Radiant floor heating provides comfort and efficiency.

LAWRENCE DRAKE
EXECUTIVE DIRECTOR
RADIANT PANEL ASSOCIATION

What exactly is radiant energy?

When you watch a campfire burn, the flames extend upward. Hold your hand over a hot cup of coffee and feel the heat. It is much warmer at the ceiling than at the floor in most buildings. The logical conclusion then is that heat rises. Logical maybe, but incorrect! Hot air rises but heat can travel in any direction. That is why you can feel the heat of a campfire even though you are not directly above it. The radiant energy transfer is caused by a warm surface (the campfire) giving up its heat to a cooler surface (your body). Whenever there is a temperature difference between two surfaces, both surfaces will attempt to equalize. One surface will give off heat and the other will absorb it. This radiant energy travels through space without heating' the space itself. It only turns into heat when it contacts a cooler surface. The interesting thing is that our human comfort relies just as much on radiant heat transfer as it does on air temperature, yet the majority of heating and air-conditioning professionals think only in terms of air temperature. As a result, Americans are missing out on a truly comfortable living environment in their own homes or places of business. Radiant panel heating and/or cooling systems address the other half of the comfort equation which is virtually ignored by the HVAC industry. By controlling both the air temperature and the mean radiant temperature, radiant panel systems deliver a comfort that is unsurpassed in the heating and cooling industry.

Walls, ceilings or floors

A radiant panel can be effectively mounted on any flat surface. The only; requirements are that the surface be sufficiently large enough. The larger the surface as compared to the space it is heating, the lower the actual surface temperature required. Four feet of wall radiator two feet high with a surface temperature of 180 [degrees] F would put out approximately the same amount of heat as a floor heated to 81 [degrees] F in a 15' by 15' room. A heated ceiling in the same room would require a surface temperature of 91 [degrees] F if the entire ceiling where heated. If only half the ceiling where covered with heating panels, a surface temperature of 107 [degrees] F would be required.

As we said, radiant energy travels in any direction and does not convert to actual heat until it strikes an object. The radiant energy emitted from the heated panel will heat the surfaces of all objects which are in a direct line of sight from the panel; that means all walls, floors, ceilings, chairs, tables, or people which can be "seen" by the radiant panel. Therefore, a heated ceiling will raise the surface temperature of floors and walls, while heated floors will raise the temperature of ceilings and walls.

Body temperature and radiant energy

The human body is a walking furnace. We fuel it with food which is converted to heat, hence the term "warm blooded." Our skin surface temperature, about 85 [degrees] F, is generally warmer than the surrounding surfaces. This makes us a radiant panel, Stand by a large picture window in midwinter and you will feel the heat leaving your body. If the rate at which we radiate heat is correct, we feel comfortable. When the temperature difference between our body and the surrounding cool surfaces becomes too great, we have to put on a sweater to slow down the rate at which we are radiating. If the sun begins beating down on us through the window, we receive heat instead of radiating it and off comes the sweater. In fact, we may have to draw the curtain because we can only absorb radiant heat for a short time and then we become overheated. Our normal state is to lose heat at a constant and regulated rate. We are also designed to lose heat in other ways as well. The air coming in contact with our skin conducts away heat. Our skin is moist and moving air also causes evaporative cooling. A truly comfortable environment is one designed to draw heat away from our bodies at precisely the correct rate.

With this in mind you can now understand why a heated floor may not "feel" warm at all, but only neutral. Its surface temperature is usually less than our body temperature. Only on the coldest days when the floor is called on for maximum output will it actually "feel" warm. Heat coming from a wall radiator can be felt the closer you get to it because its surface is much warmer than your body. If you spend much time close to a wall radiator you will become overheated. Radiant ceiling panels are also generally warmer than your body so you will feel some warmth on your head and shoulders. All these radiant systems are designed to match the heat loss of the human body in a way not possible by any other form of heating. Any of these radiant experiences are far more pleasant than being buffeted by the hot-then-cool breezes of a forced air furnace.

How panels are heated

Whether your choice is floor, wall or ceiling panels or a combination, they are heated in one of three ways: electric circuits, water pipes or air ducts embedded in the panel. Of these three, air is seldom used, leaving electric circuits and water pipes (or channels) as the most prevalent. Obviously, electric circuits have electricity as their sole utility. Water, on the other hand, can be heated by almost any utility, be it natural gas, propane, oil, wood, solar, or electricity. Electric circuit panels are quite simple in construction and generally have a lower "up-front" cost than water systems which have more components. Your choice will probably hinge on the energy costs of the available utility.

