Water Pressure Explained:
Low pressure gravity system
This is the most common of all domestic water systems. The system consists of a cold water tank (or cistern) normally in your loft and a hot water tank (or cylinder) normally in your airing cupboard.
It is called a low pressure gravity system because the water is kept under pressure via the gravity-fed water delivered from the cistern.
How much pressure is produced is determined by the height at which the base of the cold water cistern is situated above the outlet. The greater the height of the cistern above the outlet the more pressure is created.
Mains Pressure Combination Boiler
A combination boiler heats cold water direct from the cold mains supply as soon as you turn on the hot tap in your home.
The boiler is normally wall mounted and you will not have a cold water tank (cistern) in your loft or a hot water tank (cylinder) in your airing cupboard.
As the water supply to the system is mains fed this system should produce good pressure.
Mains Pressure Unvented
An unvented mains pressure system, stores mains pressure water in a large strengthened hot water tank (cylinder), usually found in the airing cupboard.
The hot water will be heated either by immersion heaters fitted in the side of the cylinder or by a central heating boiler. This type of system doesn't require a cold water tank (cistern) in your loft and provides a high water flow rate and pressure.
Other systems also producing hot water at mains pressure are available, these are generally known as thermal store systems.