Combination Boiler System
Unlike more conventional boilers, the combi boiler normally contains the pump and, frequently the hot water/heating timer (programmer). Because it is a sealed system, Central Heating (CH) water expansion (due to heat) and contraction has to be accommodated in a pressure /expansion vessel; which is also within the casing. In the case of sealed systems using existing 'older' radiators it is often advisable to fit a supplementary expansion vessel to help prevent damage to the radiators or valves. Instead of an open safety vent pipe looping back over the header tank in the loft space, there will be a pressure relief valve.
A pressure gauge is usually fitted to give an indication of whether or not the system pressure is being maintained and no air is being admitted.
Occasionally the system will require re-pressurisation, to facilitate this a 'filling loop' (flexible braided pipe) is provided.