Controlling comfort

Have no fear, you will not be faced with a space shuttle control panel. A simple wall thermostat is generally all that is required. Working in the background may be an "outdoor reset control" which adjusts the panel temperature based on the outdoor temperature for increased comfort and economy, but you will not have to fiddle with that. A big advantage is the option of a thermostat in every room. This provides additional comfort as well as energy savings because you can turn down those rooms that are not in use or that you prefer to have cooler. Keep in mind additional features like these also increase the cost just like adding power windows and locks bump up the sticker price of an automobile. But unlike automobile options, these comfort features will pay back in energy savings.

Panel construction

All radiant panel systems have three basic elements, (a) a way to deliver heat to the panel, (b) a way to spread the heat within the panel, and (c) a radiating surface. Panels can come factory assembled or constructed on site. They can be surface mounted on floors, walls or ceilings or embedded. Wall and ceiling panels generally come pre-assembled and are primarily metal with a decorative surface. Floor panels are usually part of the floor construction. They consist of electric cable or warm water tubes embedded in or attached to the floor. The cable or tubes may be buried in a concrete slab, covered with concrete or gypsum on a wood subfloor, sandwiched between layers of a wood floor or attached beneath the subfloor. Often metal plates which act as fins to disperse the heat within the floor are used in wood floor applications.

Thermal mass 

The term "thermal mass" doesn't mean a heated congregation on Sunday morning, it refers to the ability of a material to retain heat. For instance, a heated stone will remain warm much longer than a block of wood. This is because the stone is denser, thereby containing more mass. Mass can be both a blessing and a burden for a radiant panel system. The mass of the earth can be used as a flywheel when it is heated under a radiant concrete slab. This storage of heat can carry a building through a time when energy is not available. Where "off-peak" electrical rates are offered, using a radiant floor in conjunction with the thermal storage of the earth beneath the slab can produce some very low electric bills.

The down side to using a thermal mass storage is that it is like rolling a stone wheel. It takes a lot of energy to get it rolling and once it is rolling it is difficult to stop. Controlling the amount of heat delivered to the building at any particular time is difficult but manageable. This rolling wheel does bring some good news with it. The thermal mass in a heated shop or hangar floor responds rapidly to the change of air temperature when a big overhead door is opened. All the heat that has been "trickled" into the slab over time is released quickly to combat the cold air rolling in over the floor. This happens because of the sudden, dramatic increase in temperature difference between the slab and the air. Once the door is closed the building returns to its normal comfort setting almost immediately. It is a joy to experience.

A system which has very little thermal mass will respond quickly to the needs of the room environment. The key to any radiant panel system is to provide an even surface temperature so some mass is required to spread the heat across the panel. This mass may be in the form of a gypsum or other cementitious material or metal plates in the panel construction. Some underfloor systems simply rely on air currents within the joist space and the mass of the wood subfloor to spread the heat. While lower installation costs make this system attractive, particular care must be used in designing these systems because they have limited output and require substantially higher water temperatures.

Response time

As a rule, wall and ceiling panels have far less mass than floor panels and therefore respond quicker to changes in the room environment. This is particularly true when recovering from setting back a thermostat at night or when returning from vacation. On the other hand, floor systems are very stable and maintain a uniform climate because the floor surface remains at a constant temperature.

Energy savings

Increasing your comfort and, at the same time, saving money on your utility bill is a winning combination. We have already talked about two ways energy savings are accomplished with radiant panel heating; multiple zoning to allow unused rooms to be turned down, and using thermal mass for off peak storage. Another factor is lower thermostat settings. When both air temperature and radiant transfer are compensated for, you feel comfortable at room temperatures which are lower. You no longer have to force yourself to turn down the thermostat to save, you will do it automatically to be comfortable.

Heat loss from any building is driven by the temperature difference between the inside of the structure and the outside. Ask yourself this question. In a forced air system, where are the hot air registers located? The answer, of course, is along outside walls, under windows and in front of sliding glass doors. Why is that done? To compensate for all those cold surfaces. And where does that hot air go? Up those cold outside walls, across the ceiling and down to the cold air return. In other words, every place where heat loss occurs, that is where the hot air goes. This is a great setup for heat loss. Take 130 [degrees] F air from a register, blow it up the front of a sliding glass door when it is 10 [degrees] F outside and you have a 120 [degrees] F temperature difference between inside and outside the glass! If you have ever tried holding onto a drinking glass filled with hot coffee you will understand this heat transfer. The same holds true of the hot air that clings to the ceiling, transferring heat to the cold attic above. The bats may like it but it doesn't do you any good. Radiant panels direct the heat to the interior of the space and reduce or eliminate the excessive temperatures on outside walls and ceilings. This results in energy savings of 10% to 30% in most residences and up to 60% or more in shops, hangars and warehouses.

