A system boiler is quite like a combination boiler in that it houses the pump, expansion vessel, pressure relief and other system safety features inside the boiler itself.

Unlike a combination boiler it heats your hot water indirectly through a gravity, unvented or thermal store cylinder and uses an external divertor valve (either 2- or 3-way valve) to send heated water through the cylinder or to provide space heating to radiators, UFH etc.

This type of system is normally sealed and under pressure (normally 1 to 1.5 bar). Like a combination boiler you will need to check the pressure and occasionally allow water into the system through a wras approved filling link or loop.