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Combi Boiler Hot Water Flow Rate Explained

A combi's flow rate tells you how much hot water it can deliver. It's the figure to check if you want strong showers.

3 min read · Updated October 2026

Shower running with a combi boiler and a display showing 12 litres per minute

What the figure means

Combi boilers list a hot water flow rate in litres per minute, usually at a stated temperature rise. Higher-output combis can heat more water per minute: as a rough guide, many combis deliver somewhere around 10 to 16 litres per minute depending on their output.

Mains pressure and flow matter too

A combi can only deliver as much water as your mains supply provides. If your incoming flow is low, a larger combi won't help. Engineers test mains pressure and flow before recommending a combi.

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Choosing for your household

Consider:

  • How many showers or baths are used at the same time
  • Whether you have a power or mixer shower
  • Your incoming mains flow rate
  • Whether a system boiler with a cylinder would suit better

Common questions

Will a bigger combi give a better shower?

Only if your mains supply can provide the extra flow.

Why does my shower go cold when someone runs a tap?

The combi's flow is shared between outlets.

Can I fit a power shower with a combi?

Pumped power showers usually aren't used with combis; mixer showers are common instead.

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