The header tank, otherwise known as the feed and expansion tank or cistern, is an essential part of an open vented central heating system with a regular boiler. The role of the header tank is to supply water to the central heating system and also maintain the level of water, which expands when heated.

What is the purpose of a header 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*.

Why is hot water feeding back into header tank?

Another frequent cause is a blockage where the feed / vent pipes from / to the F&E tank join the main pipework. That causes hot water to go up the vent into the small tank whence it is dragged back into the system again via the feed.