Unvented hot water cylinders: why they need an annual check

Many larger homes in Surrey and South West London have an unvented hot water cylinder, usually alongside a system boiler or heat pump. These cylinders give strong mains-pressure hot water to several bathrooms at once, but they store hot water under pressure, so safety devices matter.

How an unvented cylinder works

Cold water comes straight from the mains into the cylinder, where it is heated by the boiler, a heat pump or an immersion heater. Because the water is under pressure, the system has several safety devices, including a pressure reducing valve, an expansion vessel and temperature and pressure relief valves.

Why an annual service matters

Most manufacturers recommend an annual check, and many require it for the warranty. A service typically includes:

• Checking the incoming pressure and the pressure reducing valve.

• Checking and re-charging the expansion vessel.

• Manually testing the temperature and pressure relief valves.

• Checking the discharge pipework is clear and correctly routed.

• Inspecting controls, thermostats and the immersion heater.

Installation and maintenance of unvented systems should be carried out by an engineer with the relevant unvented hot water qualification.

Warning signs

• Water dripping or running from the discharge pipe, often outside or into a tundish.

• Hot water pressure changing noticeably.

• Banging or noisy pipework.

• Water that is much hotter or cooler than usual.

Book a check

The Essential Connection services boilers and hot water systems across Walton-on-Thames, Weybridge, Cobham, Esher, Kingston and South West London. Get in touch or call 07719 664615.