Cooling and air-conditioning

Most people these days require cooling and dehumidification in areas of high humidity. Some radiant panel systems are capable of cooling by circulating cool water through the panel. All the principles of radiant transfer still apply. By providing a cool surface, all other surfaces, including our bodies, will give up their heat to the panel. Just as in heating, this is extremely pleasant. Radiant panel cooling is most often done with ceiling panels although it can be accomplished using walls or floors. Floors become enjoyably cool, not cold. It is only when humidity is a factor that steps must be taken to avoid condensation on the panel. In this case, an auxiliary air system is used to dry the air.

When panel cooling is not available or practical, a separate air-conditioning system is installed. This could be a simplified central system or a split system with strategically placed units. The advantage of this system is zoned cooling. By using zoned cooling and placing air registers in the ceiling (where they should be), the extra cost will be returned in summer energy savings.

Finding an installer

The hardest part about installing radiant heat in your next project is finding an installer. You may be fortunate enough to live in an area where radiant panel heating and cooling is well known and installers abound. If not, contact a few manufacturers. Have them send you their literature and the name(s) of installers or dealers in your area. The Radiant Panel Association, [phone], can provide you with a list of manufacturers. Contact several of them. One manufacturer may not have a dealer in your area while another may have half a dozen. Choose an installer who either has experience or has the proper training. Avoid those who appear to be flying by the seat of their pants. The industry has been around long enough that "design as you go" is not acceptable. The installer should have a room-by-room heat loss analysis of your building done as well as a step-by-step system sizing process. Many companies have these on computer and can provide you with a printout, although work done by hand is perfectly acceptable. Guesses and estimates are okay for preliminary work, but nothing replaces good planning.

Final results

You will encounter objections. The construction trade does not like to do anything out of the ordinary. Anyone involved in the building or remodeling process who is not experienced with radiant panels is likely to balk. This is a normal, self preservation reaction encountered in architects, engineers, builders, plumbers, electricians, mechanical contractors and anyone else who has input into your project that might be affected. By their objection they are really saying, "I don't know enough about radiant panels and therefore do not feel comfortable working with them, but I don't want you to know that." Your choices are, either find someone else who does know radiant panels or educate the people you are working with. Working with a professional or trades people who are unfamiliar with the systems will only result in inflated costs and a potentially poor installation.

Remember, you are the one who will live or work in the building, not the contractors. It all comes back to your desire for comfort and energy savings. Do your homework, ask a lot of questions, use a reputable installer and you will end up with a level of comfort you may not have thought possible. People who have experienced radiant panel heating seldom, if ever, return to conventional heating and cooling systems. Make the decision for radiant panel heating and you will not be disappointed.

RELATED ARTICLE: Radiant panels aren't the same as baseboard heaters

If you are working for NASA you may think the term "radiant panel" describes a protrusion on a satellite designed to collect solar energy. As a Boeing 747 pilot, you might think it means a well-lit instrument duster. You might even think it refers to twelve apostles sitting down at a long narrow table for supper, but Webster's dictionary has an altogether different definition for "radiant panel" heating. According to Mr. Webster, this term was coined in 1928 to define, "space heating by means of wall, floor, baseboard, or ceiling panels with embedded electric conductors or hot-air or hot-water pipes." The term stuck because engineers have been using "panel heating and cooling" ever since. Technically, to be a true heating or cooling panel, the surface temperature of the panel must be controllable and not exceed 300 [degrees] F. (Most panels operate under 150 [degrees] F; floors generally less than 85 [degrees] F.) And finally, it is only a radiant panel if 50% or more of the heat transfer is by radiant energy. You may be more familiar with terms like "radiant floor" or "radiant ceiling" heating, or "radiators." These are not to be confused with hot water or electric baseboard heaters which actually produce only a small percent of radiant energy and primarily heat air by convection.

Old shower curtains can be hung behind lightweight drapes in the winter to prevent drafts. The extra layer creates a pocket of dead air that helps to insulate the room from the cold air outside the window.