A combi boiler’s flow rate tells you how much hot water it can deliver in a minute, usually measured in litres per minute (L/min). Generally, a higher flow rate means the boiler can supply more hot water, which can improve shower performance and fill baths faster.

However, the figure on the boiler specification isn’t necessarily what you’ll get from your taps. Your actual hot-water flow is also limited by the water entering your property. A boiler capable of 15 L/min can’t provide that flow if the incoming mains supply can only deliver 10 L/min.

What does combi boiler flow rate mean?

Unlike a system or conventional boiler with a hot-water cylinder, a combi boiler heats water directly from the mains when you open a hot tap. It doesn’t normally keep a cylinder of hot water ready for use.

The boiler’s domestic hot-water flow rate describes how much water it can heat and deliver in a given period under specified conditions.

For example, a combi with a rated flow rate of 12 L/min can potentially provide approximately 12 litres of hot water per minute under the conditions used for its rating.

A higher flow rate can be useful if you want a stronger shower, need to fill a bath quickly or expect greater hot-water demand.

Why does temperature rise matter?

Flow-rate specifications are normally based on the boiler raising the incoming water temperature by a specified amount. This is known as the temperature rise, or ΔT.

For example, if incoming mains water is 10°C and the boiler heats it to 45°C:

45°C − 10°C = 35°C temperature rise

The boiler has a limited amount of heating power. The greater the required temperature rise, the less water it can heat to the required temperature in a given time.

This means a boiler’s stated flow rate needs some context. When comparing different models, check whether their figures are quoted at the same temperature rise.

Incoming water temperature can also change throughout the year. Colder incoming water requires the boiler to add more heat to reach the same hot-water temperature.

What’s the difference between boiler kW and flow rate?

Boiler output and flow rate are closely related, but they measure different things.

Kilowatts (kW) measure the boiler’s heating power.

Litres per minute (L/min) measure how much hot water it can deliver per minute under specified conditions.

Higher-output combi boilers can generally heat more water per minute, which is one reason larger combis often have higher domestic-hot-water flow rates.

Typical figures might look approximately like this:

Combi hot-water output	Illustrative flow rate
24–25kW	9–11 L/min
28–30kW	11–13 L/min
32–35kW	13–15 L/min
40kW+	16 L/min or more

These are illustrative ranges rather than specifications that apply to every boiler. Check the manufacturer’s figures for the individual model and the temperature rise at which the flow rate was measured.

A combi boiler may also have different outputs for hot water and central heating. Its high maximum output may be needed to produce hot water quickly even though the property requires much less power for space heating.

What is a good flow rate for a combi boiler?

There isn’t one ideal flow rate for every home. It depends on your hot-water demand, the number of outlets you’re likely to use simultaneously and what the incoming mains supply can provide.

As a rough guide:

Household requirements	Flow rate to consider
Lower demand / one hot outlet at a time	Around 9–11 L/min
Typical one-bathroom home	Around 10–12 L/min
Greater hot-water demand	Around 12–15 L/min
High demand / faster bath filling	15+ L/min

These aren’t strict sizing rules. Which? notes that around 9–12 L/min may be appropriate for many one-bathroom homes, while 12–16 L/min can provide greater capacity for homes with two bathrooms.

However, simply choosing the boiler with the highest number isn’t necessarily worthwhile. If your mains supply or pipework can’t provide enough water, you may not benefit from the boiler’s additional capacity.

What happens when two taps or showers run at once?

This is one of the main limitations of a combi boiler.

Because a combi heats water as it’s required rather than drawing it from a stored supply, its available hot-water output has to be shared between outlets.

Suppose your boiler can deliver 14 L/min. If one shower is using most of that capacity and somebody opens another hot tap, the boiler doesn’t suddenly produce another 14 L/min. The available hot-water flow has to be divided between the outlets.

You may therefore notice a reduction in flow when several people use hot water simultaneously.

A higher-flow combi can provide more capacity, but even a powerful combi has limits. Homes where several showers or baths are regularly used at the same time may be better suited to a system that stores hot water.

Boiler flow rate vs mains water flow rate

The boiler can only work with the water supplied to it.

