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System boilers

System boilers, sometimes known as a sealed system provide central heating and hot water that is stored in a separate hot water storage cylinder, usually in an airing cupboard. They are suitable for larger family homes with multiple bathrooms, as they allow multiple baths / showers to be run at the same time. Instead of a feed and expansion tank in the loft, a pressure vessel is used which isn't open to the atmosphere, minimising evaporation, meaning the pipework only needs topping up once a year, via a filling loop. As the system is sealed, airlocks are unlikely, and easily cleared. System boilers are more efficient than conventional boilers and are easier and quicker to install as the major components are built in.

System boilers require a hot water cylinder, and there are two types: vented and unvented.

vented system boiler

Vented cylinder

With a vented hot water cylinder, the cylinder is fed by a cold water storage tank in the loft. The pressure that forces the water out of the taps is generated by gravity, which suits a property with low mains pressure.


  • Simple system, with low maintenance
  • Easy to install
  • More than one outlet can operate at the same time


  • The cold water tank in the loft takes up space
  • Hot water flow rate is lower than with an unvented cylinder
unvented system boiler

Unvented cylinder

Unvented cylinders, such as megaflo, suit a properety with good water pressure, as they get their water directly from the mains. This means that the hot water system operates at mains pressure, making it possible to have power showers without the need for a pump.


  • High flow rate for domestic hot water
  • No tanks in the loft
  • More than one outlet can operate at the same time


  • Not suitable for properties with low water pressure