Knowing which hot water cylinder you need for your system, can be daunting to say the least, I found the following information on the discounted heating website and was impressed to see they have given information about each type of cylinder separately and even have information on solar cylinders which are relatively new in the cylinder world.
If you are looking to install a solar cylinder I would recommend the Worcester Greenskies range as they are one of the leading manufactures in this area and have a good aftercare service.
The unvented cylinder is run off the mains water supply which, in turn pressurises the cylinder allowing the hot water to be distributed around the system when required keeping a high flow rate. Unvented cylinders are available in both direct and indirect (please see below) models. The unvented direct cylinder heat’s the water through an electric immersion heater, while the unvented indirect cylinder uses a gas, oil or electric boiler.
Open vented cylinders are used in conjunction with a header tank, this is normally located in the loft and stores the cold water, the water is then gravity fed from the header tank to the open vented cylinder, which is then heated as it passes through and then onto the outlet in use. Open vented cylinders should be fitted lower than the header tank, as this will create better water pressure however in some cases the use of a pump is required to maintain a higher flow rate.
The combination cylinder is ideal for properties where space is limited, it is a vented cylinder that has a built in cold water header tank fitted on top and is available as both direct and indirect models.
Direct cylinders have a built in immersion heater which heats the water “directly” as it passes through the cylinder, eliminating the need for a separate heat source, the water is then sent from the cylinder to the outlet in use.
Indirect cylinders are designed to work in conjunction with a boiler. The water is heated by the boiler and then passed through an internal coil inside the cylinder which heats the water stored in the cylinder to the desired temperature.
Solar cylinders work by liquid that has been heated through solar panels is passed through a coil inside the cylinder, and then in turn, heating the water stored in the cylinder. Solar cylinders are usually 'twin coil' this means it has an additional internal coil, this is used to heat the water when the solar energy is not sufficient. Initially solar systems are more expensive to install, however it is more beneficial and cost effective in the long run.