Unvented DHW

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Like a conventional hot water cylinder fed from a tank in the roof, an unvented cylinder contains hot water (heated by a boiler or immersion heater) which directly supplies the hot taps. Unlike a conventional cylinder, however, the water in the unvented cylinder comes directly from the cold water main and is at (nearly) mains pressure. To contain this pressure the cylinder has to be physically much stronger than in a gravity-fed system. Unvented cylinders are made of thick copper or stainless steel.

Water expands as it is heated. Since the outlet of the cylinder is to DHW taps which are normally closed, and the inlet is from the cold water main which may incorporate non-return (check) valves or other devices preventing expansion back into the supply pipe, measures have to be provided to accomodate the expansion of the hot water which could otherwise give rise to enormous pressure in the cylinder. These take the form of some type of container of gas which can be compressed as the water expands. This may be arranged as a bubble of air in the cylinder or a separate expansion vessel.

Safety of unvented systems

At normal atmospheric pressure water boils at 100C. At higher pressures the boiling point increases so that pressurised water can be heated to over 100C and remain liquid. However if the pressure is released it will rapidly turn to steam, expanding greatly and causing a steam explosion. In the days of steam the explosion of boilers or pipework in locomotives, shipping and industry was rightly feared for the destruction and loss of life it caused. If the water in an unvented hot water cylinder is allowed to rise above 100C then the reduction of pressure when a tap is opened could cause a steam explosion. For this reason unvented systems must have safety systems to control the temperature and pressure of the water, these systems must be checked annually, and their installation and servicing must be carried out by engineers with specific qualifications for these tasks. The matter is potentially very serious. [[1]]

Small unvented storage cylinders of less than 15 litres don't have these and the small amount of expansion is accomodated by backing the water back into the mains. The only requirement is to check that all stop valves upstream are not of the 'loose jumper' type which could act as a non-return valve and block the expansion.

Otherwise the safety equipment normally consists of the following:

  • A check valve which is usually implemented as part of the pressure reduction valve.
  • A pressure reduction valve typically set to around 3 bar (although other pressures are sometimes used).

(There may also be a cold water outlet point after the pressure reducing valve to supply e.g. shower mixing valves which work better when their hot and cold supplies are at the same pressure).

  • A pressure relief valve typcially 6 bar, this is mounted on the inlet to the cylinder.

The above items are often found combined into a single multifunction unit.

  • A combined pressure and temperature releif valve. This is always on the side of the cylinder it will typically be set to 7 bar and 90C.

The reason for the temperature limiting valve is that if a heating source fails to cut out then it is possible for the pressure rise to remain quite modest until the water reaches well over 100C (even 1 bar will raise the boiling point to around 120C). Neither pressure relief valve will open and yet the contents of the cylinder are lethal and will 'flash' boil to steam when pressure in the system is reduced by opening a tap. This could cause serious injury to a user, hence the need for a temperature safety valve. Should this operate it will dump the bulk of the contents of the cylinder containg water at 90C into the relief discharge pipework. The valve will only close when cooler water from the mains supply has reached it. This is also why there are stringent requirements for the size, length and routing of the discharge pipe.

Both the thermostats for the immersion heater (if fitted) and the boiler (if connected) will contain a second higher temperature cut-out requiring manual resetting if it operates.

The indirect heating coil connection must contain at least one 2-port zone valve.

The use of solid fuel heating is forbidden.