Understanding water pressures and bar rating

When fitting new taps and shower valves water pressure can be a very important factor in choosing which products will be suitable. Normally the cold water pressure doesn't present a problem as this should be at the pressure of the mains, but the pressure of the hot water must be taken into consideration when deciding which tap style to choose. This is because a tap that is designed for a high pressure water supply can take an excessively long time to fill up a bath if it is fitted on a gravity fed system, and likewise a high pressure shower valve can be reduced to little more than a trickle.

Before choosing the taps or shower valve it is important to know the current pressure of the hot water system. Water pressure can be measured in three common units, bar, psi and Head (m). 1 bar = 10 metres Head = 14.5 psi.

There are two types of domestic hot water systems Pressurised and Unpressurised. Below is some help in understanding more about water pressures.

The water pressure available in a gravity-tank fed system is directly related to the height of the cold water storage tank. To calculate the available pressure simply measure the distance, in metres, from the bottom of the cold water tank to the outlet of the tap or shower and multiply by 0.1. This will calculate the pressure in bar...

for example : 0.1 Bar (1.45 psi) is equal to approx one metre of height between the bottom of the water tank and the outlet of the tap or shower.

1 metre height of water = 0.1 bar
2 metre height of water = 0.2 bar
3 metre height of water = 0.3 bar
4 metre height of water = 0.4 bar
etc

Therefore to achieve even 1.0 bar of water pressure from gravity fed hot water systems you would need to have a cold water header tank in the loft which is 10 metres above the height of the tap you are using. Clearly not possible for most houses.

Mains pressure: This is the pressure of the water supply entering the house measured from the stop tap. Water companies will maintain a minimum requirement of 1 bar water pressure, and a flow rate of around 9 litres per minute.