a header tank
is a water tank, usually located higher than any tap in the house (in the loft or attic space). It is usually kept topped up with cold water from the ordinary water supply, and in normal conditions it never gets fully emptied.

I'm not really sure why / if we need them; but there are several advantages:
# Bathroom tap water pressure is reduced from the standard mains blast.
# In the event of a Mains water failure, you have a small backup supply.
# In the event of a Mains water failure, your boiler is less likely to run dry / explode.

There are some (water quality) drawbacks:
# The tank is never fully emptied, meaning the chlorine in the water will evaporate, meaning the water will have higher algae/bacteria levels.
# Unless the tank is well sealed, dust (or worse) fall in; as Big Smurf can attest.

Being realistic, it's unlikely to kill you. It's more of an "ick"-factor.
( The Incredible Sulk Finally remembered his password on, Tue 23 Mar 2010, 1:37, 1 reply)

Header tanks have to be sealed to Byelaw 30 spec
With the outlet in the base not the side, so stagnation can't occur. You only find indirect cold water systems in remote houses high up in the hills where the mains water pressure is below 1 bar (about 9 litres/minute flow rate), otherwise all your cold taps will be fed directly from the main.

Different rules apply to commercial/industrial premises, including schools, which is where a lot of misconception arises.

You can tell what I'm doing at college, can't you?
(WillI'mNot logs in twice a year, every year, Thu 25 Mar 2010, 1:28, closed)