Why mixers and combis go together
A combi boiler heats water on demand from the cold mains, so both hot and cold supplies reach the shower at similar pressure. That's ideal for a thermostatic mixer shower, which balances the two and holds a steady temperature.
Pumps and power showers are not suitable: they're designed for tank-fed systems, and water regulations restrict pumping directly from the mains. If a weak shower comes from a combi, the answer is never a pump – it's finding what's limiting the flow.
Flow is the limiting factor
A combi can only heat so many litres per minute, depending on its output. If the shower – especially a large rainfall head – asks for more than that, the water will be cooler or weaker than you hoped. The incoming mains supply matters too: a narrow or old supply pipe can starve both the boiler and the shower.
- Check the boiler's hot water flow rate in its manual
- Choose a shower head and valve suited to that flow
- Be cautious with very large rainfall heads
- Expect the flow to drop if a tap is run at the same time
- Consider an electric shower for a second bathroom
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Two bathrooms on one combi
Combis are best suited to homes where one shower is used at a time. If two showers run together, both will suffer. For larger households, options include an electric shower in the second bathroom, a higher-output combi, or a system with a hot water cylinder.
Installation tips
When fitting a new mixer to a combi, plumbers check the incoming pressure and flow, use pipe sizes that don't restrict the supply, and set the thermostatic valve's maximum temperature. Any change to the boiler itself must be done by a Gas Safe registered engineer.
