Is this a open vent or system boiler?

If the water storage in the airing cupborad is metal clad and flat topped, not domed, it sounds as if you have an unvented hot water system with a traditional open vented system for the rads. Although the hot water tank is mains pressure and sealed (this provides greater capacity and better pressures in a large house with multiple outlets) the hot water is heated by the same heating circuit as the rads and is an open system.

Tony B
January 2009

A system boiler can be used on an open vented system. A system boiler has an expansion vessel and pump built in.

Are you sure that the tank is for the heating and not the domestic hot water.

A sealed system is sealed and doesn't have a header tank, it is filled by a a mains supply and will be pressurised to approx 1.5-3 bar, unless your tank is 15m up from the boiler you will not get this pressur.

Richard George Hall - Senior Mechanical Engineer
July 2008