A storage boiler comprising: a core comprising a heat storage medium ; a base to which the core is mounted, and a fluid system extending through the core and comprising: a heat exchanger ; a cooled fluid passageway extending from the heat exchanger to the core ; a heated fluid passageway extending from the core to the heat exchanger ; a fan for circulating a transfer fluid through the cooled fluid passageway , the core , the heated fluid passageway and the heat exchanger ; and a bypass arrangement for selectively communicating at least part of the transfer fluid from the cooled fluid passageway to the heated fluid passageway without passing through the core and mixing cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger , wherein the bypass arrangement is mounted inside the base ; and a control system configured to operate the fan , by varying the speed of the fan , for controlling the flowrate of transfer fluid through the core and fluid system and to operate the bypass arrangement for controlling the flowrate of transfer fluid through the bypass arrangement . a core comprising a heat storage medium ; a base to which the core is mounted, and a fluid system extending through the core and comprising: a heat exchanger ; a cooled fluid passageway extending from the heat exchanger to the core ; a heated fluid passageway extending from the core to the heat exchanger ; a fan for circulating a transfer fluid through the cooled fluid passageway , the core , the heated fluid passageway and the heat exchanger ; and a bypass arrangement for selectively communicating at least part of the transfer fluid from the cooled fluid passageway to the heated fluid passageway without passing through the core and mixing cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger , wherein the bypass arrangement is mounted inside the base ; and a control system configured to operate the fan , by varying the speed of the fan , for controlling the flowrate of transfer fluid through the core and fluid system and to operate the bypass arrangement for controlling the flowrate of transfer fluid through the bypass arrangement . a heat exchanger ; a cooled fluid passageway extending from the heat exchanger to the core ; a heated fluid passageway extending from the core to the heat exchanger ; a fan for circulating a transfer fluid through the cooled fluid passageway , the core , the heated fluid passageway and the heat exchanger ; and a bypass arrangement for selectively communicating at least part of the transfer fluid from the cooled fluid passageway to the heated fluid passageway without passing through the core and mixing cooled transfer fluid with heated transfer fluid prior to the mixed transfer fluid entering the heat exchanger , wherein the bypass arrangement is mounted inside the base ; and a control system configured to operate the fan , by varying the speed of the fan , for controlling the flowrate of transfer fluid through the core and fluid system and to operate the bypass arrangement for controlling the flowrate of transfer fluid through the bypass arrangement . <EOS>