TradeTheory
PLUMBING • TRACK: LEVEL 2
Cold Water
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Micro-lesson 1 of 2

Cold water storage cistern basics

6 minsMicroPractical

What it is

A cold water storage cistern (CWSC) stores cold water and supplies outlets by gravity in a typical vented system. It’s commonly sited in the loft and feeds cold outlets and the hot water cylinder cold feed (depending on system design).

Why it exists

  • Provides stored water where mains flow/pressure is limited.
  • Supplies a steady gravity head to the hot water cylinder and some cold outlets.
  • Allows a vented hot water system to operate without being fully dependent on mains pressure.

Key components you should recognise

  • Float-operated valve (ball valve): controls incoming mains fill.
  • Warning/overflow pipe: safely indicates overfilling if the inlet valve fails.
  • Lid/cover: reduces contamination risk.
  • Insulation: helps reduce freezing risk and heat loss in nearby pipework.
Common fault pattern
Warning
If you hear constant running water or see discharge from the warning/overflow, suspect a faulty float valve or incorrect float level setting.

Quick install checks (practical)

  • Cistern is supported properly (no rocking, correct platform).
  • Lid fitted and intact.
  • Float valve shuts off cleanly and water line is correct.
  • Warning/overflow route is visible/obvious and not terminated into a hidden waste.
  • Pipework is clipped and protected where required (especially in loft spaces).
Quick check
Q: Checkpoint: What mainly determines pressure at a gravity outlet?
A: The vertical head (height difference) between cistern water level and the outlet.
Remember
Tip
Gravity systems = head height matters. If the cistern is too low relative to an outlet, performance will be poor even if pipe sizes are correct.