Louise France - domestic heating, plumbing and gas engineer
'static' pressure (taps closed)

'dynamic' pressure (with a tap running)

The pressure and flow rate of your cold mains supply can vary from street to street, house to house. Mains supplies can be upgraded to bigger pipes, but there is an alternative.
It's possible to improve your pressure & flow rate with the use of an accumulator. Essentially, these are giant potable expansion vessels, made better with a high quality mains booster pump.

As water utilities companies across the UK reduce their supply water pressures down to as little as 1 bar or demand in certain areas outstrips available supply capacity, a problem that many householders are now facing is reduced mains water dynamic pressure and flows which in some cases can result in complete failure of the supply at times of peak demand.
The best way to describe an accumulator is like a battery storage system but for water. Accumulators take high pressurised water
from the mains supply at times of low demand and store it in a 
vessel under pressure.
An accumulator is essentially an expansion vessel that is installed open to the incoming mains water supply. It has an internal butyl diaphragm that expands as it fills with water from the mains. It
will stop filling when the pressure within the vessel is equal to the available mains pressure. If the mains pressure rises the pressure
in the accumulator rises. A double check valve on the mains supply prevents any loss of pressure.
When any taps within the dwelling are opened, the water from the accumulator supplements the water drawn from the mains. The
effect of this is stable pressure and flow to all taps with minimal
drop off when more than one tap/ outlet are opened simultaneously.
The accumulator has an air space above the diaphragm, the air pressure within this space is important to the efficiency of the accumulator and is set to optimum pressure via a pressure valve
on top of the vessel. A system can have one or more accumulators
to satisfy demand.
The pressurised water in the accumulator can then be used at times
of high demand to provide plenty of flow and pressure whilst other homes are having to cope with low flow and low pressure.
In real terms an accumulator acts as your own personal mini mains pumping station and reservoir resuscitating your system.