Combi or system boiler under ECO4: hot water questions UK households should ask (2026)

Guide by ApplyFreeBoiler.uk (Reliant Energy Solutions Ltd), a heating and insulation installer in Bury, Greater Manchester. We are not the government, Ofgem or an energy supplier.

When an old or broken boiler is replaced under ECO4, one of the first practical questions is whether the new one will be a combi boiler or a system or regular boiler with a hot water cylinder. The right answer depends on the home and how hot water is used. This guide lists the questions to raise with the surveyor. It is general information from a Bury-based heating and insulation installer and does not guarantee funding.

1. Write down what you have now

Note whether you have a hot water cylinder in an airing cupboard, a cold water tank in the loft, or neither. Take a photo of the boiler data badge if you can find it. This tells the surveyor where they are starting from.

2. Count bathrooms and showers

A combi heats water on demand and can struggle if two showers run at once. Homes with more than one bathroom, or several people showering at the same time, may suit a stored hot water system better. Explain your daily routine to the surveyor.

3. Ask about water pressure and flow

Combi boilers rely on the mains supply. If your cold water flow is weak, ask the surveyor to test it and explain how it affects the choice of boiler.

4. Think about space

Moving to a combi can free the airing cupboard and loft tank. Keeping a cylinder uses that space but can store hot water. Decide whether the space or the stored hot water matters more to your household.

5. Ask about electric showers and immersion heaters

If you rely on an electric shower or immersion heater, ask whether it will stay, be removed or be disconnected, and what that means for running costs and back-up hot water.

6. Check the controls

Ask which thermostat, timer and radiator valves come with the new boiler, and ask the engineer to show you how to set them before they leave.

7. Ask about flue and pipe changes

A different boiler type can mean a new flue position, a new condensate pipe or moved pipework. Ask where these will go and whether any of it will be visible outside.

8. Keep commissioning paperwork

Keep the commissioning checklist, the Building Regulations compliance certificate for the new boiler and the warranty details together. Check the engineer's Gas Safe ID card on the day.

9. Compare local delivery data

See ECO measures by council on ApplyFreeBoiler.uk, built from DESNZ Household Energy Efficiency statistics, to see how many ECO measures have been installed in your local authority area.

10. Check eligibility

Use the ApplyFreeBoiler.uk eligibility check to see whether an ECO4 heating or insulation pathway may be available for your home in England, Scotland or Wales. ECO4 is due to close on 31 December 2026.