Abstract: A hot water storage unit 100 having a vessel 102, in which hot water is storable, an outer casing 104 enclosing the vessel 102, a hot water outlet 114 for heated water to exit from inside the vessel 102, and a relief device 106. The relief device 106 relieves excess pressure and/or temperature from inside the vessel 102 by allowing water to escape from inside the vessel 102 and exit from the relief device 106. The relief device 106 comprises a housing 108 containing the operational components of the relief device 106. The relief device 106 is removably retainable relative to the vessel 102 such that the housing 108 of the relief device 106 is located substantially behind the outer casing 104 of the hot water storage unit 100 on the inner side 122 of the outer casing 104.