What is boiler flow rate?
Put simply, the flow rate, measured in litres per minute (l/min), is the amount of hot water your boiler can deliver every minute.
In general, higher flow rates are needed for:
- Homes with multiple bathrooms
- Powerful showers
- Simultaneous hot water use
However, your boiler can only deliver as much water as your home’s mains supply allows. This is known as the mains supply flow rate.
For example, if your mains can supply 9 litres per minute but your boiler is capable of 16 litres per minute, the boiler won’t be able to reach its full hot water output.
- You won’t get the performance you paid for
- Hot water flow may feel weaker than expected
- Efficiency gains will be limited
Your boiler’s flow rate should always match your mains water supply.
How to measure your mains flow rate
Measuring your mains flow rate is simple and doesn’t require any specialist equipment. All you need is:
- A measuring jug
- A stopwatch (your phone will do)
Place the jug under a cold tap and turn it on fully. Run the tap for exactly six seconds. Measure the amount of water collected (in litres) and multiply it by 10.
Example: 0.9 litres × 10 = 9 litres per minute.
As a general guide:
- Below 10 l/min – Low flow
- 10–15 l/min – Average
- Above 15 l/min – Good
Once you know your flow rate, you can choose a boiler that suits your home’s water supply.
How to boost your mains flow rate
As more homes are built on existing infrastructure, water pressure has reduced in some areas. This can affect shower performance and hot water flow, especially in larger households.
There are two common solutions:
- Booster pumps – Help push water through your system (but can’t create water)
- Break tank systems – Store water and provide a stronger, more consistent supply
Regulations limit pumped flow rates in many areas to protect the water network. Homes with multiple bathrooms may need a break tank system for reliable performance.
Typical guidance:
- Average home: ~200 litre tank
- Larger homes: Bigger tanks required
Conclusion
Choosing the right boiler isn’t just about efficiency ratings and power output. Your mains water flow rate plays a major role in how well your hot water system performs.
- Measure your flow rate first
- Choose a boiler that matches it
- Upgrade the supply if needed
A higher-flow boiler won’t fix water pressure issues on its own, but the right setup can deliver reliable, efficient hot water for your home.
It’s always recommended that you speak to a qualified boiler specialist before making a final decision. They can assess your home’s water pressure, heating demand, and overall system requirements to help you choose the right boiler size for reliable performance and long-term efficiency.
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