Stored-water protection, inlet control and safe warning/overflow arrangements.
In a typical indirect domestic arrangement, a cold-water storage cistern supplies selected outlets and feeds the vented hot-water cylinder. Trace the actual connections: not every outlet except the kitchen tap is always cistern-fed. Keep this stored domestic water separate from the primary heating water, which has its own feed-and-expansion cistern.
Protect the stored water
Fit a close-fitting cover and screen the openings to keep contamination out. Arrange the storage and draw-off so the water turns over regularly, and leave access for inspection and cleaning. Support the cistern well enough that it does not distort when full. Site and insulate it to limit warming and frost risk, though insulation cannot guarantee it will not freeze.
Think of the cistern full, not empty. A firm, level platform under its whole base passes that weight to the structure. A plastic cistern resting straight across spaced joists has parts of its base unsupported, so it can bow and strain its walls or outlet connections. The level platform also keeps the water line where it was designed to be. Gaps left underneath for ventilation carry no load.
Insulate a cistern's sides, lid and connected pipework, but leave the loft floor directly beneath it uninsulated. A little warmth rising from the heated rooms below is what helps keep the stored water from freezing; insulating underneath it blocks that warmth and makes freezing more likely.
Check the inlet control
The float rises with the water and operates the valve to stop filling at the set level. Choose the valve and seat to suit the supply and the flow needed. BS 1212 Part 1 covers piston-type valves and Part 2 covers diaphragm-type valves with copper-alloy bodies. These are construction types, not pressure ratings: both have high- and low-pressure variants. Keep the right air gap at the inlet: the label on a valve does not give backflow protection.
On a traditional lever-operated float valve, movement of the float operates the shut-off mechanism: rising water closes the inlet and falling water reopens it. How it works inside, and how you adjust it, depends on the valve type: identify the model before you choose parts or change the setting.
Overflow capacity and warning
Provide an overflow pipe and a suitable warning arrangement. A combined warning/overflow pipe may be acceptable for a cistern of 1,000 litres or less; alternative warning arrangements need agreement with the water undertaker.
The discharge must be safe and easy to see, and never into another cistern, where an overflow would go unnoticed. The pipe falls all the way to its outlet, and a discharge to a drain needs a visible air break giving fluid category 5 protection. Do not let it make a path slippery or icy. Size the overflow to carry the largest inflow a fault could cause, allowing for any screen: the minimum bore may not be big enough. If the warning pipe runs, investigate the inlet valve; never plug the discharge.
The guidance typically places the shut-off level at least 25 mm below the overflow where a warning pipe is fitted, increasing to at least 50 mm where there is none, and the warning or overflow pipe itself needs a bore of at least 19 mm. Where an insect screen is fitted over it, the openings should be no greater than 0.65 mm. Fit the screen upright and make it removable; allow for its effect on discharge capacity.
Servicing valve and outlet position
Fit a servicing valve on each distribution pipe leaving the cistern, close to the cistern, so a tap or a length of pipework can be isolated and worked on without draining the whole cistern. Water Regs UK's outlet-valve rule does not apply to vent, overflow and warning pipes, which must always stay open to do their job. The inlet needs one as well: guidance to the Water Fittings Regulations asks for a servicing valve on the inlet to every float-operated valve, fitted as close to it as is reasonably practical, so that valve can be repaired or replaced while the rest of the house keeps its water. That includes the fill valve of a WC cistern, and the same guidance asks for one on the supply to appliances such as washing machines, dishwashers and water heaters.
An outlet set a little above the base draws in less of the sediment that settles below it, but the cistern will not drain completely through it by gravity. The water below the outlet still mixes with the rest. Put outlets low and, where you can, opposite the inlet so the water turns over; follow the cistern instructions and keep access for cleaning.
Before removing a cistern
Ask the water undertaker before taking a storage cistern out. The break between it and the incoming supply may be protecting the fittings after it from backflow, so connecting them straight to the mains can remove that protection as well as the storage.
Linking two or more cisterns
Avoid linking cisterns where you can, because stored water can stagnate. Where the storage has to come from two or more cisterns, keep their number and total volume to a minimum. Connect them in parallel, with each cistern's inlet and outlet at opposite ends, so that every draw-off moves water through every cistern instead of short-cutting past most of them. Delayed-action float valves are recommended, and the inlets and outlets need balancing.
Float valve faults and access
Float-operated valves must be adjustable and kept accessible, because the shut-off level needs checking and resetting from time to time. Do not box one in without leaving room to reach and service it.
If a cistern keeps overflowing and the float is sitting unusually low in the water, suspect a punctured, waterlogged float: it cannot rise far enough to lift the arm and close the valve. A damaged seat or washer can also cause overflow, but neither makes the float itself sit low.
After draining and refilling, a gravity-fed outlet that spurts and then stops may have trapped air. Check the refill, open valves, water level and pipe route before deciding the cause. Mains pressure can help push air through pipework, but an outlet on the mains can still have an air lock or another restriction.