Cistern storage, warnings, float valve types, and overflow pipe sizing.
Under C&G Unit 204, a cold water storage cistern feeds domestic outlets and vented heating systems in an *indirect* cold water system — only the kitchen tap and any drinking point stay on the mains, and everything else is gravity-fed from the stored water. Storing water gives a reserve if the mains is interrupted and lowers the pressure and noise at the outlets.
What a storage cistern must have
Once water is stored it stops moving, so the Water Fittings Regulations and Approved Document G require it to be protected so it stays wholesome:
- A close-fitting, insect-proof lid to keep out light, dust and insects — light and warmth encourage bacterial growth in standing water.
- Insulation to stop the water freezing in a cold loft and to limit warming from the space around it.
- A screened warning/overflow pipe (and, on modern installations, a screened air path) so the cistern can breathe without letting insects in.
- Rigid, level support across the whole base, because a full cistern is heavy and a sagging base stresses the connections and the cistern itself.
How the float valve works
The float valve shuts the incoming supply off as the level rises: the float lifts on the water surface, the arm presses the washer onto the seat, and flow stops at the design level.
- Part 1 float valves are the older higher-pressure pattern.
- Part 2 (diaphragm) float valves are quieter and discharge above the water line, giving better protection against back-siphonage of stored water into the incoming main.
Warning pipe vs overflow
Every cistern needs a warning pipe set *below* the cold feed inlet but *above* the normal water level. If the float valve starts to pass, water escapes — and is seen — through the warning pipe before the cistern can overflow internally, and it discharges somewhere conspicuous outside so a drip is an obvious sign the valve needs attention. On larger cisterns a separate overflow may be fitted as well as the warning pipe.
*Original revision guidance; confirm exact sizes and positions against the current Water Fittings Regulations and Approved Document G.*