Radiant Panel Heating Systems
Radiant Panel Technology – What is It?
Ceiling radiant panel heating systems are similar to other air-water HVAC systems with respect to the arrangement of components. The important difference is that room thermal comfort is maintained primarily by radiant heat transfer instead of convective heat transfer (as is the case with tube and fin baseboard radiation). The significance of this is best illustrated by an analogy:
You are walking outside on a brisk fall day in bright sunshine. Suddenly the sun goes behind a cloud. Although you feel an immediate chill, the outside air temperature did not change in the instant that the sun was hidden; it is just that you were, until that instant, bathed in a warming radiant field.
Radiant ceiling panels create a radiant field, similar to the sun’s but of lower intensity, inside the occupied space. This radiant field heats the solid objects within the space without appreciably heating the room air. In addition, since most interior finishes have high emissivity, all surfaces within the room absorb and reradiate the heat to give a generally even warming effect.
How Is It Different From Conventional “Radiation”?
Radiant Panel Heating
Heat may be transferred by conduction (warmth transferred by objects that are touching each other), or convection (heat that is carried from a warmer to a cooler body by an intermediate fluid such as air) or by radiation. Most conventional tube and fin “radiators” are in fact, convectors. They heat a space by warming the air that passes by the heated fins. This air, now warmer than the surrounding air, rises due to buoyancy. This is known as free convection. Where the air is moved past the fins by a blower or fan it is called forced convection. In either case the idea is the same, to compensate for perimeter heat losses by circulating warm air within the space.
By contrast, between 75 and 90 per cent of the energy from a radiant panel heating system is propagated by thermal radiation. Thermal radiation is an electromagnetic radiation propagated due to a difference in temperature. Therefore, the heat is transferred to all bodies “seen” by the panel that are at a lower absolute temperature than the panel itself.
Why Are The Panels In the Ceiling?
Radiant Ceiling Panels
It is often asked “Why put the panels in the ceiling? Heat rises, doesn’t it?” The answer is no, heat does not rise. Warm air rises. The panels are most effective in the ceiling because from there, they are best able to “see” the objects in the room. Again, for clarification, consider the following analogy:
For maximum effect, lighting is placed in the ceiling. This is because from above, it is not obstructed by room objects and its light can be most effectively cast on the surroundings. Light is a type of electromagnetic radiation which has a shorter wave length than thermal radiation. Therefore, as with a light, the optimal location for radiant panel heating is in the ceiling.
Another reason for placing the panels in the ceiling is so that higher surface temperatures can be used. The rate of radiant heat transfer from the panel is governed by the Stefan-Boltzmann Law, q = S Ts4 , which demonstrates that the radiative heat transfer from a body increases dramatically as its temperature is increased. Radiant ceiling panels are typically operated at 79-85oC (175-185oF). On the other hand, wall mounted radiators cannot be operated at these high temperatures. This makes radiant panels comparatively more effective radiators.
Is Radiant Panel Heating New?
Exposed Radiant Panel Heating
Radiant panel heating technology is not new to the HVAC (Heating, Ventilating and Air Conditioning) industry. It is a mature, proven technology that has been in use in Europe for over 100 years. Over the last 35 years the Canadian and United States HVAC industry has employed radiant panels in many commercial and institutional settings. In fact, since the much publicized outbreak of Legionnaire’s disease (spread by contaminated central air systems), radiant heating systems have been the choice for U.S. Veteran’s Administration Hospitals and, in the last 10 years, over 80% of the new hospitals constructed in the provinces of Western Canada have employed radiant panel ceiling heating systems.
What Are Some Of The Advantages Of This Technology?
The principal advantages of radiant panel heating systems are listed in Chapter 6 of the 1992 ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) Handbook as:
- Because not only indoor air temperature, but also mean radiant temperature, can be controlled, total human thermal comfort may be better satisfied. Additionally, because the operative temperature for required human thermal comfort may be maintained by primarily controlling the mean radiant temperature of the conditioned indoor space, dry-bulb air temperature may be lower in heating or higher in cooling, reducing sensible heating and cooling loads.
