A combi heats water on demand. It’s compact, efficient and needs no cylinder, which makes it perfect for flats and homes with one bathroom.
A system boiler heats a stored cylinder of hot water. Paired with an unvented cylinder, it delivers mains-pressure hot water to several outlets at once.
Rule of thumb
- One bathroom, 1–3 people: combi.
- Two bathrooms or an en-suite, or two showers at once: system plus unvented.
- Three or more bathrooms: system plus a large unvented cylinder (250 litres or more).
- Weak mains pressure: a vented or thermal-store option may be better. We test before we recommend.
Try our instant boiler quote to see what we’d recommend for your home.