A storage boiler

The storage boiler 10 has a core 11 comprising a heat storage medium (e.g. steel fragments or magnetite dust) and a fluid system extending through the core and comprising a heat exchanger 51, a cooled fluid passageway 60 extending from the heat exchanger to the core and a heated fluid passageway 61 extending from the core to the heat exchanger. A fan 52 is operable to circulate a transfer fluid (e.g. air) through the core via the passageways and through the heat exchanger. A bypass arrangement 80 (e.g. bypass passageway 86 and gate/flap 83) selectively communicates at least part of the transfer fluid from the cooled fluid passageway to the heated fluid passageway without passing through the core. The boiler ideally includes one or more v-shaped baffles (90, figure 3) to distribute air leaving the core over the heat exchanger. A storage boiler comprising a core containing an aggregate bed of material elements, and a storage boiler having a core fluid passageway passing through and sealed from an interior of a core are also claimed.