If your mains supply delivers 10 L/min, installing a combi capable of heating 16 L/min won’t make 16 litres of water arrive at the boiler each minute.

The incoming supply can therefore become the limiting factor.

Pipework can also affect the flow available at individual outlets. Restrictions, small pipe diameters and simultaneous demand elsewhere in the property can all affect what you experience at a tap or shower.

This is why the incoming water supply should be assessed when choosing a combi rather than sizing the boiler solely from the number of bedrooms or bathrooms.

What’s the difference between water pressure and flow rate?

Water pressure and flow rate are related but aren’t the same measurement.

Water pressure describes the force pushing water through the pipework and is normally measured in bar.

Flow rate describes the volume of water passing a point over a period and is normally measured in litres per minute.

A property can therefore have apparently reasonable water pressure while still delivering insufficient flow for a high-output combi.

Static pressure, the pressure when no water is being drawn, also doesn’t tell the whole story. Dynamic pressure, measured while water is flowing, is particularly relevant when assessing whether the supply can support a combi boiler.

An installer can assess both pressure and flow before recommending a boiler.

How can you check your home’s water flow rate?

You can make a rough flow-rate measurement using a measuring container and a stopwatch.

Run a suitable cold mains tap fully and measure how much water it delivers over a set period.

The calculation is:

Volume collected (litres) ÷ time (seconds) × 60 = flow rate in L/min

For example, if you collect 5 litres in 20 seconds:

5 ÷ 20 × 60 = 15 L/min

This gives you an approximate flow rate at that particular outlet and at that moment.

It isn’t a substitute for a professional assessment before installing a boiler. An installer can assess the incoming mains supply, dynamic pressure and how flow changes when other outlets are opened.

Can you improve a combi boiler’s flow rate?

It depends on what’s limiting it.

If the boiler itself can’t heat water quickly enough, a higher-output combi may provide a greater hot-water flow rate. But this only helps if the incoming supply can support it.

If the mains flow is the limiting factor, installing a more powerful combi won’t solve the underlying problem.

Likewise, if you’ve previously had good hot-water flow and it has suddenly deteriorated, don’t automatically assume the boiler is too small. A fault, restriction or another change to the system may need investigating.

When might a combi boiler not be suitable?

Combi boilers can work particularly well in homes where hot-water demand is relatively modest and several outlets aren’t routinely used simultaneously.

They become less suitable when a household needs large quantities of hot water from multiple outlets at the same time.

A system boiler with a hot-water cylinder can store a larger volume of hot water ready for use. This may make it more suitable for properties with several bathrooms or high simultaneous demand, provided the system and incoming supply are appropriately designed.

The trade-off is that a cylinder requires additional space, while a combi provides heating and instantaneous hot water from a single appliance.

Choosing between them should therefore be based on the property’s actual hot-water requirements rather than assuming a larger combi will always solve a flow problem.

Frequently asked questions

Is 10 L/min a good combi boiler flow rate?

Around 10 L/min may be adequate where hot-water demand is relatively modest and one outlet is normally used at a time. Whether it’s sufficient depends on the household and the type of shower or other outlets being used.

Is 12 L/min enough for a shower?

It can be. Many showers operate comfortably at less than 12 L/min, but requirements vary. Remember that the boiler’s rated figure isn’t necessarily the flow you’ll receive at the shower.

Is 15 L/min a good flow rate?

A 15 L/min combi offers relatively strong hot-water performance, provided the incoming mains supply can support it. It can be useful for higher-demand households or faster bath filling.

Will a bigger combi boiler improve water pressure?

Not necessarily. A higher-output boiler may be able to heat more water per minute, but it doesn’t create additional mains water pressure or flow. The incoming supply may still limit performance.

Can a combi boiler run two showers at once?

Potentially, but both showers must share the boiler’s available hot-water capacity and the incoming mains supply. Flow at each shower may therefore fall. If two or more showers are regularly needed simultaneously, a stored hot-water system may be more suitable.

Sources

Which?, How to buy the best boiler –  https://www.which.co.uk/reviews/boilers/article/buying-a-new-boiler/how-to-buy-the-best-boiler-aOGcH4m3mSJt

Accurate as of: 11 September 2026