Architectural design championed the monolithic ceiling, leading to the development
of several approaches for concealing electric and hydronic radiant heating in the ceiling.
These systems included the use of resistance wire, gypsum panels, and flexible
radiant panel elements.The engineering compromises in efficiency, control, technical
reliability, and service accessibility have culminated in the virtual disappearance of
electric concealed ceiling heating.
Concealed radiant ceiling systems merit coverage in this Handbook because the
systems are still in use in many areas in the United States and abroad. It is also
important to understand that the circumstances, which existed during the period of
their greatest popularity, were very different than those that exist currently. For a
heating system selection and design to stand up over time, global factors that impact
design engineering must be considered. In addition, there are a lot of lessons to be
learned from the experience history of concealed radiant ceiling heating, which can
be a viable design alternative when all performance factors are addressed.
One of the early techniques for providing ceiling heating involved stapling or
adhering insulated resistance wire directly to the ceiling surface before finish plastering
was applied.The wire was generally placed in parallel pattern about 1⁄2 to 1 in apart
running from one side of a room to the other, covering about 80 percent of the ceiling
surface. System life expectancy was 15 to 25 years, or longer. Failures were caused
by breaks in the circuit caused by wire failure, moisture, or building stress breaks.
When these systems were popular, repair service was available through electric utilities
and electricians.
When circuit interruption occurred, current detection wands were used to locate
the break; special wire bridge connectors were installed; and the ceiling was patched
and repainted. Concealed resistance wire repair has become a lost art. However, as
these systems age, wire failures increase and repair ceases to be cost-effective.
Occasionally, an outline, or ghost line, related to the concealed wire would appear
on the ceiling surface.This may occur through a process known as historesis, where
particles in suspension are attracted to surfaces in relation to the nature of the electrical
magnetic field, such as may exist on the ceiling surface below the concealed
resistance heat wires or hydronic metal conduit.
As the cost of plastering increased, cheaper ways of installing a radiant ceiling
system were sought. As sheetrock became economical, embedding resistance wire
with backside termination connectors resulted in prefabricated gypsum radiant panels.
The entire assembly became both the ceiling and radiant heating system. Users
of concealed radiant systems generally found them to be very comfortable. Complaints,
when they did occur, related to localized discomfort and slow response time.
Problems relating to localized discomfort sometimes occurred because the general
construction and insulation practices at the time were very different from those
defined by current building codes. Specifically, glazing was single-pane alone or
combined with single-pane storm windows.There was no insulation under the floor
above a basement or crawl space, so the floor would likely be cold under a table with
any heating system, as would a space by a window or a masonry fireplace.
Two concerns are inherent in concealed systems. Response time is related to the
Btu limitations of the gypsum or plaster ceiling materials, which generally results in
warm-up recovery times from setback that are too long to make setback practical
except for long vacancy periods. As a result, these systems are normally turned on at
the desired temperature in the fall and turned off in the spring, which should be factored
in to design and operation analysis.
The second area of concern is that watt density or Btu output is normally uniform
across the ceiling surface being heated.The Btu limitations of concealed systems may
not adequately address areas of high heat loss, such as in front of sliding glass doors,
or in rooms where the ceiling area is inadequate to meet the heat loss, such as bathrooms.
Concealed system design often incorporates supplemental or hybrid design to
ensure uniform occupant thermal comfort and energy-efficient system operation.
Original owners were usually happy with operating costs: electricity was very
inexpensive during the days of the “Gold Medallion Home,” and all-electric homes
were given an additional rate discount.When energy prices escalated and rates were
revised, the level of insulation and construction standards of the time made operating
cost comparisons with tighter, better insulated buildings less attractive, and electric
heat was universally singled out as being expensive. In reality, for concealed
radiant systems, the main cause of high operating cost was inattention to the importance
of minimizing radiant panel backloss and edge loss. A second cause was the
failure to recognize that the floor is the primary recipient of ceiling radiant energy
and, therefore, must be insulated from an unheated space below.
Electric radiant ceiling heating products also include resistance wire–embedded
gypsum panels of varying widths and lengths that are laid between the ceiling joists
directly upon the top of the gypsum ceiling board.This type of panel is normally connected
by routing a connecting wire from the thermostat through an encapsulated
feed located at the end of each panel on the circuit.
As with all radiant panels, installation design must incorporate insulation to maximize
frontal panel output. Use of loose cellulose or any potentially combustible
insulation is not recommended. Loose insulation, like cellulose, can migrate between
the panel and the ceiling, where the highest temperatures occur. Product life is similar
to ceiling cable heat, but failure occurs on a panel-by-panel basis rather than
an entire room, as may be the case when a single ceiling cable comprises the entire
heating element.
Repair is virtually impossible for between-floor installation. Replacement of
panels that could be up to 10 ft long is very difficult, even with attic access. Due to
the weight of the gypsum panels, the use of at least 5⁄8-in sheetrock is essential to
avoid the potential appearance of a wavy ceiling surface. Normally, panel support
strapping is installed to position the panels prior to sheetrock installation. However,
the panels normally rest directly on the sheetrock to ensure maximum heat conduction
to the sheetrock when the installation is completed.
Sheet resistive elements stapled to the ceiling joist are designed to provide heat
in the area between the joists, which is conducted directly to the gypsum ceiling that
is fastened directly to the same ceiling joists. Common elements include graphiteimpregnated
fiberglass material and graphite-encapsulated plastic film. Both elements
encountered occasional electrical problems.The failures were usually traced
to buss bar electrical failure due to manufacturing defect or to wire crimp connection
failure during installation.
A third element constructed with etched resistance foil embodied a heat-limiting
feature that prevented dangerous overheating by breaking the resistance circuit.
However, its use in very tight homes, with as much as R-70 insulation placed upon
the element, resulted in sufficient temperature buildup to cause premature element
failure, thereby leaving the building owner with an inaccessible and virtually unsalvageable
or unrepairable heating system.
Flexible elements were popular with installers, who found installation to be easy,
quick, and profitable. Builders and owners embraced the technology and marveled
at its simplicity and nonmechanical effectiveness. Architects were attracted to the
invisibility of the heating system and the resulting monolithic ceiling.
When concealed electric radiant heating systems fail, the most cost-effective option
is usually the installation of surface-mounted panels. Room or area thermostats and
ceiling wiring are already in place, and electrical circuitry is adequate as installed
capacity is normally reduced by 30 to 50 percent. Operating cost should be reduced by
15 to 20 percent, and can be further reduced by the use of temperature setback that is
possible with fast-acting surface- or wall-mounted panels.
Architectural design championed the monolithic ceiling, leading to the development
of several approaches for concealing electric and hydronic radiant heating in the ceiling.
These systems included the use of resistance wire, gypsum panels, and flexible
radiant panel elements.The engineering compromises in efficiency, control, technical
reliability, and service accessibility have culminated in the virtual disappearance of
electric concealed ceiling heating.
Concealed radiant ceiling systems merit coverage in this Handbook because the
systems are still in use in many areas in the United States and abroad. It is also
important to understand that the circumstances, which existed during the period of
their greatest popularity, were very different than those that exist currently. For a
heating system selection and design to stand up over time, global factors that impact
design engineering must be considered. In addition, there are a lot of lessons to be
learned from the experience history of concealed radiant ceiling heating, which can
be a viable design alternative when all performance factors are addressed.
One of the early techniques for providing ceiling...
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© 2009 McGraw-Hill Companies, Inc. (The)
