A Brief History Of Radiant Heating
June 1, 2006
Like many aspects of modern life, we all owe a debt of gratitude to the ancient Romans. Up until the time Roman armies conquered the world, about the best you could hope for was a fire in a fireplace. If your hut burned down in the middle of the night or you died of smoke inhalation, well, your fate could have been a lot worse before you died of old age at 30.
The Romans practically invented central heating with their version of underfloor radiant heating called a “hypocaust.” Basically, here’s how it worked: First, a slab floor was raised about 2 feet off the ground supported by a series of tiled pillars. Spaces were also left inside the walls of the building, too. A “praefurnium,” or wood-fired furnace, was placed on the outside of the house, below the floor. Hot gases from the praefurnium were naturally drawn through the floors and up and out the walls, radiantly heating the space along the way.
While the system worked like a charm to warm your villa, it really did the trick for public baths. The Romans were active people and, when they weren’t running about senator-ing, gladiator-ing or conquer-ing, they needed baths.
Now, things could get pretty chilly inside a marble building, especially if you were running around naked and wet. (A case of “shrinkageius Seinfeldium,” if ever there was one.) In this case, the praefurnium was placed next to the “caludarium,” the hottest room in the place in which the air temperature could reach 120 degrees F. Other rooms controlled the heat by only allowing heat to draw up one wall. So let’s chalk up the notion of zoning to the Romans, too.
As you might imagine, this modern convenience was borne on the backs of slaves who had to constantly tend to the fire. So let’s not give the Romans credit for the “thermostat.”
After the barbarian hordes sacked Rome, its culture and hypocausts faded into history and we were all back huddled in front of fires wondering when someone would invent the cast-iron stove.
It was a long wait until 1742 when Benjamin Franklin turned his ingenuity toward cast-iron stoves, making improvements to existing designs that all but replaced the fireplace as a building’s main source of heat. Franklin, in fact, called his invention the “Pennsylvania Fireplace,” but even some 250 years later, we can still find Franklin stoves heating homes.
While versions of a “circulating stove” predated his invention by 70 years, Franklin designed a freestanding stove that provided radiant warmth in all directions rather than just one from a wall-bound fireplace. Thanks to advancements in metallurgy, the stove radiated warmth even after its fire died down.
Like his other inventions, Franklin didn’t patent the stove. “As we enjoy great advantages from the invention of others,” he wrote in his autobiography, “we should be glad of an opportunity to serve others by any invention of ours; and this we should do freely and generously.”
As a result, competitive manufacturers continued to improve on his original design. Franklin, himself, designed a stove for coal in the 1770s.
The Franklin stove may be one of the first instances of thinking “green” in terms of heat. Franklin lived in Philadelphia, one of the largest cities in the Colonies. Much like today’s recent increase in gas and heating oil, wood was becoming costly and scarce as demand literally outstripped supply. Thanks to his innovations, Franklin’s stove was said to give off twice the amount of heat as a normal fireplace for a third the amount of wood.
Everything was great until someone decided to move the stove to the basement and duct the heated air up through vents to the living areas. Technology, as it has a way of doing, marched forward and, by the early 1900s, a blower was added to the setup, creating forced-air heating.
Meanwhile, during the 1800s, someone else got the bright idea for a hot water heating system - heat the water, make some steam and pump it up into cast-iron radiators positioned strategically around the living area. And so it was for a long while.
Then came 1907 and a British professor named A.H. Barker came up with the first real radiant heating system, the “radiant panel,” which used a large surface like a floor or ceiling to radiate heat into a building’s living space. Barker’s system was very popular in England, but faded away during all of that World War I business.
After the war, the Bank of England got a nice new hydronic radiant heating system that was installed under the direction of a fellow named Dr. Oscar Faber. Dr. Faber’s system used copper pipes embedded in concrete floors and plaster ceilings and it was used to cool the building in the summer and heat it in winter.
And that’s just about where things stood until a fellow named Frank Lloyd Wright got into the architecture game and started incorporating radiant heating in his homes and office buildings. At this point in our story, we come full circle since Wright took his cue from his visits to Asia. There, he experienced buildings warmed in much the same way as the Romans had perfected with their hypocausts. (This form of heat is still common in Korea, where it is called “ondol.”)
