1 minute read
UNVENTED CYLINDERS FROM
WHY SO MANY SAFETY DEVICES?
Paul Millard, Technical Manager at Water Regs UK shares technical insights relating to the regulations around installing unvented hot water cylinders.
I
nstallers are responsible for ensuring compliance with the Water Fittings Regulations or Scottish Byelaws. One of the key requirements for hot water cylinders (Schedule 2 paragraph 17-24) is ensuring it is maintained below 100°C.
VENTED CYLINDERS
For gravity-fed cylinders this is achieved with an open vent, which is placed over the cold-water storage cistern which feeds it.
UNVENTED CYLINDERS
Mains fed cylinders, termed ‘unvented’, pose a greater risk. They are maintained under mains pressure and can provide the conditions for water to flash to steam. The regulations, therefore, require these cylinders to be installed using a number of additional safety features.
SAFETY DEVICES
On the cold mains feed there must be a pressure-reducing valve and an expansion valve. The heated, expanded water must be accommodated within an expansion vessel. Usually this is built into the top of the cylinder or provided separately on the hot water outlet.
There must also be a thermal energy cut out to stop the heat source in case the thermostat fails. Temperature and pressure relief valves are also required. When activated, these valves allow cold water to be drawn into the cylinder which helps reduce the temperature. For more information visit: www.waterregsuk.co.uk
Unvented hot water system diagram
IN SUMMARY
Maintaining the cylinder below 100°C is not just about avoiding a scalding risk, it is the point water turns to steam. When this happens, the expansion ratio is approximately 1:1700 and can occur almost instantaneously in the right conditions. If the whole cylinder were to flash to steam this would have devastating consequences.
HANDY HINT
It is also worth noting that the installation of these cylinders must also comply with section G3 of the Building Regulations.
