The Header Tank is the smaller of the two water tanks usually found in the loft, the larger one being the cold water storage tank. The header tank is used to fill the heating system and accommodate the expansion of water during the heating cycle.
It has an overflow pipe running to outside, a mains water supply connecting to the tank via a ball valve used to fill the system and a cold feed pipe* and an open vent pipe*.
The cold feed and open vent pipe are both crucial to the operation of the heating system. It is essential that they connect in to the system in the correct way. In many systems which have been altered over the years it is not uncommon to find these two pipes connected incorrectly, especially with today’s high efficient low water content boilers.
*On some systems the cold feed and vent pipes may be combined as one or even just have a cold feed pipe. This only acceptable if the boiler manufacturer says so in their literature.