Cover, screened openings, inlet air gap, turnover, insulation, support and materials that will not contaminate stored drinking water.
A cold water storage cistern that feeds drinking water must keep that water wholesome. Check each of these:
- Cover: fit a rigid, close-fitting cover that excludes light, insects, vermin and dust. It must not be airtight; the cistern breathes through screened openings. Limiting light discourages algae; suitable temperature, turnover and maintenance remain necessary for wider microbial control.
- Screened openings: screen the vent and the warning or overflow pipe so insects cannot get in. An uncovered cistern with an open overflow does not meet the requirements.
- Inlet air gap: the float-operated inlet discharges through an air gap above the stored water, so contamination in the cistern cannot be drawn back if the supply pressure falls. For a type AG inlet, measure to the critical water level under fault conditions, not the normal float setting or the overflow's edge.
- Keep the gap clear: no hose, splash or later alteration may bridge it. The float valve's label does not show that backflow protection is in place: check the gap.
- Turnover: place the inlet and outlet on opposite sides to encourage circulation and reduce stagnant zones. Storage capacity and actual use must also allow adequate turnover.
- Insulation: insulate the cistern and nearby pipework in an unheated loft. Insulation slows unwanted cooling or warming and reduces frost-damage risk. It supplies no heat and may need other protection where prolonged freezing is possible; follow the specified arrangement.
- Support: stand the cistern on the base its manufacturer requires, so its full weight does not distort or damage it.
- Materials: use materials of suitable quality that will not contaminate the water. Non-metallic materials such as plastics, rubbers and jointing compounds are normally shown suitable by testing to BS 6920. A material must not taint the water or support microbial growth, even if it is not toxic.
- No lead: never use new lead pipe or lead-based solder on drinking-water pipework; lead dissolves into the water.
- Known materials only: decline unknown second-hand pipe and fittings, and use materials rated for the temperatures they will carry, because unsuitable materials soften or fail when hot.
- Access: leave access to inspect and maintain the inlet, cover and overflow.