Underfloor central heating has been popular in Europe for decades, and is making a way into many US homes today. Underfloor systems tend to be more efficient. Since heat rises, the heat will move from the floor first, instead of being forced downwards by a fan with above head systems. These systems aren't ideal in areas of the country where the temperature gets frigid, because they produce slightly less heat than fan forced systems.
In a sub-floor electric heater, cables are placed underneath the floor. This is most commonly seen in homes where removing the floor, or adding a new layer of flooring isn't feasible. Electrical cables are placed under the existing floor, and spaced apart depending on the type of floor. When the heating system is turned on, the cables are heated and generate heat through the floor itself. These systems can be installed in their simplest forms with electric mats. An electrician will need to be part of the installation process to avoid potential deadly fire issues later.
Much like a hot water heater, an underfloor heater can be installed that utilizes a series of pipes and water to generate heat. The pipes are installed under the floor, usually in a concrete slab. The flooring material, such as tile, is placed above the system. When the system is turned on, a boiler is used to create heated water that circulates through the pipes and heats the floor and surface air. These are much more time consuming and expensive to install, but generally produce a more reliable source of heat.
Using an underfloor heat system can be much cheaper than other types of heat. If water is used, only the gas or electricity to heat the water is needed. This is different than a typical central heating system, since they tend to use much more energy to create heat.
Since these systems are installed under a floor, the heat is more evenly dispersed throughout the home. With fan forced heating systems, it's common to have 'cold spots' throughout the house where heat vents are not accessible. With an underfloor system, the entire floor is generating heat, eliminating these cold spots. It also frees up the design of any home, allowing you to place furniture wherever you wish without worrying about blocking heat registers.
Underfloor central heating systems don't work as quickly as other types of heat. Depending on the material of the floor, it can take as long as 45 minutes for heat to generate in to the room. It can also take that long for the heat system to cool down. In the winter this may not seem like much of a disadvantage.
Some flooring materials are not suited for underfloor heat. Wood can warp with constant temperature changes. Laminate flooring will need less underlay with these systems. Carpet can act as an insulator and prevent the heat from escaping in to the room as it should.