Cylinders are an essential component of your heating system but many of our clients ask for advice to better understand how they work. So we have prepared a brief guide to cylinders below to help you get up to speed.

Good to know: Oxbridge Plumbing and Building’s engineers are trained and certified by NICEIC in the installation and servicing of domestic unvented hot water storage systems in compliance with G3 of the Building Regulations Document G and the Water Supply (Water Fitting) Regulations 1999. So you can be sure you are in safe hands.

Certificate Number: 14/UVHW270/21919

Guide to Cylinders

What is a cylinder?

This is normally found in your airing cupboard or a similar location. A cylinder is a container for heating and storing your domestic hot water (the water for your bathroom, kitchen tap etc.). Alternative names for this container are a Hot Water Cylinder or Calorifier.

They come in different sizes depending on your demand for hot water.


Why do I need a cylinder?

Plain / Heat Only Boilers and system boilers do not directly heat your tap water. You will need a cylinder to enable your boiler to heat and store your domestic hot water unless you are using a Combi Boiler. If you have a high demand for hot water, such as more than two bathrooms in use simultaneously, even a Combi Boiler will not provide enough hot water. In these circumstances you would need to use a Plain / Heat Only Boiler or System Boiler in conjunction with a cylinder to supply your hot water.


How does a cylinder work?

Cylinders typically work in two ways – Indirect and Direct (See below for details).


Indirect Cylinders

The most common type is known as an indirect cylinder. Here the cylinder is filled with water which originates from the cold mains and is connected to your hot water taps via pipework. The cylinder also has a coil of pipe inside it which is connected to the central heating system and hot water is pumped through it. The hot water circulating through the coil heats the water in the cylinder without ever mixing with it. This is known as a heat exchanger.

As a back-up the cylinder normally has an electric element inside it. This enables the cylinder to heat the water if the boiler breaks down.


Direct Cylinder

The other type of cylinder available is called a direct cylinder. This is a cylinder which is filled with water originating from the cold mains. It is solely heated by an electric heating element and has no relationship with the central heating system or main boiler.

All cylinders are insulated with a dense foam on the outside in order that the water inside retains its heat for a long time.


Unvented (Pressurised) and Vented (Gravity Fed) Cylinders

Whether a cylinder is heated directly or indirectly, they both require water pressure so that the hot water will flow from the cylinder, though the pipework and out of the hot tap. The water needs to come out of the tap at a satisfactory speed when the tap is turned on.

Cylinders can be pressurised in two different ways. They can be mains pressurised or gravity fed. In the business a mains pressurised cylinder is called an unvented cylinder and a gravity fed cylinder is called a vented cylinder.

Older cylinder systems tend to be vented (gravity fed). This means that they have a large tank in the roof which provides the pressure for the domestic hot water.

New cylinder installations are often installed as unvented (mains pressurised) systems. These do not have a tank in the roof. Instead they are connected directly to the mains water supply with a safety control valve. The mains pressure ensures that the water flowing from the cylinder to the tap is at mains pressure or thereabouts. This solution often creates a more powerful flow of water from the hot tap than achieved by vented (gravity fed) systems.