PANEL heating and cooling uses controlled-temperature sur faces
on the floor, walls, or ceiling;
the temperature is maintained by circulating water, air, or electric
current through a circuit embedded in the panel. A controlled-temperature
surface is called a radiant panel if 50% or more of the heat transfer
is by radiation to other surfaces seen by the panel. Radiant panel
systems may be combined either with a central station air system
of one-zone, constant temperature, constant volume design or with
dual-duct, reheat, multizone or variable volume systems. These
combined systems are called hybrid HVAC systems.
PRINCIPLES OF THERMAL
Thermal radiation (1) is transmitted at the speed of light; (2)
travels in straight lines and can be reflected; and (3) elevates
the temperature of solid objects by absorption but does not noticeably
heat the air through which it travels.
Thermal radiation is exchanged continuously between all bodies
in a building environment. The rate at which radiant heat is transferred
depends on the following factors:
• Temperature (of the emitting surface and receiver)
• Emittance (of the radiating surface)
• Reflectance, absorptance, and transmittance (of the receiver)
• View factor between the emitting surface and receiver
(viewing angle of the occupant to the radiant source)
A critical factor is the structure of the body surface. In general,
rough surfaces have low reflectance and high emittance/absorptance
characteristics. Conversely, smooth or polished metal surfaces
have high reflectance and low absorptance/emittance characteristics.
One example of radiant heating is the feeling of warmth when
standing in the sun’s rays on a cool, sunny day. Some of
the rays come directly from the sun and include the entire electromagnetic
spectrum. Other rays from the sun are absorbed by or reflected
from surrounding objects. This generates secondary rays that are
a combination of the wavelength produced by the temperature of
the objects and the wavelength of the reflected rays. If a cloud
passes in front of the sun, there is an instant sensation of cold.
This sensation is caused by the decrease in radiant heat received
from the sun, although there is little, if any, change in the
ambient air temperature.
Principal advantages of radiant panel systems are the following:
• Comfort levels can be better than those of other conditioning
systems because radiant loads are satisfied directly and air motion
in the space is at normal ventilation levels.
• Space-conditioning equipment is not needed at the outside
walls; this simplifies the wall, floor, and structural systems.
• Almost all mechanical equipment may be centrally located,
simplifying maintenance and operation.
• No space within the conditioned room is required for mechanical
equipment. This feature is especially valuable in hospital patient
rooms and other applications where space is at a premium, where
maximum cleanliness is essential, or where it is dictated by legal
• Draperies and curtains can be installed at the outside
wall without interfering with the space-conditioning system.
• When four-pipe systems are used, cooling and heating can
be simultaneous, without central zoning or seasonal changeover.
• Supply air quantities usually do not exceed those required
for ventilation and dehumidification.
• The modular panel provides flexibility to meet changes
• A 100% outdoor air system may be installed with smaller
penalties in terms of refrigeration load because of reduced air
• A common central air system can serve both the interior
and perimeter zones.
• Wet surface cooling coils are eliminated from the occupied
space, reducing the potential for septic contamination.
• The panel system can use the automatic sprinkler system
piping (see NFPA Standard 13, Chapter 3, Section 3.6). The maximum
water temperature must not fuse the heads.
• Radiant panel heating and cooling and minimum supply air
quantities provide a draft-free environment.
• Noise associated with fan-coil or induction units is eliminated.
• Peak loads are reduced as a result of thermal energy storage
in the panel structure, exposed walls, and partitions.
• Panels can be coupled with other conditioning systems
for heat loss (gain) compensation for cold or hot floors, windows,
• Response time can be slow if controls and/or heating
elements are not selected or installed correctly
• Improper installation of pipe or element spacing and/or
incorrect sizing of heat source can cause nonuniform surface temperatures
or insufficient heating capacity
Metal Ceiling Panels Attached to Pipe Laterals
Three types of metal ceiling systems are available. The first
consists of light aluminum panels, usually 12 in. by 24 in., attached
in the field to 0.5 in. galvanized pipe coils.
. Aluminum ceiling panels are clipped to these pipe
laterals and act as a heating panel when warm water is flowing
a cooling panel when chilled water is flowing.
The second type of panel consists of a copper coil secured to
aluminum face sheet to form a modular panel. Modular panels are
available in sizes up to about 36 in. by 60 in. and are held in
by various types of ceiling suspension systems, most typically
standard suspended T-bar 24 in. by 48 in. exposed grid system.
Figure 14 illustrates metal panels using a copper tube pressed
aluminum extrusion, although other methods of securing the copper
tube have proven equally effective.
Metal ceiling panels can be perforated so that the ceiling
becomes sound absorbent when acoustical material is installed
the back of the panels. The acoustical blanket is also required
thermal reasons, so that the reverse loss or upward flow of heat
the metal ceiling panels is minimized.
The third type of panel is an aluminum extrusion face sheet with
a copper tube mechanically fastened into a channel housing on
the back. Extruded panels can be manufactured in almost any shape
and size. Extruded aluminum panels are often used as long, narrow
panels at the outside wall and are independent of the ceiling
system. Panels 15 or 20 in. wide usually satisfy the heating requirements
of a typical office
Performance data for extruded aluminum panels vary with the
copper tube/aluminum contact and test procedures used. Hydronic
ceiling panels have a low thermal resistance and respond quickly
changes in space conditions. Table 1 shows thermal resistance
values for various ceiling constructions.
Metal radiant ceiling panels can be used with any of the all-air
cooling systems described in Chapter 2. Chapters 25 through 28
the 1997 ASHRAE Handbook—Fundamentals describe how to calculate
heating loads. Double glazing and heavy insulation in outside
walls have reduced transmission heat losses. As a result, infiltration
and reheat have become of greater concern. Additional design considerations
are as follows:
1. Perimeter radiant heating panels not extending more than 3
into the room may operate at higher temperatures, as described
under item 8d in the section on Hydronic Panel Systems.
2. Hydronic panels operate efficiently at low temperature and
suitable for condenser water heat reclaim systems.
3. Locate ceiling panels adjacent to the outside wall and as close
possible to the areas of maximum load. The panel area within
3 ft of the outside wall should have a heating capacity equal
greater than 50% of the wall transmission load.
4. Ceiling system designs based on passing return air through
perforated modular panels into the plenum space above the ceiling
are not recommended because much of the panel heat transfer is
lost to the return air system.
5. When selecting heating design temperatures for a ceiling panel
surface or mean water temperature, the design parameters are as
(a) Excessively high temperatures over the occupied zone will
cause the occupant to experience a “hot head effect.”
(b) Temperatures that are too low can result in an oversized,
uneconomical panel and a feeling of coolness at the outside wall.
(c) Give the technique in item 3 priority.
Metal radiant panels can also be integrated into the ceiling
design to provide a narrow band of radiant heating around the
perimeter of the building. The radiant system offers advantages
over baseboard or overhead air in appearance, comfort, operating
efficiency and cost, maintenance, and product life.
Radiant Floor Heating