General Info



  • The hypocaust is one of the most ancient forms of an HVAC system.  Like many great innovations, it originated with the Romans over 2000 years ago.  A hypocaust is both a primary system and a secondary system, as it creates heat and distributes it as well.



  • The main use for hypocausts was found in the large public bathhouses.  Sauna rooms were created by adding a pool of water, heated by the same fire heating the air below.  This created a hot, humid space to clean oneself and converse with friends.  The temperature could have easily reached 100 degrees Fahrenheit and the humidity could have reached 100% due to the pools.  These parameters are not exact, as the system has not been used is many, many years.



  • Its purpose was to evenly heat the room in the most efficient way possible.  A hypocaust was composed of a raised floor (typically about two feet), supported by columns or pedestals of stone every few feet, with the space below left open.  A furnace, composed of a continuously burning fire, created heat, which was then allowed to flow through the space below the raised floor, thus heating the floor and rest of the room.  Once cooled, the air escaped through flues in the wall and out of vents in the roof.  The furnace takes up a fair amount of space, so it was usually located in a separate room.  The flues were built directly into the walls so they did not take up useful space.



  • Connecting the aqueducts (which brought water into the city from the mountains), water tanks and pools were a series of tunnels.  These were constructed of brick and mortar (very crude for todayís standards, but they obviously worked).



  • Like the water system, the air flow system was connected by ducts, consisting of stone or brick tunnels under the floor, into the large open space beneath the raised floor, and into the wall flues.  These flues also provided a source of insulations for the room.  The hot air rising created a barrier, keeping the warmth inside the building.



  • As mentioned above, the typical materials used in this system are stone, handmade brick and mortar.  An early form of concrete may also have been used, as the Roman had used it for many other buildings of the age (for example, the dome of the Pantheon).  Due to the second floor and pillars, this was difficult and laborious to construct.  Costs were much higher than a simple fireplace due to the larger amount of materials used.



  • The main disadvantage of the hypocaust system is a very dangerous one.  The fumes created by the fire in the furnace easily crept out of the holding space below the false floor and into the main space.  This silent killer is now known as carbon monoxide.  Although it is easily detectable and preventable today, the Romans probably had no idea of this concept.



  • Another disadvantage with this system is the possibility of the fire becoming unmanageable and getting out of control.  A stone or concrete building may survive, but the occupants may not.



  • Needless to say, the hypocaust is no long used today.  However, we do have a modern version of this concept.  Radiant floor heating uses the basic concept to evenly heat the room, without the dangers of an open fire and fumes.  It is also much cheaper to do this system than a modern hypocaust.



  • As an aside, the hypocaust is similar to a Korean Ondol.  It is basically set up in the same way, with sub-floor heating providing warmth for occupants.  This is also no long used, due to the same carbon monoxide and overheating problems.


Typical Uses

  • This system had a use in ancient times, but with todayís safety codes it is virtually impossible to use it in current time.  This is due to the fact that hypocaust systems have, in comparative terms, a high probability that the occupant will become ill or possibly die from carbon dioxide poisoning.  However, as a last resort to help struggling countries and villages, it might not be a bad idea to teach them to use these systems and maybe apply modern day synthetics or textiles to help avoid the danger (at least this is the only way we can see a modern day application for this system).



  • Temperature ranges for a hypocaust system are not user controlled due to the fact that you canít control the heat coming off of an open flame.  For this reason there are only two real settings for a hypocaust system; on and off.  This makes the low temperature that can be reached depend on the outside weather.  The maximum temperature that a hypocaust system can reach is about 100oF and humidity can reach about 100%.  The humidity can be added by heating pools of water.  Again there is not much control that this method provides.



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