When choosing the right combi boiler for you, one of the key considerations comes down to the water flow rate. A good installer will ask or check this for you before recommending a boiler.
What is high water flow rate?
Water flow rate is the amount of water that comes out of your taps, measured in litres per minute, the more water comes out, the higher the flow rate. It is dependent on pipework, valves and taps in the plumbing system installed in your house. Factors such as water pressure (the force of the water) can also impact the flow rate.
Flow rate is different from the water pressure in that the force at which the water comes out is called the pressure rather than the amount that comes out.
How is water flow rate measured?
It is measured in litres per minute (l/min) and this is an important factor when choosing the right combi boiler. The higher the number the more hot water you get through your tap. It is measured with a weir cup by plumbers and heating engineers.
How to measure water flow rate in your tap
Turn the cold tap nearest to where your boiler is or will be on full, and use a 1 litre measuring jug and a stopwatch. Press start on the stopwatch as soon as you put the jug under the running tap, and stop the timer when 1 litre is filled.
Use the following chart to easily find what the flow rate is from your tap.
Water Flow Rate Chart
| Time to fill 1 litre jug (in seconds) | Litres per minute |
|---|---|
| 2 | 30 |
| 3 | 20 |
| 4 | 15 |
| 5 | 12 |
| 6 | 10 |
| 7 | 8.5 |
| 8 | 7.5 |
| 9 | 6.7 |
| 10 | 6 |
| 11 | 5.4 |
| 12 | 5 |
| 13 | 4.6 |
| 14 | 4.3 |
| 15 | 4 |
What’s a good water flow rate for a combi boiler?
With combi boilers, you are always limited in how much hot water you will get per minute. But for a hot bath to fill quickly or a good shower, 14 to 16 litres per minute is considered a good rate. To make it higher, you will likely need a storage combi or a conventional boiler with a separate storage tank.
What is the minimum flow rate for a combi boiler?
Because combi boilers take water straight from the mains cold supply, they need a minimum of 10 litres per minute to function adequately. The hot water flow coming out of the taps will be less than this because of the resistance within the boiler. You can increase the rate with a mains pump or an accumulator tank.
When do you need a high water flow rate combi boiler?
Having a good performing boiler helps to fill a hot bath quicker and gives better shower output.
Combi boilers can struggle when there is more than one hot water tap open. But as they save space and are efficient, they will continue to be popular. If you have two or more bathrooms, or one bathroom and two showers and more than a couple of occupants, you will likely need a high flow rate boiler.
The alternative to having a more powerful combi is a system boiler. They are better for homes with two or more bathrooms because they store hot water in a cylinder. When the cylinder is full, two baths or showers can be run at the same time. Most combi boilers will struggle to do this.
Do you need a powerful boiler for higher water flow rates?
A higher water flow rate usually requires a high kW output boiler. A lower output combi of 24kW provides up to 10 litres of hot water per minute. When looking for a boiler that can supply more than one hot tap at the same time comfortably, you will need a boiler between 34kW and 42kW.
Now we have looked at some factors that go into choosing a good boiler that has a high water flow rate, let’s look at some of the best, in our opinion, combis on the market:
Best combi boilers with high water flow rates
| Combi boiler | Water flow rate (in litres per minute) |
| Viessmann Vitodens 111-W 35kW | 18.4 |
| Alpha E-Tec Plus 38 kW | 15.9 |
| Baxi 600 36 kW | 15 |
| Ideal Logic C35 | 14.5 |
| Worcester Bosch 8000 Style 35kW | 14.3 |
Alpha E-Tec Plus 38kW Combi
This 38 kW boiler has a massive 15.9 l/min hot water flow rate. It has quality parts inside and comes at a very reasonable price compared to the more well known brands.
Baxi 600 36kW Combi
This is a powerful yet compact boiler built in the UK providing a powerful 15 l/min hot water flow rate
Ideal Logic C35
The very popular Logic range also features the most installed boiler in the country. With a powerful 14.5 l/min flow rate, it’s great for a large household.
Worcester Bosch 8000 Style 35kW Black
A stylish and compact boiler from one of the best known brands in the UK. Delivers a good 14.3 l/min flow rate, and has many other features included.
Storage combi boiler
A storage combi provide space saving benefits and powerful water flow rates in one. You won’t need a separate storage tank with them and they provide instant hot water like a regular combi. They are larger in size than a normal combi with the extra storage capacity.
Viessmann Vitodens 111-W 35kW
A wall mounted storage combi specially designed for families with a high demand for hot water. It has a 46 litre hot water capacity storage tank integrated into the unit and substantial water flow for large homes at 18.4 l/min.
*The information in this article should be used for general guidance only and not as financial advice. Full details are on the link in the footer to our disclaimer page. Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.
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