Modern unvented hot water systems require a range of ancillary fittings to operate safely.  When heated, water will expand and increase the pressure within a closed system. If not controlled the increase in pressure may cause damage.  To counter this, expansion vessels and safety valves are fitted to accommodate the increase in volume.  Expansion vessels contain a balloon shaped bladder or diaphragm that is open to the expanding water on one side, whilst a pre-set air charge counters any expansion of the bladder on the other.  In operation, any increase in a systems volume is accommodated within the bladder by pushing against the compressed air.  When the water cools it is expelled from the vessel by the air charge.  This operation results in a small volume of water being exchanged during each heating and cooling cycle.  However, if the tidal volume is insufficient the water can become stagnant and allow sediment to build up at the base of the vessel.  To prevent this expansion vessels must always be mounted vertically with the outlet at the bottom. If mounted directly onto the supply pipework a special diverter valve can be fitted to produce circulation within the vessel.  If mounted remotely then vessels must be installed on pipework that rises off the supply pipe and vessels with a volume greater that 2 litres must be fitted with a tamper-proof isolation valve and a drain cock.  The drain cock permits the content of the vessel to be flushed to drain every six months.  Expansion vessel bladders should also be replaced as per manufacturer’s instructions.