A heat distribution system, method and computer program product, including an unpressurized tank configured for holding a heat transfer fluid; and one or more submersible heat transfer fluid pumps configured to pump the heat transfer fluid to one or more heat load loops respectively connected to the one or more heat transfer fluid pumps.

A system includes: a heat generator obtaining heat from a heat source and having a generator outlet port and a generator return port; a central heating circuit having a heating feed port and a heating return port; a tank having a top portion and a bottom portion, the tank containing a heat storing medium; a first tapping coil immersed in the heat storing medium in the bottom portion of the tank; and a second tapping coil immersed in the heat storing medium in the top portion of the tank The heat generator, second tapping coil, central heating circuit, and first tapping coil are fluidly connected in series to allow fluid to flow from the heat generator via at least one of the second tapping coil, the central heating circuit, and the first tapping coil back to the heat generator The system includes first, second and third three-way valves.