Combi is short for 'combination' boiler and is labelled as such since it provides both hot water and heating from a single unit. They are the most popular type of boiler in the UK. In fact, they're used by 74% of households.
Combi boilers are efficient and cost less in heating bills.That's because:
They only heat water when you need it, rather than storing hot water up
They're small and compact, as they don't use an external hot water tank