A fluid heater (100) comprising a storage reservoir (1) for storing heating fluid, a burner (5) for heating the heating fluid and a first heat exchanger (11) located outside said storage reservoir (1). The heater (100) also comprises a first fluid circuit (200) connecting the storage reservoir (1) with the first heat exchanger (11) for carrying heating fluid from the storage reservoir (1), through the first heat exchanger (11) and back to the storage reservoir (1), and a second fluid circuit (300) connecting a first inlet (19) for mains fluid to a first outlet (27) for heated mains fluid via the first heat exchanger (11). The first heat exchanger (11) is adapted to exchange heat between said heating fluid and said mains fluid.