Choosing one or the other is dependent on whether they have a direct or indirect system. These terms when related to hot water cylinders refer to how the mains water system is supplied – either directly into a pressure vessel that is situated inside a cylinder or indirectly into a cold water storage tank – or how the water is heated inside hot water cylinders – directly by use of an electric immersion heater or indirectly by the use of a heat exchanger served by a boiler.
Many households utilise a combination of both types with indirect for common/normal heating and an immersion heater as back-up or storage for hot water emergencies.
Up to 1989, unvented hot water cylinders were not permitted in domestic central heating systems. Only vented cylinders were allowed, this system has a vent pipe that is subjected to atmospheric pressure. The mains water is directed to and stored in a water storage tank which is usually situated in the loft and from there the water is fed to all the appliances inside the house. The appliances requiring the hot water are served indirectly via the storage tank to the hot water cylinder where it is heated by a heat exchanger. As the water supply is gravity fed and low pressure it is sometimes important to have a pump installed to maintain a strong water flow for both heating and hot water usage.
Unvented hot water cylinders work directly from the main water source. The water is heated by an immersion heater and not by a heat exchanger. As the pressure comes directly from the mains water source the flow is much stronger. The system is fitted with many safety triggers to help minimise the high pressure and expansion of the water. Here, you will find the tank in the loft for venting and supplying the central heating as there is no need for a cold water storage tank.
What are the pro's and con's of vented hot water cylinders?
The number one advantage of vented hot water cylinders over the unvented variety is that they are very much easier to install and maintain, reducing the cost and the time servicing the system.
The biggest disadvantage is that you will need more available space to house the vented system as you will require a cold storage tank
An indirect heating system heats the water by means of a primary heat exchanger within the unit that is supplied by a boiler. The exchanger system is a much more cost efficient method of heating the water. The water is fed to the hot water cylinders which have a series of coils that are used to heat the water. Unfortunately, if the boiler suffers from a breakdown, the house will be left without any hot water.
What are the pro's and con's of unvented hot water cylinders?
The unvented hot water cylinders produce hot water at mains pressure and there is no need for a cold storage tank and the extra pipe work required to link it all up.
And because the system's pressure is delivered via the mains rather than relying on gravity you can place the hot water cylinder anywhere around the home.
The biggest drawback is that you are limited to your choose of showers as some of the power showers and mixers cannot be used with these types of hot water cylinders. They will also require a specialist to install the system making this method of hot water cylinders a costly option.
The use of immersion elements to heat up the water means that you are less reliant on the boiler and immersion elements are very easy to get hold of. The downside is that costs are high due to the heavy reliance on electricity.