Replacing a cylinder is not a straight volume swap. A heat battery stores energy in another medium and may heat mains water through a heat exchanger, so its capacity and flow claims need different evidence.
The short answer
Record how the existing cylinder is heated, its volume, recovery pattern, water pressure and the household's busiest period. Then ask the new system to meet a stated outcome: simultaneous showers, a bath, or a full morning routine.
Survey the existing connections
Photograph cold feed, hot outlet, primary coil connections, immersion supply, expansion components, discharge pipework and controls. Ask which parts remain and which are replaced.
Do not use litres alone
A maker's cylinder-equivalent figure depends on test temperatures and draw pattern. Compare usable kWh and water delivered at the required temperature rise.
What happens to the old system
The quote should cover draining, safe removal, disposal, redundant pipework and making good. If the cupboard will be repurposed, confirm the final floor, walls and ventilation rather than assuming they are included.
Keep a fallback during the work
Ask how long hot water will be unavailable and whether unexpected electrical or plumbing work could extend that period. Arrange access and washing needs accordingly.
Check the boost behaviour
When stored heat runs out, the system may wait for the next charge, use an electric boost or call another heat source. The comfort and cost implications should be written into the design.
Your next step
Photograph the cylinder label and all visible connections before the survey. Add a seven-day record of baths, showers and times when hot water ran short.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026
- Sanitation, hot water safety and water efficiency: Approved Document G — GOV.UK Checked 10 Nov 2026