A heat battery can hold enough energy for a day's hot water yet still disappoint at a high-flow shower. Storage capacity, heat-exchanger output, mains pressure and pipework all affect the experience at the outlet.
The short answer
Measure dynamic flow and pressure while water is running, not only static pressure at rest. Ask the supplier for delivered litres per minute at a stated incoming and outlet temperature.
Temperature rise changes the claim
Heating water from 15°C to 40°C needs less power than heating it from 5°C to 50°C. A flow figure without temperatures is incomplete, especially through a UK winter.
Simultaneous outlets
Two showers divide the available water and thermal power. Add kitchen taps or a filling bath and the peak becomes higher. State which outlets must run together.
Find the limiting component
The incoming main, stop tap, internal pipe size, filters, valves, heat exchanger and shower fitting can each restrict flow. Replacing the store does not cure a weak main.
Compare with today's performance
Time a bucket fill at a representative cold tap and note shower performance while another outlet runs. An installer can take proper measurements, but the household test provides useful context.
Mixing and safe temperatures
Outlet controls must manage scalding risk while delivering useful flow. Do not bypass thermostatic or safety devices to chase a brochure number.
Your next step
List the two most demanding outlets that may run together. Ask for the predicted flow and temperature at both, with the assumed incoming water temperature.
Sources
Primary and independent references checked by our editorial team.
- Sanitation, hot water safety and water efficiency: Approved Document G — GOV.UK Checked 10 Nov 2026