A plain-English guide to the main boiler types.
Start my quoteHeats water on demand straight from the mains – no hot water tank or loft tanks. Compact and common in smaller homes with one bathroom. Can struggle if several hot taps run at once.
Works with a hot water cylinder (usually in an airing cupboard) but no loft tanks. Suits homes with more than one bathroom.
Uses a hot water cylinder plus cold water and expansion tanks in the loft. Often found in older homes with older pipework.
Replacing like-for-like is usually simplest. Switching (for example from a regular boiler to a combi) means removing tanks or a cylinder and changing pipework, which usually costs more. Your engineer will advise what suits your home and hot water use.
General information only – not advice for your home. Sources: Checkatrade, checked October 2026.