Unvented Hot Water Cylinders
Indirect Hot Water Cylinders are heated from an external source like a gas boiler. The water from the gas boiler is heated up, then sent around a coil that runs through the cylinder, which in turn heats the water up inside. Most indirect cylinders also have an electric element inside for backup. Heat Pump Cylinders are designed for use with an air or ground source heat pump as the main heat source.
Direct Hot Water Cylinders simply heat the water DIRECTLY themselves from an in built electric immersion element. No outside source or boiler can be used for this type of cylinder.