Water Pressure and Flow

The difference between water flow and water pressure sometimes causes confusion.

Water Flow

Water Flow is a measurement of how much water is delivered at a particular outlet over a set period of time. For example if you place a 10 litre bucket under the tap in a sink and it takes 10 seconds to fill the bucket you can see that the flow rate is 1 litre per second .

Water Pressure

Water Pressure is a measurement of the force exerted by the water. We can see how this works by looking at an example. A cold water storage cistern in the attic may be used to supply water to a basin in a bathroom and a basin in a downstairs cloakroom. Assuming everything else is equal you will notice that the pressure at the downstairs tap is considerably more than that at the one upstairs. The increased pressure is due to the height of the cistern in relation to the tap.

A good way of understanding the relationship between the two is to consider a garden hose with an adjustable nozzle on the end. With the nozzle only open a small amount, there will be a high pressure jet of water but the amount of water coming out per minute will be relatively low. Open the nozzle fully and the pressure will be far less but the amount of water coming out – the flow rate – will be far greater

There are devices available to measure water pressure and water flow. You may see a plumber using these. Where a new plumbing installation is being designed, measurements of both the incoming mains water pressure and flow may be needed to help with the design and specification process.

Mains Water Pressure Gauge

For example, the pressure for the outlet supplying a washing machine may need to be checked to see if it meets the manufacturer’s specified limits. Using a mains water pressure gauge, the pressure at an outlet can be measured. This device has a threaded connection to fit the outlet and will measure the water pressure in bars

Water Flow Weir Cup

Although in the example earlier, a rough measurement of water flow may be taken with a specific size bucket, accurate measurements of water flow may be required. For example a boiler installer may need to check there is a sufficient flow rate for installing a combi boiler. The device used is called a weir cup. Readings on these are usually shown in both litres per second and gallons per minute. If a flow rate needs to be reduced, a weir cup may be used to measure and then accurately adjust the flow rate to meet the operating flow rate required to suit an appliance manufacturer’s specification.