This month's College class will adhere to the question and answer format of "How Come?"

Q: What makes a radiant-floor heating system different from a convective heating system such as finned-tube baseboard?

A: A radiant-floor heating system first heats the objects in the room. Then, the objects heat the air to a certain degree, but the movement of the air is relatively slow because the objects in the room don't get that hot.

The key difference between a radiant heating system and a convective heating system is that a radiant system strives to control the rate at which your body loses heat. A convective system, on the other hand, tries to replace the heat being lost by the building.

Air temperature has little to do with radiant heating. If the air is still and your body is near objects that are as warm as you, you'll feel comfortable even though the air in the room is cooler than what you're normally used to. The air also feels fresher when it's cooler.

Q: How does a hydronic radiant-floor heating system work?

A: By circulating warm water through pipes embedded in the floor (or, in some cases, the walls or ceiling), the floor becomes a large "radiator." Radiant waves of energy move off the floor and travel out in all directions to warm the walls, ceiling, and furniture. As these objects become warm, you experience less heat loss because you're standing next to warm things. You become comfortable.

Q: Is radiant-floor heating new?

A: Not at all! In fact it's probably the oldest method of central heating there is. The Romans used a crude system of radiant heat to warm their famous baths as early as 80 BC. They built fires and let the heat travel through passages under the marble floors. Europeans heated their castles in much the same way during the Dark Ages.

Q: Does a radiant heat system work differently when the coils are in the ceiling?

A: Theoretically, no. Radiant energy doesn't know up from down. It moves in a straight line, away from its source until it hits something solid. One of the advantages of heating from the ceiling down is that you can often use hotter water. People don't come in contact with the ceiling so you can make the surface temperature hotter than you would a floor.

Q: Are there drawbacks to heating from the ceiling down?

A: Yes. Objects such as tables stop and absorb the radiant energy. So, if you're sitting down at the breakfast table, your head and upper body will be warm, but everything below the table will feel somewhat cooler. Also, if you're standing under a low, radiantly heated ceiling, you might begin to feel a bit light-headed.

Q: Does the same thing hold true if the pipes are in the walls?

A: Yes, the side of your body facing the heated wall will always feel warmer than the side turned away from that wall, just as it would if you stood next to a hot oven or an open fire -- or a freezer chest in your local supermarket.

Q: Is this the main reason why most of the systems people install today have the tubing in the floor?

A: It sure is! People walk around on the floor, not on the walls or ceiling (hopefully!).

Q: When the tubing is in the floor, what determines the temperature of the water you need inside the tubing?

A: The temperature depends on three things: how close together and how deeply buried the tubing is, what the floor is made of, and how well constructed the building is.

The typical water temperature for radiant floor heating will vary between 90 to160 degrees F, depending, in large part, on whether the tubing is in a slab or stapled under a wood floor, and whether or not the floor is covered with a rug or other insulating material.

In general, the more the flooring material surrounds the tubing, the lower the water temperature can be. In other words, if you bury the tubing in concrete there will be more tubing-to-floor contact than there will be if you staple the tubing to the underside of the wood floor. The result is the water temperature can be lower when the tubing is in concrete.

Q: What company was at the forefront of radiant-floor heating for homes?

A: Chase Brass and Copper Co. They installed a radiant heating system in the ceiling and the floor of a New England house in 1942. They used the house as a laboratory during the winter of 1942-43 and performed extensive tests. In 1945, they published their results in their classic Chase Radiant Heating Manual and set the stage for the post-World War II building boom.

Q: What type of pipes did American builders use during that building boom?

A: Copper, steel and wrought iron. Of these, installers and manufacturers of the time considered wrought iron to be the most durable.

Q: Were there problems with these systems?

A: With many of them, yes. But most of these problems were caused by the materials and the "Get it done fast!" work habits of the time. Consider, for example, William Levitt who built 50,000 mass-production "Levittown" homes in New York and Pennsylvania at the astonishing rate of one every two hours. Every one of them had a radiant-floor heating system.

