Radiant heating, a popular modern form of central heating, is actually as old as the Roman Empire. The hypocaust, a system of sub-floor tunnels connected to a room where a fire was kept constantly blazing, warmed the Roman baths and was used as a form of central heating in well-to-do villas. Heat from the fire was drawn through a void under floors or around water supply pipes from the aqueduct system in the baths, providing both hot water and warm floors for the facility. The hypocaust's main drawback was its inability to vent carbon monoxide. The Romans used ceiling vents to regulate temperature and must have also designed fresh air vents to deal with the problem since the baths were hugely successful over a long period of time. Twelfth century Muslim engineers improved on the Roman system by containing the hot air in pipes running under the floor, thereby removing the soot and carbon monoxide from the building to be heated. Modern radiant heating uses the same concept of sub-floor piping to warm the space above the floor.
Modern central heating comes in two types--convection and hydronic. Convection heating uses the movement of air to heat space. The most common kind of convection heating is "forced air," using pipes to push heated air throughout the building. Hydronic (water) heat uses radiators, baseboards or radiant heat in floors or walls to put heat where it's needed. Given the proper heat producer and distribution materials, radiant heat can be as efficient any other form of heating. Its main advantages are its uniform effect and comfort. Modern radiant heat may use either hydronic or electric coils and can be installed outdoors as well as inside the house. Most radiant heat is installed during construction but it can be part of a remodeling retrofit. It is installed by laying coils on a completely smooth prepared subsurface covered with a thin layer of cement-based composite material or by threading the coils through floor joists. Either type installation must be tested at various stages and properly insulated underneath so that all of the heat rises through the floor on top of it. Kits are available for the do-it-yourselfer or professional.
Once the coils are installed and connected, hot water or electricity warms them and the heat rises through the flooring surface above. Since the surface comes into contact with feet, furniture and wood flooring, warming is modest--no more than 100 degrees F--just a bit above body temperature. Outdoor installations like warmed driveways can be kept just warm enough to melt ice and snow. The radiant nature of the heat warms only what needs to be warmed, minimizing air movement and keeping the warm air where the people are rather than forcing it to pool near the ceiling. Systems that use wall-mounted tankless water heaters can also provide hot water for household use, eliminating the need for an inefficient traditional water heater.