Water Heater Installation

Unvented Water Heater Cylinders Explained

Unvented hot water heater cylinders store water supplied directly from the mains water supply, and heat it using either electrical heating elements or with heat from a boiler. This provides high pressure hot water supplies capable of feeding outlets with high flow rates of hot water.

The cylinders store heated water under relatively high pressures, typically in the region of 2 to 3 bar (20 to 30 metres head), and as such can supply heated water to outlets at high level, such as in a loft space, and at similar pressures to the mains cold water supply.

The increased pressures also make for better quality showers, without the need for pumps. With both hot and cold water services fed from the mains, there is no more need for a cold water storage tank, keeping loft spaces clear, and pipework to a minimum.

Mains Water Supply for Water Heater Installations

Unvented water heater cylinders are limited by the ‘power’ of the mains water supply that feeds them, and in order to function properly require both adequate mains pressure and flow rate. Supply pressure should typically be in the region of 2 bar, preferable more, and capable of supplying the required flow rates.

A Pressure Reducing Valve is required on all unvented water heater cylinders to limit the pressure of the incoming mains water to a safe level at which the cylinder is approved to operate. The difference between pressure and flow is important to understand – a mains water pressure of 5 bar is no good if it is supplied through 100 metres of 15mm supply pipework. Mains supply pipe sizes and flow rates, as well as pressures, must be checked.

Tank-Fed Unvented Systems: What You Need to Know

It is not always the case that an unvented water heater cylinder needs be fed from the mains. They may equally be supplied with pressurised water from high level water tanks, or from a pumped water supply. Indeed, where the mains water supply is not adequate to match the instantaneous demands of the user, cold water storage tanks will be a necessity.

The use of a booster pump will provide the required water pressure to feed the unvented water heater cylinder. This has the advantage over a pumped vented system of not requiring a vent pipe to be run from the cylinder back up to the cold water storage tank.