Open vented hot water cylinder or tank has a cold water storage cistern in the loft, which supplies the hot water tank when needed. This type of system uses gravity to supply cold water to the cylinder and the head of water in the supply cistern to create the pressure in the system. The water pressure therefore tends to be lower than in an unvented system. It is possible to connect a new cylinder to an open system if it is of the correct type, i.e. a vented cylinder.
 

Unvented – This tends to give much higher pressures than an open system as the heating system is completed sealed and water is drawn into the cylinder from the mains. Unvented hot water cylinders are connected to the cold mains supply so the hot water supply system operates under higher pressures. No cold water feed cistern is required, however there are a number of safety and functional control valves that must be incorporated in the system.

A single coil cylinder has just one heat exchanger coil and is designed to be connected to just one heat source i.e. a boiler or a ground source heat pump.  They are usually also supplied with an electric immersion heater which can be used should there be a problem with the boiler. 

As the name suggests, this type of cylinder includes twoheat exchanger coils. One is connected to a boiler or heat pump, the second can be connected to solar thermal panels to store any solar gain collected during the day. A boiler or heat pump is required as backup should the solar gain not be great enough to heat the tank completely. In this case the boiler or heat pump will step in to make up the difference. Again these can be supplied with an immersion heater.

Often used to refer to the number of showers, taps and bathrooms which will be connected to the hot water supply.

An essential measure of the efficiency of the cylinder. Heat loss tends to be measured in kWh/24 hours, but may also be expressed in Watts (W) so it is worth comparing before buying. The level of insulation will have an effect on how long the cylinder takes to cool down. A well-insulated cylinder will only cool by a few degrees overnight.

A measure of how quickly the cylinder will re-heat to the storage temperature following a draw off of hot water. If large amounts of water need to be supplied to multiple outlets at the same time, the quicker the re-heat time the better the continuous flow of hot water.

Powered by electricity, the immersion heater will ensure the cylinder can be heated even if there is a problem with the external heat source.