AE390 - Architectural Engineering Design I

Assignment # 5 - HVAC Systems


Hydronic System






Hypocaust System

Hypocaust System:

The hypocaust is a heating system that was invented by the Romans.  In the period when each room was heated individually by fire, the Romans created a heating system to heat multiple rooms simultaneous by heating the floors and walls.  This very simple system consists of furnace, raised floor supported by columns or multiple tiles, and chimneys.  The raised floor creates duct-like channels for heat to travel.  Heat from the fire in furnace is transferred to the floors by outside air coming entering the furnace and traveling underneath throughout the building.  From this method, the air carrying the heat could provide warmth to many rooms and hotness to the baths in Roman buildings.  The primary system of the hypocaust is the furnace where it produces the heat required in this system from fire.  The secondary system is the channels created by the columns or tiles, and the chimneys which are the distributing system that will circulate heat around the building.  The most common materials that were used in this system were stone and bricks which were for the tiles, furnace and the chimneys of this system.



Typical Uses:

Even though this system is no longer used nowadays, it was used efficiently in the Roman times.  The typical use of this system was heating multiple rooms at the simultaneously.  As this system provides heated air to the floors and walls of multiple rooms, it is much more efficient than heating each individual room separately.  A different type of hypocaust is still used in Korea, Ondol.  This system is considered to be much safer, using different heat source than wood, but still caused much death from the poisonous gas.

The other use of this system is heating water.  The Roman baths used this system to heat their baths to wash themselves.  The baths would be the closest room to the furnace and had three walls that were constantly heated by the system.  The heated wall and floor transferred it heat to the water which were used to bathe.

Even though it is not exactly the same system, this system can be used as Radiant floor heating.  The concept of the system is very similar but it uses water instead of air.  The radiant floor system won’t be raised as high and the quantity of water heated is relatively lower than air but is much safer.  Making the hypocaust system much safer can lead to using the system where radiant floor system is being used, residential housing and other floors.

Although this system is dangerous and expensive operating, studies are being done to find ways to make better use of this system.  This is extremely hard to believe as this system will be expensive to use and maintain in the case of failure.  The diagram below shows the idea of using the hypocaust in the way radiant floor heating systems are used.

     Radiant Floor Heating System                                   Hypocaust System

                       Diagram from Type 160: Floor Heating and Hypocaust by Karel Fort


Numeric Parameters:

The hottest room in Roman buildings could reach up to temperature of 120 degrees F.  For this temperature, at least three walls had to be heated in addition to the floor.  The number of walls heated and the distance from the furnace determined the temperature of the room.  The lowest temperature this system will reach is the room temperature as this is strictly a heating system.

For the maximum efficiency of heated air to travel, the floors are raised 2 feet above the ground.  The most efficient height of the fire that will allow more air to enter and carry the heat is half the furnace height.

The following table gives the different Thermal conductivity and U-values of each material.

Full Size Table

            Table from Building and Environment Volume 34, Issue 3.

This system is not as efficient in cold weather as it uses the outside air to transfer the heat.  The suitable condition will be areas that are generally warm and cool, but not too cold.

The hypocaust will last long depending on the material used.  Generally, the system fails when poisonous gas sneaks into the room, or the entire house.  This happens when the material cracks, and allowing the warm air to rise from the floor into the room.

For the maximum efficiency of heated air to travel, the floors are raised 2 feet above the ground.  The most efficient height of the fire that will allow more air to enter and carry the heat is half the furnace height.



The major disadvantage of this systems is that the furnace produces a very poisonous gas (carbon monoxide) which travels with the heated air.  Air transferring the heat will also have many other particulates that are harmful to people.  As these are traveling with the air, there is a high probability that the will leak into rooms and poison the people inside.

One limitation of this system is its control capability.  Pretty much, the only parameter that the hypocaust can only control temperature and even that has limited control.  It cannot control humidity, air quality or pressure.  It does however effect humidity and air quality, but all in a negative way.  The method of controlling the temperature of this system is to burn more wood, or burn no wood, thus have a slow response rate.  Controlling the fire is the only method of controlling the temperature and everybody knows how easy it is to control fires.

Another limitation of this system is that each room will have different temperature.  As air carrying the heat travels underneath the floors, it is bound to lose the heat.  This means that each room will get different amount of heat, the furthest room from the furnace getting the least.

The materials for this system is very limited to stones, bricks and concrete.  Wood structures are out of question as flames will burn the building with ease.  Material as steel are more sensitive to heat than bricks, stones, and concrete and may deform easier.




Erik Humes, John Balletto, Tae Kim                                                                                  Spring 2005