High and low pressure water supply
The water pressure under which a ball-valve operates is an important factor, as the size of the hole in the nozzle of the valve will be either smaller or larger according to whether it is under high pressure (ie, mains pressure) or low pressure (ie, supplied by water from a storage tank). Older Portsmouth valves have either HP (high pressure) or LP (low pressure) stamped on their bodies, and will only operate satisfactorily under the pressure for which they are designed. Modern valves, on the other hand, have interchangeable nozzles which allow you to convert them from low to high pressure or vice versa.
If the plumbers fits a high pressure valve (or nozzle) in a situation where a low-pressure one is required this will result in an agonisingly slow re-fill. A constantly dripping overflow may be the sign of a low pressure valve that has been fitted to a cistern that is fed by the mains. In some areas, mains pressure varies considerably throughout a 24-hour period. During the day, when demand is high, pressure will be low, whereas in the evening as demand falls off the pressure increases. These fluctuations in pressure don’t affect low pressure valves but they do affect high pressure ones, which can perform erratically as a result. You can overcome this problem if it affects you by replacing your high pressure ball-valves with equilibrium valves.
Equilibrium ball-valves
The plumber can use Portsmouth and diaphragm equilibrium valves. These are both designed to allow a small quantity of water to pass through or round the washered piston (or diaphragm) into a watertight chamber beyond. Acting as it does on the rear of the piston, and being at the same pressure as the mains, the water in the chamber ensures that the piston is held in equilibrium. What this means in practice is that the valve is operated solely by the movement of the float arm, rather than by a combination of the movement of the float arm and the pressure of the incoming water as is the case in an ordinary high-pressure valve. In addition to re-filling your cistern promptly regardless of any fluctuations in mains pressure, equilibrium valves also eliminate the ‘bounce’ as the valve closes – a common cause of water hammer. A diaphragm equilibrium valve will give you a particularly rapid and silent refill.