Firstly lets consider the pressure aspect of incoming cold mains water.
Water pressure is usually measured as Standing Pressure when no water is running and operating pressure when water is running through a tap or other device.
The pressure can vary throughout the day as the demand on the mains network varies with degree of usage especially at peak times.
If the pressure coming into your property is too high this can be a serious problem.
It will tend to manifest itself in the form of noisy pipe work and apparatus at worst giving rise to water hammer.
Water hammer arises when high pressure causes vibration I the pipe work or apparatus. For example, a toilet inlet valve cannot close properly. As it closes it is forced open by the water pressure the valve then tries to close again and this process is repeated rapidly causing a vibration in the pipe work.
Tap head gear can cause the same problem with the head gear vibrating within the tap. Quarter turn tap heads also suffer from this just at the point of closing.
If the pipe work is badly installed or insecurely fixed high pressure can also cause it to oscillate and vibrate
Most plumbing apparatus such as Boilers, Cylinders, Taps and Valves are designed to work up to a maximum working pressure typically 10 bar.
Continued unrestricted high pressure will cause a shortening of the working life of much of the cold water apparatus. It may even cause immediate damage to pressure sensitive devices such as Boilers and Cylinders.
Low pressure can present as many problems as high pressure. The amount of mains water pressure a water authority are required to provide may vary from region to region.
Usually they need to provide at least 1.5 bar working pressure. This is relatively low given that this equates to 15 metres head pressure.
The water flow will also be low especially upstairs where the pressure has fallen due to height.
Low pressure and flow restrict the use of some appliances.
Traditionally this problem was overcome by using a cold water storage cistern in the loft to provide a head of cold water. However, this frequently results in low pressure and flow at the taps and poor hot water provision.
This then means that provision for showering is difficult especially as the demand for more powerful higher flow showers is common nowadays.
Combi boilers may accept operating at little as 1.5 bar but if the flow rate is low the performance of the boiler will be affected. If the incoming flow rate is only 9 litres per minute there is little point installing a boiler that will provide 12 litres per minute hot water.
The sam problem will also apply to the newest incarnations of multipoint water heaters.
Unvented cylinders also require a minimum flow and working pressure.
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