just a hot water storage cylinder with two heat exchanger coils, designed to work with a pair of heat sources – in most cases, these would be solar thermal and a boiler.