Panel Heating

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 RADIATION

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 requirements.
• 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 in partitioning.
• A 100% outdoor air system may be installed with smaller penalties in terms of refrigeration load because of reduced air quantities.
• 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, etc.

Disadvantages

• 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 or as a cooling panel when chilled water is flowing.

The second type of panel consists of a copper coil secured to the 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 position by various types of ceiling suspension systems, most typically a standard suspended T-bar 24 in. by 48 in. exposed grid system. Figure 14 illustrates metal panels using a copper tube pressed into an 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 on the back of the panels. The acoustical blanket is also required for thermal reasons, so that the reverse loss or upward flow of heat from 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 to 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 of 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 ft 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 are suitable for condenser water heat reclaim systems.
3. Locate ceiling panels adjacent to the outside wall and as close as possible to the areas of maximum load. The panel area within 3 ft of the outside wall should have a heating capacity equal to or 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 follows:
(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