How Does An Unvented Cylinder Work?

An unvented cylinder is a hot water storage cylinder that is directly connected to the cold water supply from the mains. It will store hot water under pressure and supply taps, showers, baths and other hot water outlets without the need for a cold water storage tank in the loft. This makes unvented cylinders a popular choice for modern homes, larger properties and buildings where excellent hot water performance is required.

A standard vented cylinder is usually supplied from a cold water tank located above it (typically in the loft). Gravity accounts for most of the pressure at the taps. An unvented cylinder uses the incoming mains water pressure, so it works differently. When you open a hot tap or shower, cold mains water enters the bottom of the cylinder and pushes the stored hot water out of the top of the cylinder to the outlet. This means that, with a sufficient feed from the mains, the system can supply hot water at a good pressure.

Direct or indirect unvented cylinders?

There are different ways of heating the water inside the cylinder. An unvented direct cylinder has electric immersion heaters inside the cylinder. These heat the stored water directly and are often used in properties with no gas or oil boiler. An indirect unvented cylinder is heated by an external heat source, such as a gas boiler, an oil boiler, a heat pump, or a solar thermal system. We offer a dedicated range of unvented solar cylinders, available in single- or twin-coil versions, which are listed here: solar hot water cylinders. In this type of cylinder, hot water from the boiler or heat source flows through a coil inside the cylinder. The heat transfers from the heat exchanger’s coil wall to the domestic hot water stored, with no mixing of the two waters.

Water expansion

Water expands when heated, so an unvented cylinder needs a way to allow for this expansion. This function is typically done by an external expansion vessel or an internal air gap, depending on the cylinder design. The expansion vessel can safely absorb the increase in water volume as the cylinder heats.

Safety systems on unvented cylinders

Safety controls are also important on unvented cylinders. These usually consist of a pressure-reducing valve, non-return valve, expansion relief valve, temperature and pressure relief valve, thermostat, thermal cut-out and discharge pipework. The purpose of these controls is to prevent the build-up of excess pressure or temperature in the cylinder. If a safety valve opens, water is discharged via a tundish and discharge pipe to a safe area.

Tundish

The tundish is a critical visual warning point. If water is seen passing through it, the flow may indicate a fault such as a failed expansion vessel, a passing safety valve, excessive pressure or overheating. It is not to be ignored or capped off.

Conclusion

In simple terms an unvented cylinder produces and stores hot water at mains pressure and delivers it to outlets on demand. The main advantages are high hot water flow rates, no loft tank needed and the capacity to supply multiple outlets. However, it must be correctly sized, installed, commissioned and maintained by a competent person, as the pressure and safety controls are essential to the safe operation of the system.