A storage cistern is the only place in a domestic system where wholesome water sits still, in bulk, at room temperature, in a space nobody visits. Everything Schedule 2 asks of a cistern follows from that one sentence — keep things out, keep it moving, keep it cool, and make it possible to look inside.
This article covers Module 10 of the PlumbMate Water Regulations course: inlet valves and their closing levels, warning pipes, covers and screens, the access and clearance requirements, support, and the circulation rules that stop water standing. There is a 10-question mock test at the end.
The inlet
The inlet valve must be adjustable, and must shut off at a suitable level below the overflowing level. Two requirements in one sentence, and the adjustability is the one that decides which valve you can use.
The closing levels:
- 25 mm below the overflowing level where a warning pipe is fitted;
- 50 mm below where an approved alternative device is used instead of a warning pipe.
The larger figure goes with the alternative device because there is no warning pipe to catch a small overfill and show it to somebody. Less feedback, more margin.
A float valve acting through a lever must also withstand twice its ordinary operating force without bending or distorting. A lever that bends is a valve whose closing level has silently changed.
Why BS 1212 Part 1 valves are not acceptable
This follows directly, and it is worth being able to explain rather than just assert. BS 1212 Part 1 valves are not acceptable in practice, because the close level cannot be adjusted without bending the lever — and bending the lever is precisely what the strength requirement forbids. The valve cannot be set correctly by any legitimate means.
Use Part 2. A Part 2 valve has an adjustment that does not rely on deforming a component that is supposed to be rigid.
Servicing valves
From Module 5, restated because this is where it is applied:
- A servicing valve on the inlet, adjacent to every storage cistern, combined feed and expansion cistern, WC flushing cistern and urinal flushing cistern.
- A servicing valve on the outlet of every storage cistern — except one feeding a primary circuit, for the reason given there: nobody should be able to close the route by which a primary circuit expands and vents.
Warning pipes
A warning pipe is an overflow pipe positioned where the discharge can readily be seen. That definition is the whole point of the fitting. It is not there to remove water — it is there to tell somebody. A warning pipe discharging into a gully, a soil stack or a flat roof nobody looks at satisfies the plumbing and defeats the purpose.
A warning pipe is required on all cisterns except automatically operated urinal flushing cisterns, which are designed to be fed at a controlled trickle and would be reporting a condition that is normal for them.
Sizing and installation
- 19 mm minimum internal diameter — but that is a floor, not the answer. It must be sized for any flow arising from failure of the inlet valve. On a large cistern with a big inlet, 19 mm is nowhere near enough, and a warning pipe that surcharges is a cistern that overflows into the building.
- The outlet end must not be higher than the inlet end, and the pipe must run on a downward incline.
- No flexible hose — it sags, and a sag holds water and defeats the fall.
- Never discharge into another cistern.
And from Module 8: overflows and warning pipes must terminate with a type AA air gap.
Covers and screens
The cover must be rigid, close fitting, securely fixed, and — importantly — not airtight. It must exclude light and insects.
The “not airtight” requirement surprises people who assume that sealing a cistern completely is the safest thing to do. It is not. A cistern needs to breathe as its level rises and falls; an airtight cover would draw a partial vacuum on emptying and pressurise on filling, and something would give. What the cover has to exclude is light, which drives algal growth, and insects, which are a biological contaminant.
So the ventilation that is required has to be screened: corrosion resistant mesh with a maximum opening of 0.65 mm. Small enough to stop insects, and corrosion resistant because a screen that rusts away has a hole in it and nobody notices.
Insulation guards against both freezing and undue warming, and it should be sealed all round except at the air vent. Note the two directions again — the loft is below freezing in January and can be well above 25 °C in July, and both are problems.
One more that is easy to overlook: the cover material must not contaminate water condensing on its underside. Condensation forms on the inside of every cistern lid and runs straight back into the stored water, so the underside of the lid is in contact with the supply as surely as the walls are.
Position, access and clearance
The general duty: every cistern must be installed so as to minimise contamination, be sized appropriately, and have its connections arranged to allow free circulation and prevent stagnant areas.
Then a positioning requirement with two limbs, and both have to be satisfied: the position must allow the inside to be readily inspected and cleansed, and the controls to be readily repaired or adjusted. A cistern you can reach the float valve on but cannot see inside fails, and so does the reverse.
Clear space above
- 350 mm for a conventional cistern up to 1,000 litres;
- 500 mm for a bolted-lid cistern above 1,000 litres.
And a rule that explains the second figure: a cistern over 1,000 litres must be inspectable and cleanable without being wholly uncovered. On a large sectional cistern you cannot lift the whole lid off, so you work through a hatch — and 500 mm is the room to get an arm and a light through it.
Siting
Site cisterns where no surface, ground or foul water can enter. That rules out a good many convenient positions — below a flat roof outlet, next to a soil stack, at low level in a plant room with a floor gully.
