Warning! This site has now been taken over by EPHG Limited, an electrical, plumbing and gas company and by using this site, you agree to the terms located here. Please note that some of the content may no longer be relevant.

Hot water storage systems

Hot water storage vessels are used to store water used for domestic uses such as laundry, bathing and heating the home.

There are two types of hot water cylinder. These are direct and indirect cylinders.

With direct cylinders, water that has been heated in the boiler is circulated inside the storage cylinder until it is drawn off by the taps.

There is a continuous change of water with direct cylinders. This is fine for domestic use but will not work with central heating.

Water for the central heating needs to come from an indirect cylinder so that the water is separate from that used by the taps.

Using an indirect cylinder prevents the boiler and surrounding pipes from getting a build up of fur due to temporary hard water.

It also reduces the risk of radiators from corroding through contact with the ever changing water supply.

Indirect systems do not change the water in the boiler and primary pipe work because water is only lost through evaporation.

Water storage systems are cylindrical in shape so that they can cope with the internal pressure.

They are generally made of copper but some newer unvented storage cylinders are made of stainless steel because they can withstand higher pressures.

Storage vessels should be kept properly insulated so that the water inside stays hot and is not affected by a change in external temperature.


Older cylinders should have factory-fitted insulation jackets. According to regulation, all hot water storage vessels should now be labelled.

Information on the label will include their type, storage capacity in litres, heat loss per day in kilowatt hours and heat exchange in kilowatts.

Storage cylinders should be stood on battens so that air can flow under the vessel. This will prevent condensation from building up and damaging the flooring. The hot water storage should always be close to the boiler so that heat is not lost between the two.

The run of pipes between the water storage and the drain off point are known as ‘dead legs’. Dead legs should be kept short so as not to waste water or heat.

Central Heating Systems

If your water storage unit is too big, then an unnecessary amount of hot water will be stored. This will increase the amount of heat that is lost, the amount of fuel used and the cost of fuel.

You should check the recommended size cylinder for your type of property before having one installed.

In the 4 Hot Water Storage System Diagrams I have included the hot water storage system, hot water heated from roof, hot water storage 1 way and the central heating system.