Chapter 5.2.5 - Metal-Encased Radiant Heat Panels

Metal-encased electric radiant heat panels have been in use for more than 50 years.
The most common application used is in suspended ceiling grids as either primary or
perimeter heating (Fig. 2.12).The revealed-edge and flat lay-in T-bar grid or fine-line
grid units are normally lightly textured or silk-screened to match the ceiling tiles.For
many years, choice was confined to two wattage densities. System sizing and design
was usually based upon manufacturer recommendations. Users experienced oversizing
when following traditional design methodologies.

2.5.1 Features and Sizing

The most common sizes are 2 × 2 ft in 250- and 375-W models, and 2 × 4 ft in 500- and
750-W models, and custom wattage in between these ranges.Voltage choice includes
the most common 120 and 240 V, which can be manufactured as a dual-voltage
model, and commercial 208- and 277-V models. Although sizes as large as 2 × 8 ft are
made, metal encapsulation makes these panels heavy to ship, handle, and install.
Each 2- by 2-ft increment is approximately 15 lb in weight so that a 2- by 8-ft panel
weighs 60 lb or more. It is important to confirm that the weight of the panels specified
conforms to the ceiling support and structural weight rating.

The choice of wattage and watt density is related not only to the heat loss, but
also to the comfort parameters of the design application. Even though it might be
appropriate to use a high–watt density panel, a 750- or 1000-W, 2- by 4-ft panel over
an open-floor-plan factory workstation at an 8-ft height, it might be uncomfortable
for someone sitting at a desk in an office with an 8-ft ceiling. The key factor is that
the radiant asymmetry, or the temperature difference that the occupant feels, be
within the guidelines for human thermal comfort in each case.

In general, lower watt densities are more commonly used in well-insulated building
environments with average 8-ft ceilings and controlled air distribution. The
industry offers a range of watt densities that provide design flexibility.The deciding
factor is not cost per watt, which was common field design methodology when
energy was cheap.The incentive to oversize is fed by the relationship between capacity
and price per unit of capacity with products like baseboard convection heaters
that are the cheapest heaters on the market. In well-insulated buildings that are
designed to provide occupant thermal comfort, radiant panel sizing is precisely
defined. The result usually is that proper watt density and lower installed capacity
provide better occupant thermal comfort and operating economy.

In fact, both oversizing and the use of higher watt density than conforms to comfort
parameters can result in uneconomic operation and an unsuccessful installation.
In early installations, it was not uncommon to find a high-watt-density panel specified
because it was placed directly adjacent to an air diffuser.The result was, in effect, air
washing of the panel face, significantly lowering the surface temperature due to the
increased convection heat transfer while the diffuser was active, and uncomfortable
asymmetric radiation due to the much higher panel surface temperature when the
diffuser was inactive. Similarly, oversizing at the perimeter results in short cycling as
the heat loss is satisfied too quickly, thereby negating performance of the design
objective of reducing downdraft due to cold window surfaces. Short cycling will occur
with the use of high-density panels in most buildings using code-conformant glazing,
as opposed to the plate glass that was common in buildings built before energy prices
escalated. Lower-watt-density panels will have a longer “on” cycle, thereby eliminating
discomfort problems caused by window downdraft.

Metal-encased electric radiant heat panels have been in use for more than 50 years.
The most common application used is in suspended ceiling grids as either primary or
perimeter heating (Fig. 2.12).The revealed-edge and flat lay-in T-bar grid or fine-line
grid units are normally lightly textured or silk-screened to match the ceiling tiles.For
many years, choice was confined to two wattage densities. System sizing and design
was usually based upon manufacturer recommendations. Users experienced oversizing
when following traditional design methodologies.

2.5.1 Features and Sizing

The most common sizes are 2 × 2 ft in 250- and 375-W models, and 2 × 4 ft in 500- and
750-W models, and custom wattage in between these ranges.Voltage choice includes
the most common 120 and 240 V, which can be manufactured as a dual-voltage
model, and commercial 208- and 277-V models. Although sizes as large as 2 × 8 ft are
made, metal encapsulation makes these panels heavy to ship, handle, and install.
Each 2- by 2-ft increment is approximately 15 lb in weight so that a 2- by...