Wright first called his system “gravity heat.” It was a basic system in which a boiler located in the basement near the center of the building was plumbed with copper or steel piping that went through the slab.
Wright practiced architecture from 1887 to his death in 1959. While his styles varied over those years, Wright always followed a concept he called “organic architecture.” If form followed function, then Wright said form and function are the same thing. That’s a highfalutin’ way of saying that “it’s all good” if you could merge a heating system not just with a floor plan, but with the floor itself.
Somewhere in there, World War II happened and, once it was over, 16 million returning servicemen needed homes for themselves and their families. Enter William Levitt who basically invented the American suburb. Levitt and Sons built 17,447 homes in just four years in Levittown, N.Y. before heading to Pennsylvania for their second development.
Like Henry Ford, Levitt took the approach of turning home construction into an assembly line. All the homes had the same footprint and teams of workers moved from lot to lot, performing the same tasks over and over as trucks drove through the area with supplies. By 1948, Levitt boasted that he could complete a house every 15 minutes.
One way Levitt built so fast was the cookie-cutter homes came without a basement. As a result, the slab-on-grade construction afforded Levittown homeowners the comfort of radiant heat.
Irwan Jalonack, Levitt’s executive vice president, designed a water-heating unit specifically for the Levitt houses. It consisted of a 50,000 Btu/hr. oil-burning heater and a 30-gallon combination water heater, air eliminator, expansion tank. The 30-gallon tank was placed above the oil-burning heater and 1 1/2-inch copper lines connected the two allowing gravity-flow heating of the tank.
Water for floor heating was pulled from the bottom of the tank and circulated through the floor back to the inlet of the oil burner heater. A single thermostat turned the pump on when the house needed heat. A heat exchanger provided domestic hot water. The system was simple, compact, inexpensive and low in maintenance. Balancing valves were the only controls supplied to adjust heat delivery to each circuit.
For a time, the American radiant industry was on the upswing. Trade associations and manufacturers promoted the use of copper, wrought iron and steel piping for underfloor systems throughout the 1940s and 1950s. There were installation instructions for concrete slabs, wood subfloors, ceilings, walls, even driveways and sidewalks for snowmelt.
But the interest wasn’t to last. The copper-and-concrete radiant system wasn’t the best combination for all those Levittown homes. Workers on such tight construction timetables had no room for error either. Most Levittown systems failed within 15 to 20 years.
These pioneering American radiant systems, whether it was for Wright’s artistic pursuits or Levitt’s mass appeal, may have been ahead of their time anyway. Lack of insulation, controls, tubing materials and general know-how meant radiant was impractical for conventional construction of the day. Once air-conditioning became the norm in the 1960s, forced-air systems became the norm. Radiant systems kept a low profile.
At least they did in this country. In the meantime, Europeans and Scandinavians never gave up on hot-water systems and began introducing plastic tubing around the time the last Levittown was built.
Sweden’s Wirsbo, for example, got its start in 1620 making steel weapons. By the 1900s, however, the company had switched to making steel piping for the plumbing and heating market in Northern Europe. By the 1950s, the company began manufacturing polyethylene tubing.
A key development was made in 1968 when Thomas Engel, a German engineer, developed a method to chemically crosslink polyethylene, better known as PEX. A year later, Wirsbo (which changed its name this year to Uponor) perfected the means to manufacture PEX.
Plumbing & Mechanical named it one of the Greatest Plumbing & Heating Inventions in 1992 for one of our popular “History of Plumbing” issues. PEX solved a number of problems associated with metal and other plastic tubing, namely that it does not corrode and stands up to poor water quality. Wirsbo and other PEX manufacturers also added an oxygen barrier in the 1980s.
Couple the development of PEX and EPDM rubber tubing with improvement in insulation, construction techniques and controls, and the stage was set for a radiant Renaissance in this country.
It’s been a long trip since the days of the hypocaust.
(Editor’s note: A version of this article first appeared in Reeves Journal, January 2006 and January 2004.) RHR