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CLICK ON A TOPIC TO LEARN ABOUT RADIANT HEAT- Why Radiant Heat?
- How Radiant Energy Works
- Where Radiant Panels are Located
- Temperature and Radiant Energy
- How Radiant Panels are Heated
- Controlling a Radiant Heating System
- Radiant Panel Construction
- What is Thermal Mass?
- Radiant Panel Response Time
- Cooling and Air-Conditioning
- Typical Radiant Heating System Designs
Increasing your comfort and, at the same time, saving money on your utility bill is a winning combination. Reduce your energy bills by multiple zoning that allows unused rooms to have the thermostat turned down.The use of thermal mass for off peak storage can further reduce energy bills. Another energy saving feature is lower thermostat settings. When both air temperature and radiant transfer are compensated for, you feel more comfortable a lower room air temperatures. 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. Conventional systems locate registers along outside walls, under windows and in front of sliding glass doors to compensate for all those cold surfaces. That hot air goes up those cold outside walls, across the ceiling and down to the cold air return. This is a great setup for wasting energy. Radiant panels direct the heat to the interior of the space and reduce or eliminate the excessive temperatures on outside walls and ceilings. This can result in energy savings of 10% to 30% in most residences and up to 60% or more in shops, hangars and warehouses.
"What exactly is radiant energy?" Hold your hand over a hot cup of coffee and feel the heat. The logical conclusion 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 the cup when you place your hand to the side of it. Radiant energy transfer is caused by a warm surface giving up its heat to a cooler surface. Whenever there is a temperature difference between two surfaces, both surfaces will attempt to equalize. Radiant energy travels through space without heating the space itself. It only turns into heat when it contacts a cooler surface. 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. By controlling both the air temperature and the radiant transfer, radiant panel systems deliver a comfort that is unsurpassed. A radiant panel can be effectively mounted on any flat surface. The only requirement is the surface be sufficiently large enough. The larger the surface, the lower the actual surface temperature required. A wall radiator may have a surface temperature of 180'F while an 8 1'F floor will do the same job.
Radiant energy will heat the surfaces of all objects which are in 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. Air coming in contact with these surfaces is also gently heated.
Temperature and Radiant Energy
Skin surface temperature, about 85'F, is generally warmer than the surrounding surfaces. This makes us a radiant panel. Stand by a large picture window in mid-winter 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. When the sun beats down on us through the window, we receive heat instead and off comes the sweater. 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. 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. A heated floor normally "feels" neutral. Its surface temperature is usually less than our body temperature, although the overall sensation is one of comfort. Only on very cold 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. 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 which are often associated with a forced air furnace.
Whether your choice is floor, wall or ceiling panels, they are heated in one of three ways, electrical 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. Electric panels have electricity as their sole utility, are quite simple in construction and generally have a lower 'up-front' cost. Water, on the other hand, can be heated by almost any utility be it natural gas, propane, oil, wood, solar, or electricity, and is quite versatile. Your choice will probably hinge on the energy costs of the available utility and the size of the project.
Controlling a Radiant Heating System
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. 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 to the sticker price of an automobile. But unlike automobile options, these comfort features will pay back in energy savings.
Radiant Panel Construction
Panels 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. Radiant panels can be used effectively in combination with other forms of heat distribution including baseboard convectors and forced air. For example a home may have a heated concrete floor on the lower level and forced air on the upper level.
What is Thermal Mass?
"Thermal Mass" 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. 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. 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. 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. When properly designed, these systems are a good alternative for retrofitting an existing building.
As a rule, wall and ceiling panels have far less mass than floor panels and therefore respond quickly to changes in the room environment. This is particularly true when recovering from setting back a thermostat at night or when returning from vacation. Floor systems are very stable and maintain a uniform climate because the floor surface remains at a constant temperature.Cooling and Air Conditioning
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 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-conditioning system is used to dry the air. Most often a separate air-conditioning system is installed. This could be a simplified central system or a split system with strategically placed units. The advantage is zoned cooling. Using zoned cooling and placing air registers in the ceiling (where they should be), will result in summer energy savings.
Typical Radiant Heating System Designs
Under Floor Suspended: Typically, although not exclusively, used in retrofit systems, this installation uses the space under the floor for pipe placement. Here, the Hewing PEXc pipe is attached under the subfloor and insulation is placed underneath, forcing heat upward to the living space.Under floor installation used mostly for retrofit jobs.
Slab Installation: This type of radiant heating system is installed during construction prior to pouring concrete foundations. An insulating barrier is placed adjacent to the ground and radiant pipe is fastened to a reinforcing mesh. Concrete is then poured over the top, forming a permanent room size heat distribution unit - the floor.
Information on this page courtesy of Radiant Panel Association
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