Anatomy Of A Radiant Heating System. Part 1: The Hypocaust

May 09, 2010

Hello and welcome to FloorHeat Help, the blog devoted to radiant heating systems.

In previous installments, we touched briefly upon the history of radiant heating and talked about how hydronic systems are actually very similar to the human circulatory system.  Beginning with this and continuing for the next several installments, we'll a take a little deeper look at the history of radiant heating systems.  Along the way, we'll also take a look at the anatomy of a radiant heating system and how the various components have evolved.  We'll conclude the series with a discussion of what constitutes Radiant Warmth The Right Way.

Very often, a look at the past helps us to appreciate the present.  For example, consider the automobile.  Although what we drive today shares some similarities with the Model T, there are vast technological differences.  The same is true of modern  radiant heating systems and what came before.

The idea of radiant heating isn't new.  In fact, the Romans had a system called the "hypocaust" over 2,000  years ago.  Hypocaust means "heat from below," and the system was used for both space heating and hot water production. Buildings that incorporated the hypocaust system were constructed of masonary and had a type of crawl space under the floor. There was a firebox at one end of this crawl space and a series of small chimneys that ran through the walls to the roof.  The heated exhaust gas from the firebox would draft into the crawl space and up the chimneys, thereby warming the building's floors and walls.  In Roman bathhouses, a shallow pool, called the tepidarium, was recessed into the floor.  The heated exhaust gas from the hypocaust would circulate around the floor and walls of the tepidarium, thereby warming the water.  The following diagram shows the basics of a hypocaust system.  There's also a nice video clip on the hypocaust at http://www.youtube.com/watch?v=q-32UWMCrtE

(CLICK ON IMAGE TO ENLARGE)

All indications are that the hypocaust worked fairly well; however, the system had a few drawbacks.  First, it took a long time and massive amounts of fuel to heat a building, not to mention the tepidarium.  The Romans likely didn't see this as big problem, because trees were plentiful and slave labor was used to tend the fire. Second, there was no effective way to regulate temperatures, and a building's walls and floors could become very hot -- whereas modern radiant systems are designed for bare feet, sandals were likely the rule in buildings heated by a hypocaust.  Third, there was a potential for deadly carbon monoxide to leak into the occupied space through cracks in the walls and floors.  Realistically, though, in large, drafty buildings like a bathhouse, carbon monoxide poisoning probably wasn't a very significant threat. Still, despite these drawbacks, the hypocaust was an improvement over freestanding fireplaces in that it distributed heat more evenly throughout a building and could maintain a pool of warm bath water.  In the 12th Century, Muslim engineers in Syria developed an improved version of the hypocaust whereby heat was transmitted through underfloor ducts, rather than a crawl space, which streamlined the flow of exhaust gas from the fire.  The Spanish gloria system, which came into use in the early Middle Ages, further improved upon the hypocaust by locating the firebox outside and added shutters that regulated airflow to the firebox and provided some control over heat output. By having some control over the rate of combustion, people could use hay, rather than wood, for fuel.  Moreover, locating the firebox away from the building produced two other advantages.  While the hypocaust drew its combustion air from within the building being heated, the gloria system used outdoor air.  With no fire drawing inside air, cold drafts within buildings were reduced and there was no danger of filling the building with smoke in the event of a blocked vent. As with the Muslim system, warm exhaust gas from the firebox was transmitted through under floor ducts and vented to the outside via a vertical flue, or chimney.  In the 13th Century, the Cistercian monks at the Royal Monastery of Our Lady of the Water Wheel   developed one of the first hydronic radiant heating systems when they diverted water from the Erbo River to primitive wood-fired boilers and then circulated the heated water throughout the monastery complex. Yes, despite the name, the Royal Monastery of Our Lady of the Water Wheel is a real place that you can read about by going to http://en.wikipedia.org/wiki/Real_Monasterio_de_Nuestra_Senora_de_Rueda

Although a modern radiant heating system is a far cry from the hypocaust, the two share a common architecture comprised of three essential elements: (1) each requires a heat source; (2) both need at least one heat emitter; and (3) they require a means of transmitting heat from the heat source to the heat emitter(s). In the hypocaust system, these roles were filled respectively by the firebox; the building's floor and walls; and the crawl space under the floor.  In a modern system, there's a wide variety of heat sources available, including but not limited to solar collectors, geothermal units, combination water heaters, and high efficiency gas and electric boilers.  Regardless of the heat source, to work correctly it needs a heat emitter, like FloorHeat's EasyFloor System, and an effective means of transmitting heat to the heat emitter(s), like a FloorHeat EasyPanel.  For more information on FloorHeat products and services, please visit our website http:/www.floorheatcompany.com or watch our videos "The EasyFloor System" and "Radiant Warmth The Right Way" on You Tube at  http://www.youtube.com/user/FloorHeat2010

At about the same time the Romans were developing the hypocaust, the Koreans were developing a similar system called guun-dol, or ondol, which means "heated stone."  The ondol system was actually used in many parts of the Korean Peninsula until the 1960's and played a major role in the modern resurgence of radiant heat.  More on that next time.

That's it for this installment of FloorHeat Help. Please don't hesitate to let me know if there's a radiant heat related topic you like us to discuss. So long for now, and stay tuned!

God Bless!

Jim Szalla
The FloorHeat Company
3120 Spanish Oak Dr.
Lansing, MI 48911
Ph:  517,272,4441
Toll Free:  888.265.5455
Fax:  517.708.2304
email:  jszalla@floorheatcompany.com
http://www.floorheatcompany.com/