Q: What sort of problems developed in these systems?

A: Most of the Levittown problems were a result of the hastily installed concrete slabs that shifted and cracked over the years, causing the metal pipes to stress and leak.

Q: Is concrete itself corrosive to metal pipes?

A: It can be. It all depends on what goes into the concrete. For instance, some concrete contains cinders from incinerators (that's where the name "cinder block" comes from). Cinders contain sulphur compounds that can damage metal pipe.

Q: So what material should surround metal pipe when you bury it in the floor?

A: The old-timers used broken limestone, gravel, sand or non-sulphur-bearing concrete. If their connecting lines to and from the boiler ran through cinder fill, they dug a trench and filled it with crushed limestone. They covered the trench with a slush coat of lime mortar. The limestone neutralized the acids in the cinders and prolonged the life of the pipe.

Q: Is the radiant-floor tubing that's available today better than copper, steel and wrought-iron?

A: Yes, much better. Today, there is plastic and rubber radiant-floor tubing that can take relatively high temperature and pressure. These materials won't corrode in the presence of concrete, ground water or dissimilar metals.

Q: What size tubing goes into most residential work?

A: It's usually 3/8 inch or 1/2 inch.

Q: Is there a way to estimate how much radiant-floor tubing is needed on a job?

A: A good rule of thumb is to figure on using 1.25 feet of radiant-floor tubing for each square foot of floor space (assuming the radiant-floor tubing is going in a slab).

This will allow you to place the radiant-floor tubing on 6-inch centers near the outside walls (for a distance of about 24 inches in from the wall), and on 12-inch centers for the rest of the room. This is typical residential spacing.

For instance, if you had a 20-foot by 20-foot room to heat, you'd need 500 linear feet of tubing (20-ft. x 20-ft. x 1.25 = 500 feet)

Q: Are the circulators for radiant-floor heat jobs different from the circulators used on other types of systems?

A: No, they're the same. What you're looking for are small flow rates against relatively high pressure drops. The small, water-lubricated circulators you use from day to day will usually work well.

Q: What's the main difference between a job installed with a temperature-responsive control and one installed with, say, just a three-way valve and a circulator?

A: In the temperature-responsive job, the circulator runs continuously while the water temperature varies. In the other, less-complicated, system, the water temperature is constant and the flow rate varies.

Q: Is one way better than the other?

A: If you can afford the controls, it's better to run constant circulation and vary the temperature of the water. A good rule of thumb is that the controls should not cost more than the tubing on any given job.

Q: Will I need any other special hydronic accessories for these jobs?

A: You'll need an automatic fill valve to set the initial system pressure. Remember to close the valve going to the automatic fill valve once you've set the system pressure. You'll also need a properly sized expansion tank. You may need a back-flow preventer, depending on the codes in your area.

Since automatic fill valve manufacturers now want us to keep the valve closed when you're not using it, I think you should have a low-water cutoff on the system to protect the boiler against dry firing or explosion, regardless of whether or not the code calls for one.

You'll also need a strainer to keep debris out of the tubing. This is something you wouldn't normally use on a residential hydronic system, but here, it's a good idea.

Flow-control valves or spring-loaded check valves will keep heat from migrating into zones that are higher than the boiler. You may need two, one on the supply side and another on the return side to prevent "gravity" circulation.

Q: How does a radiant-floor heating system affect the humidity level in a house?

A: It has relatively little effect. The lower air temperatures usually mean the relative humidity will be slightly higher than it will be in a convective hydronic system.

Q: Can I air-condition with a radiant floor or ceiling by circulating cold rather than hot water?

A: It depends on where you are in the country. If the relative humidity is high, the cold floor or wall surfaces will become damp (and dangerous if it's a floor). Some contractors in the western states where the relative humidity is low have been experimenting with radiant cooling with greater success. Time will tell which way this goes.

Q: Did they ever use steam for radiant heat?

A: They tried in the early days, but with no success. Steam is simply too uncontrollable for this application.