Boiler Flow Rates Explained

Why Mains Pressure Matters

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Homeowners thinking about upgrading their boiler need to consider several things including power output, efficiency and flow rate.

Of these flow rates are often left until last, with most buyers focusing on power outputs and efficiency levels first. But this is the wrong way to go about it because installing a boiler with a flow rate higher than your mains can supply, will result in inefficient heating.

What is boiler flow rate?

Put simply, the flow rate, measured in litres per minute (l/min), is the volume of hot water that passes through the boiler every minute. Generally speaking, the more bathrooms and showers you have, the higher flow rate you need.

But it’s not that simple. You can’t just install a boiler with a high flow rate because you want to. The amount of water that can flow through your boiler is dictated by the volume of water supplied from the mains, this is known as the mains supply flow rate.

For example, say you have a mains supply flow rate of 9 litres per minute. And you install a boiler with an optimum flow rate of 16 litres per minute. In this case, the mains cannot supply the boiler with enough water to maximise its output.

So when replacing a boiler you need to ensure your new boiler has a flow rate that matches your mains flow rate.

How to measure your mains flow rate

Now you understand what the different flow rates are and why they are important, how do you calculate your mains flow rate.

Fortunately, this is easier than you think. All you’ll need is a stopwatch (a mobile phone will do) and a measuring jug.

Place the measuring jug under the cold tap in your bathroom and turn it on. Run the tap for six seconds precisely. Now take the volume of water in the jug, in litres, and multiply by 10. (eg 0.9 litres x 10 = 9 litres per minute).

Anything below 10 litres per minute is considered low pressure. A reading between 10-15 litres per minute is average and a flow above 15 is considered good.

Once you have established your mains flow rate, it’s simply a case of choosing the most efficient and cost-effective boiler with a compatible flow rate.

How to boost your mains flow rate

As more homes are built on existing infrastructure, mains water pressure is continually reducing in most areas. As a result, many more homes are now suffering from low mains flow rates.

This can be a problem if you want to install a power shower or increase the number of bathrooms in your home. Well don’t panic, there are a couple of solutions to this problem.

The easiest and most cost-effective method is to install a pump into your mains supply. This will help to boost pressure, but it is not a failsafe solution. Pumps are only designed to push water, not draw it, so you still need an adequate supply of water.

There are also specific bylaws in place that limit pumps to 12 litres per min to protect the mains infrastructure. This may not be enough, considering a power shower can use as much as 9-15 litres per minute.

Homes that need a flow rate greater than 12 litres a min, will have to install a break tank to provide a reservoir of water. A push/draw pump can then be installed to supply water at a higher flow rate.

An average-size home will need a break tank of approximately 200 litres. The more bathrooms and showers you install the bigger tank you need. Combined pump and tank systems are available at a relatively low cost and can be installed very easily.


With boiler replacement costs running at £1,500 for a three-bedroom home, it’s worth taking the time to ensure your new boiler is working to its maximum output and efficiency.

The only way to do this is to establish your mains flow rate first and then choose a compatible boiler. Don’t be fooled into thinking a boiler with a higher flow rate will solve your hot water problems. It won’t unless you boost the mains flow to feed it.

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