Gas, Solar, Electric heat pump or wood fired boiler used for Hydronic heating simply circulates hot water in a closed loop system to heat instead of conventional blown hot air; water being far more efficient method of carrying heat. Similar to the way the sun heats our planet, hydronic heating provides a more even, natural radiant heat throughout the house with out moving the volume of air and dust.
Heat transfer to a room takes place in a heat ex-changer, which consists of a copper pipe (or similar) fitted with aluminium fins that are mechanically connected to the pipe. Flow of hot water through heat ex-changer heats pipe and fins, heat is passed to surrounding rising air, performing chimney effect or convective flow of warm air in heating element.
The water in a conventional hydronic heating system is re-circulated through insulated pipework commonly known as the FLOW & RETURN pipes. These normally travel past or as close as possible to each room in the house you want to heat, the flow pipe being the re-heated water leaving from your preferred heat source or boiler going out to feed the radiators with more heat. Once the heat has been absorbed by the radiator and expelled into the room it leaves the radiator via the return pipe re entering the loop system and to return to the boiler to begin this process all over again.