Caius Sergius Orata is credited, by Vitruvius, with inventing the hypocaust. The word, from the Latin hypocaustum, in a literal translation, means access from below. The hypocaust is a raised floor system on ceramic piles where, at one end, a furnace—where firewood is burned uninterruptedly—provides heat to the underground space, which rises through walls constructed of perforated bricks. Hypocausts heated, through the floor, some of the most opulent buildings of the Roman Empire (including some residences) and, above all, the famous Public Baths.
With a similar function, but in the East, there existed the ondol. It is estimated that it was developed during the Three Kingdoms of Korea (57 BC-668 AD), but researchers point out that the solution was used long before that. The system also manipulated the flow of smoke from agungi (rudimentary wood stoves), rather than trying to use fire as a direct heat source like most heating systems. It even caught the attention of Frank Lloyd Wright, as pointed out in this article, who adapted the system to use it in heating homes in the United States and in his important Imperial Hotel in Tokyo. How do radiant floor heating systems currently work?
Radiant floor heating works through air conduction, radiation, and convection. There are two main ways to assemble the system: through electrical resistances or thin hoses with a fluid running through them. The systems are incorporated into the floor during construction or renovation, remaining hidden and often inaccessible because they have been concreted to the structure. By means of hot water or electrical resistance, the floor material is heated by contact, radiating to the air of the interior environment. There, the law of thermodynamics shows that warm, lighter air flows upward, and cold air travels downward. In this way, the environment is fully heated, avoiding heat waste and the formation of “heat islands” or cold spaces in the environment, as is often the case with traditional radiators. In addition, many traditional heaters over-dry room air, triggering allergies and respiratory problems in occupants.
In the case of electric floor heating, the system is greatly simplified. It needs an electricity supply, a thermostat (which will regulate the desired temperature), and electrical resistance, surrounded by insulating layers, similar to garden hoses. These are placed on a thermal blanket and then overlaid by the floor. The main disadvantage of this solution is the high energy consumption.
The Hydronic radiant floor, on the other hand, works through coils built into the floor, where the heated water will travel. The water stored there remains in a closed system, that is, it circulates uninterruptedly, alternately heating up and losing this energy to the environment. It can be heated in several ways, such as gas boilers, diesel oil, solar heating, or even electrical resistance. In the past, steel and copper tubes were the most common. Today, most systems use flexible and resistant materials such as polyethylene and PEX (cross-linked polyethylene). In addition to being considered cheaper, this option is also more environmentally friendly, as it generally does not use as much electricity.
Despite being considered a relatively expensive system, there are cases of heating in public and even urban uses, such as on sidewalks, roads, airstrips, or even under the grass of soccer fields. When weather conditions are extreme, some antifreeze fluid is mixed with water to keep the system efficient at any temperature.
Often, low-efficiency heating systems account for significant fractions of the maintenance costs of homes in cold regions. History shows that humanity has always sought solutions to heat its buildings and, over time, increasingly ingenious solutions were developed, bringing thermal comfort and energy efficiency. Of course, a building with good insulation, without thermal bridges or air infiltration, will help to achieve a more satisfactory result. The radiant floor systems are especially useful in cold places. But they are an interesting option in spaces where environments with stable and well-distributed interior comfort are needed, especially in cases of public and collective buildings such as schools or offices.