A system and method for providing domestic hot water and/or space heating within a building. The system comprises at least one storage vessel comprising a removable cover, wherein an internal volume of the storage vessel is fluidly connected to a source of sewage and/or wastewater from one or more buildings, a water-source heat pump, at least one solar thermal collector, at least one heat transfer circuit. The at least one heat transfer circuit includes at least one fluid line containing a heat transfer fluid, and a controller for controlling an operation of the system. The system has an operation mode in which the controller of the system is configured to connect the at least one solar thermal collector to the at least one storage vessel via the at least one fluid line. The system stores a heat output from the at least one solar thermal collector in sewage and/or wastewater.