Boiler flow rate is the amount of hot water a combi boiler can deliver each minute at a useful temperature, measured in litres per minute. A typical shower uses around 8 to 12 litres a minute, so a combi that delivers 10 litres a minute runs one good shower and nothing else. Open a second hot tap and both slow down and cool. Flow rate is the single most important figure when choosing a combi, and it is limited not only by the boiler but by the pressure and flow of the water coming into your house from the mains.
This guide explains what the numbers mean, why showers go cold, how to measure what your house can deliver, and how to choose a boiler that keeps up with your household.
What the flow rate figure means
Manufacturers quote a combi’s flow rate at a given temperature rise, usually 35 degrees, which is roughly the lift from cold mains water to a warm shower. A boiler rated at 12 litres a minute at a 35 degree rise will deliver a little more in summer, when the mains is warmer, and a little less in winter. Higher-output combis reach 14 to 16 litres a minute or more; compact models sit around 9 to 11.
Why the mains matters as much as the boiler
A combi can only heat the water that reaches it. If your incoming mains delivers 10 litres a minute, a combi rated at 16 makes no difference; you get 10. Mains flow depends on the water company’s pressure in your area, the size and age of the pipe from the street to your house, and the pipework inside. Old lead or narrow pipes, long runs from a rural main and shared supplies all reduce it. An installer should measure the mains flow and pressure at the survey before recommending a combi.
Why showers go cold when a tap runs
A combi shares its hot water between every open outlet. Run the kitchen tap while someone is in the shower and the shower’s share drops. The boiler may also struggle to hold the temperature as the flow through it changes, which is the hot-and-cold effect. The fix is either a higher-output combi with a mains supply that can feed it, or a system boiler with a cylinder, which stores hot water and delivers it to several outlets at once. Our guide to a combi boiler going hot and cold covers the fault-finding side.
How to measure your own flow
Put a measuring jug under the cold kitchen tap, open it fully and time how long it takes to fill. Divide the litres by the seconds and multiply by 60. That is your mains flow in litres a minute, and it is the ceiling for any combi. For pressure, a simple gauge that screws onto an outside tap gives the reading in bar; most combis want at least 1 bar, and unvented cylinders need more.
Matching the boiler to the household
| Household | Typical recommendation |
|---|---|
| One bathroom, one or two people | Compact combi, around 10 to 12 litres a minute |
| One bathroom, family | Mid-range combi, around 12 to 14 litres a minute |
| Two bathrooms used at different times | High-output combi, 14 litres a minute or more, with a good mains supply |
| Two bathrooms used together, or a large family | System boiler with an unvented cylinder |
| Weak mains supply, any size of home | System boiler with a cylinder, or a combi with an accumulator tank |
Our guides to what size boiler you need and what a system boiler is cover the choice in more depth.
Ways to improve flow without changing the boiler type
- upgrading the mains pipe from the street to a larger modern pipe
- fitting an accumulator tank that stores mains water under pressure to boost flow at peak times
- replacing narrow or scaled internal pipework
- removing unnecessary restrictors and old shower valves
How DCI Boilers can help
DCI Boilers measures mains flow and pressure at every survey across Cornwall and Devon, and recommends a combi or a cylinder system based on what your house can actually deliver, not on the brochure figure. Our Gas Safe and OFTEC registered engineers fit the boiler at a fixed price with manufacturer warranties of up to 12 years. Read about our gas boiler replacement service, or our heating services for cylinders and mains upgrades.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
What is a good flow rate for a combi boiler?
Around 12 litres a minute suits most one-bathroom homes. Two bathrooms need 14 or more, and a mains supply that can provide it.
Can a combi boiler run two showers at once?
A high-output combi with a strong mains supply can, at reduced flow to each. For two showers at full strength, a cylinder system is the reliable answer.
Why is my new combi’s hot water weak?
Usually the mains supply, not the boiler. Have the incoming flow and pressure measured; the pipe from the street may be the limit.
What is an accumulator tank?
A pressurised vessel that stores mains water and releases it at peak times, boosting the flow available to a combi where the mains is weak.