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Boiler Flow Rates Explained

What is a boiler flow rate and how does it affect my hot water supply?


Boiler Flow Rates ExplainedWhen manufacturers & installers talk about water heaters, especially combi-boilers, a key phrase used is “flow rate”.

In most cases this is really referring to how much water can flow through the boiler & how fast it can come out of a tap or shower head once the boiler is working.

However, although you may think that getting the boiler with the maximum flow rate will be the best option, you could actually be wasting your money & time if the mains water supply to your house is less than the boiler can handle – for example, you could buy a boiler with a flow rate of 12 litres per minute but your mains pressure only supplies 9 litres per minute, so your boiler can never reach its full potential because of the lower mains supply flow rate.

Water: If it isn’t coming in fast – it’s not going to come out of your tap fast.

Flow & Boiler Power Requirements

You should note that the higher the boiler flow rates are, & the larger the rise in temperature needed from the mains water, the more powerful the boiler needs to be in order to cope with heating a faster flow of water.

If you want hot water at 9 litres per minute, what amount of heating power do you need from your boiler?

Starting with the assumption that we increase the temperature of the incoming water from 10°C so that it comes out at 60°C (a temperature rise of 50°C).

The calculation is based on:

Water weight x rise in temp x specific water heat capacity 

9 litres of water weighs 9 kilos, so the flow is 0.15 kg per second

So we get 0.15 x 50 x 4.2 = 31.5 kJ/sec (1 kiloJoule per sec = 1 kiloWatt) (rapidtables.com/convert/electric/kj-to-kw)

  • Which broadly means you need something like a 31.5 kilowatt power output on your boiler without allowing for any inefficiencies

The power requirements are really affected by the amount of temperature rise & the flow rate as if you have 10 litres per minute with a 40°C temperature rise we get the following

  • Water weight (0.167) x rise in temp (40) x specific water heat capacity (4.2) =28.56

To get the best option for your home, you really need to have an expert around to check the water pressure & flows, then discuss what you are going to need from your boiler, do a survey and then provide you with a quote based on the boiler recommended for your situation. (combi boiler prices here)

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