By far the most popular size valves we sell are 15mm (feed pipe diameter) – to suit 1/2″ (radiator thread/hole size) or 15mm-1/2″.The remainder being bigger at 22mm-3/4″. With the higher pressures and flow rates of modern heating systems, the smaller valve tends to be the favoured choice for most heating engineers even in the case of larger radiators. The 22mm-3/4″ gives a slightly larger overall appearance as does the 22mm pipe running to it. This bigger look is very occasionally favoured by our customer. Practical restrictons, ie, much larger radiators or reduced flow rates may also dictate the need for this larger choice of valve, to ensure sufficent flow of hot water through the radiator. British Imperial pipe threads have not been improved upon and as a result are still the pipe thread of choice for industry. Modern copper pipe however, comes in metric sizes hence the somewhat confusing references to both imperial and metric sizes on pipework fittings such as valves.