Hydronic heating systems have been around for decades. Remember the old radiators in grandma's house? These days, hydronic heating systems are more sophisticated and use hot water piped through tubes that are run under floorboards, through radiators or along base boards [source: HydronicHeating.net]
In hydronic systems, boilers heat liquids using solar energy and geothermal energy. Most of these boilers heat water, but some systems warm other liquids, such as antifreeze. The liquid is pumped through plastic tubing into a heat exchanger, such as a radiator [source: HydronicHeating.net].
In a hydronic system, heat is transferred in three ways: conduction, convection and radiation. During conduction, heat energy moves from object to object, such as a spoon in pot of hot water. When you touch the spoon you can feel it get warm. Radiation is the transfer of energy through electromagnetic waves. You can feel heat radiation from a heat lamp. Boiling water is an example of convection. During convection, warm water rises while the cooler water sinks [source: HydronicHeating.net].