A heat transfer system comprises a first fluid storage tank fluidly connectable to a first fluid circuit for heat exchange between the first fluid circuit and a first fluid in the fluid storage tank, a first heat exchanger positioned for heat exchange with the first fluid, and a second heat exchanger fluidly connected to the first heat exchanger via a refrigerant circuit for heat exchange with the first heat exchanger. The refrigerant circuit includes a refrigerant compressor module and an expansion valve for circulating a refrigerant therethrough for heat exchange between the first and second heat exchangers. A hydronic system is also described.