- All pumps, fans, filters and so forth can be centrally located, simplifying maintenance.
- Cooling and heating can be simultaneous, without central zoning or seasonal changeover when three and four pipe systems are used.
- In terms of simple payback period, ceiling cooling panels and chilled beams have the highest energy savings potential. (DOE 2002)
- Noise associated with fan-coil or induction units is eliminated.
- Supply air quantities usually do not exceed those required for ventilation and dehumidification.
- Draperies and curtains can be installed at the outside wall without interfering with the heating and cooling system.
- The modular panel concept provides flexibility to meet changes in partitioning.
- A 100% outside air systems may be installed with less severe penalties in terms of refrigeration load because of reduced air quantities.
- Wet surface cooling coils are eliminated from the occupied space, reducing potential for septic contamination.
- Comfort levels are better than those of other conditioning systems because radiant loads are treated directly and air motion in space is at normal ventilation levels.
- Mechanical equipment is not needed at the outside walls, simplifying the wall, floor and structural systems.
What About Cold Feet….Warm Head?
Research and testing of radiant panel heating was carried out in the mid 1950’s by ASHRAE (actually called ASHVE at that time). Although this corpus of research is over 35 years old, wives tales about radiant panel technology still persist.
Cold feet? Hardly. Tests have shown that the floors in a room heated by a radiant panel ceiling exhibit temperatures 1 to 2oC (3 to 4oF) above the ambient air temperature and actually provide a source or reradiated heat themselves. In fact, where downdrafts from cold walls or glazing present design challenges with respect to occupant comfort, radiant panels provide a solution. The ceiling panels warm the wall or window surfaces by direct transfer of radiant energy, significantly increasing the temperature of each. It has been found that even under extreme cold wall conditions (27oF, -2.7oC), the air velocities are non-draft in nature (less than 50 fpm or 0.25 m/s).
And if your head is hot, don’t blame it on your radiant panel ceiling. Typically, radiant panel surface temperatures are 170 to 185oF (about 82oC). This can cause the directional mean radiant temperature (DMRT) to be 16 to 20oC (30 to 40oF) above the ambient air temperature. By contrast, on a sunny day, the DMRT outside is 30 to 40oC (50 to 70oF) higher than the ambient air temperature. So, if you enjoy being out in the sunshine, you will enjoy the indoor warmth of a radiant panel ceiling.
Where Is This Technology Used?
Radiant Panel Usage
Radiant panel heating technology is applicable in a wide variety of building environments, most notably it is well suited for hospital patient rooms and laboratories. Radiant panels provide a silent, draft-free, thermally stable environment for sedentary patients and require no mechanical equipment or bacteria/virus collectors in the heated space. This is of extreme importance in a hospital or laboratory setting where maintenance of an aseptic environment is crucial. It has been shown that the elimination of baseboard convectors alone can reduce bacterial growth in a space by over 40%.
Radiant panel ceiling systems are not just for hospital and laboratories. The leasable space reclaimed by eliminating 6 to 8 inches of baseboard convectors around the entire perimeter of every floor of an office building can mean significantly increased earnings for the commercial owner. Also, with ceiling panels, leased areas can be easily re-partitioned since there is no wall fin to build around. Most importantly, with no mechanical equipment in the occupied space, radiant ceiling panels are almost maintenance free.
Radiant panels can also be the ideal retrofit. In areas where lay-in acoustic ceiling tiles are used, radiant panels can provide heat that is architecturally invisible. With the hot water supply piping already in the ceiling space, the panel installation is very straightforward:
- Remove an existing acoustic ceiling tile.
- Make two plumbing connections to the hot water supply and hot water return pipes.
- Lay the radiant ceiling panel in the same T-Bar opening.
There are no risers to run and no cabinets to paint. In fact, once silk-screened to match the pattern of the existing ceiling tiles, the radiant heating panels are almost indistinguishable from the surrounding ceiling design.
Radiant Panel Heating
Radiant Panel Heating Brochure .PDF