Cistern and water storage

Frequently asked questions

Why does a British house have a tank in the loft when a modern one does not?

Because of how the supply was arranged, and the change is one of the clearest generational divides in domestic plumbing. A traditional INDIRECT system fed the incoming main to the kitchen tap and then to a storage cistern in the roof, and everything else — the bathroom taps, the WC, the hot water cylinder — drew from that cistern under gravity. The reasons were historical: it buffered a supply that could not be relied on for pressure or continuity, and it protected the main from backflow. The costs are the ones that ended it: pressure limited by the height of the loft, a large vessel of standing water in an unheated space, and a serious escape-of-water risk. A modern mains-pressure system — a combi or an unvented cylinder — deletes the cistern entirely, which is why a loft conversion so often begins with removing one.

What is a header tank, and is it the same thing?

It is a cistern doing a specific job for a heating circuit, and it is much smaller. An open-vented heating system needs somewhere for the water to expand into as it heats, and somewhere to top up from as it loses a little — so a small feed-and-expansion cistern sits above the system, usually in the loft, connected by a feed pipe and an open vent pipe. It is not a water supply and its contents are not drinkable: the same water circulates through radiators and back. Calling it a header tank is common and slightly misleading, because a header in pipework elsewhere means a manifold. A SEALED system replaces it with an expansion vessel — a pressurised diaphragm vessel doing the same job in a fraction of the space and with no open water at all.

Why is there an air gap above the cistern's inlet?

To stop stored water being drawn back into the mains, which is the whole reason cistern inlet valves look the way they do. If the mains supply loses pressure — a burst in the street, a hydrant opened — pressure can reverse and siphon water backward out of anything connected to it. A cistern full of standing water, possibly in a loft, possibly with something in it, is exactly what must not travel back into a drinking supply. So the inlet discharges ABOVE the water surface across a specified vertical gap, and the gap is a physical impossibility rather than a device that can fail. It is why an inlet valve that has been set too high, or a cistern overfilled so the water reaches the inlet, is a regulatory breach and not a nuisance.