Water Pressure and Flow Rate

There’s a common misconception that flow rate and water pressure are the same thing. They are related, but they explain different aspects of your water system. 

 

Flow Rate

Flow rate is the volume of water passing through a pipe. Flow rate is a lot more variable than water pressure because it is affected specifically by friction.

The main factor in flow rate is the diameter of the pipes the water is passing through, wider pipes offer less resistance and so a higher flow rate. Other things that affect flow rate are bends in the piping, cartridges in taps etc. and any systems like water softeners or similar. Each “thing” added to the system adds friction, which will reduce the flow rate to some extent. 

A higher water pressure will increase  your flow rate and can seem like an easy fix but it’s worth considering how plumbing is being laid out when you’re adjusting or building a system.

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Simply keeping things clean will be a huge help as well. Limescale build up in pipes and taps can add a huge amount of friction and drastically reduce your flow rate. This can build up quickly in hard water areas like the South West of England where our Showroom is located.
We’ve written a handy guide to cleaning your tap cartridges and removing limescale which you can find here – Maintaining Tap Cartridges.

It’s possible to easily measure your flow rate using a bucket. Try and make sure you know how much water your bucket holds to make the final figures easier to calculate. This is measured in Litres Per Minute (LPM).  
Simply fill your bucket and time it from start to full. So if you have a 10L bucket, and it takes 1.5 minutes to fill: 10L÷1.5mins = 6.66 Litres Per Minute

Many manufacturers will list the approximate flow rate based on a suggested water pressure alongside their products. 

 

Water Pressure

Water pressure is the pressure of the water that is being forced through the pipe. It is a measurable amount of pressure being exerted on your water to push it through your system. 

The practical application of this in a building is how much pressure your water system is creating. In an older Gravity Fed system (see our blog post on different water systems here – Types of Home Water Systems) the pressure is dictated by the amount of “Head” available. Head is the vertical distance between the specific water outlet, and the water source, in this case a water tank.

For each 30 feet of “Head”, you achieve 1 BAR of water pressure. So if you have 3 feet between your shower head, and the water tank in the loft above it, you will receive 0.1 BAR of water pressure.  If you have a downstairs bathroom that has an outlet 12 feet below the tank, you will receive 0.4 BAR of pressure. Pressure in these systems can be increased by adding a pump which uses an impeller to generate increased pressure. 

In a more modern system like combination boilers and pressurized cylinders, your pressure will typically be more in line with the mains water pressure, or higher. Also the height of the system shouldn’t affect the pressure much, making things easier to measure and manage in a property with multiple floors. 

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Whilst water pressure and flow rate are related, water pressure should typically remain more or less static throughout the system. You can measure water pressure using a water pressure meter or gauge which you should be able to find at most plumbing merchants. 

Most taps, showers, and other items that fit into your water system will have a minimum BAR pressure that they require in order to function, in some cases they may have a maximum as well. This range will most often be listed in the manufacturer catalogues alongside the catalogues so that it’s easy to determine whether a product will work with your system. 

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