An Essex flange is a fitting on a hot water cylinder which prevents a device, such as a shower pump, from being damaged by air. It does this by drawing off water from the side of the cylinder.
When water in the cylinder is heated by the coil or the immersion heater, air bubbles form. The top of the cylinder is shaped like a bell in order to prevent these air bubbles from being trapped. This allows them to rise to the highest point of the cylinder and out at the top, before venting out of the system via the expansion pipe as normal. By drawing water off from the side of the cylinder using an Essex flange, the ingestion of aerated water is prevented.
An Essex flange is slightly different to a Surrey flange, which fits at the top of the cylinder, and has a tube which dips down into the main body of water. The advantage of an Essex flange is that there is no need to modify the pipework at the top of the cylinder. Depending on the manufacturer, some brand new hot water cylinders are already equipped with an Essex flange.