Support
The support must be a flat, rigid platform capable of carrying the cistern when full, and the figure to reckon with is roughly a kilogram per litre. A 230 litre cistern is a quarter of a tonne sitting on somebody's ceiling joists.
The practical consequence comes up on almost every cylinder replacement: replacing a metal cistern with a plastic one usually means upgrading the base. A galvanised cistern is rigid enough to span between bearers, so it was often supported on two timbers. A plastic cistern is not — it deforms to the shape of what is under it, and it needs continuous flat support across its whole base. Fit a plastic cistern onto a pair of joists and it will bulge between them, stress at the corners and eventually split.
Circulation
This is the stagnation half of the module, and it is the part most often ignored.
Where practicable, take all outlets from the bottom of the cistern. Water drawn from the bottom means the whole volume turns over. An outlet part-way up leaves a layer beneath it that is never drawn, never replaced, and slowly becomes something other than wholesome water.
Where two or more cisterns are linked, the inlets and outlets must be placed so that water passes through all of them and short-circuiting does not occur. The failure mode is straightforward and common: inlet and outlet both at the same end, so the first cistern does all the work and the second is a large tank of standing water permanently connected to the supply. Inlet at one end, outlet at the far end — make the water travel.
Temperature
- Keep stored water below 20 °C, with 25 °C the ceiling for cold water at a tap.
- Expansion water entering a cistern should preferably not raise the temperature above 20 °C.
That second point is worth thinking about on a combined feed and expansion cistern, where hot water from the primary circuit is discharged into stored cold water by design. It is also the reason a cistern in a warm loft above a poorly insulated ceiling is a compliance problem in summer rather than winter.
Sizing, and the stagnation trap
“Sized appropriately” cuts both ways, and the direction people get wrong is oversizing. A cistern sized generously for a family of six, in a house now occupied by one person, holds water for days rather than hours. Turnover falls, temperature drifts up towards ambient, and a cistern that was entirely compliant when installed is now a stagnation problem — without anything having been changed or broken.
Which is why a change of occupancy is a reasonable moment to look at the cistern, and why bigger is not safer.
Putting the module together
Keep things out: rigid, close fitting, securely fixed, not airtight, light and insects excluded, 0.65 mm screens on the vents. Keep it moving: outlets at the bottom, linked cisterns plumbed so water travels, sized so it turns over. Keep it cool: below 20 °C stored, 25 °C at the tap, insulated against both directions. And make it possible to work on: inspectable and cleansable, controls reachable, 350 mm or 500 mm clear above, on a base that will hold a kilogram per litre.
📝 10-Question Mock Test
Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.
25 mm with a warning pipe, 50 mm where an approved alternative device is used instead. The larger figure goes with the alternative because there is no warning pipe to catch a small overfill and show it to somebody.
The two requirements cannot both be met, so the valve cannot be set correctly by any legitimate means. Part 2 valves have an adjustment that does not rely on deforming a component that is supposed to be rigid.
The definition is the whole point of the fitting. It is not there to remove water — it is there to tell somebody. One discharging onto a flat roof nobody looks at satisfies the plumbing and defeats the purpose.
A floor, not the answer. A warning pipe that surcharges is a cistern that overflows into the building. It must also run on a downward incline with the outlet no higher than the inlet, use no flexible hose, and never discharge into another cistern.
A cistern has to breathe as its level rises and falls; an airtight cover would draw a partial vacuum on emptying. What must be excluded is light, which drives algal growth, and insects — which is why the vent needs a corrosion resistant screen of 0.65 mm maximum opening.
The underside of the lid is in contact with the supply as surely as the walls are. Easy to overlook, and a genuine route for contamination from an unsuitable material.
On a large sectional cistern you cannot lift the whole lid off, so you work through a hatch, and 500 mm is the room to get an arm and a light through it. Conventional cisterns up to 1,000 litres need 350 mm.
A galvanised cistern is rigid enough to span between bearers; a plastic one is not. Reckon on roughly a kilogram per litre when assessing whether the platform will carry it — a 230 litre cistern is a quarter of a tonne on somebody's ceiling joists.
The common failure is inlet and outlet at the same end, so the first cistern does all the work and the second is a large tank of standing water permanently connected to the supply. Inlet at one end, outlet at the far end — make the water travel.
Nothing has been changed or broken — the household changed. Which is why a change of occupancy is a reasonable moment to look at the cistern, and why bigger is not safer. Stored water should stay below 20 °C, with 25 °C the ceiling at any tap.
Nothing in this module is difficult. It is simply a list of the ways stored water goes wrong, each with a requirement attached — and it is the module where a technically compliant installation can drift out of compliance without anyone touching it, because the household changed and the turnover fell.
The one to carry away: the cover must be rigid, close fitting, securely fixed and NOT airtight. A cistern has to breathe. What it must exclude is light and insects, which is what the 0.65 mm screen is for.