To ensure a swift flow of water to taps or radiators, a 22mm pipe is commonly used instead of a 15mm one, which may decrease volume flow despite increasing pressure. In situations where a significant amount of hot and cold water is required, such as for baths and immersion tanks, a 22mm pipe is also preferred. Central heating boilers need to transmit heat rapidly to the radiators, which is why a 22mm pipe is laid down to reach under the rads, followed by a 15mm one to the valves.
This allows for a speedy flow of water to the radiators before slowing it down so that heat can be emitted into the room. Moreover, water expands when heated, so the larger diameter of a 22mm pipe provides sufficient room for the water to expand. While 15mm pipe can be suitable for short distances as it is close to its pressure source, microbore typically employs 10 or 8mm pipes to supply rads individually from manifolds nearby, but these are often fed by at least 22mm pipes from the boiler, which can even be 25mm.