Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.59
the warmer supply water may be routed to the perimeter or exposed walls and the
cooler return water brought to the interior zones.
Panel and Room Temperatures. Heat from the embedded pipes is transmitted to
the panel, which in turn supplies heat to the room by two methods: (1) convection
and (2) radiation. The amount of heat supplied by convection depends on the temperature
difference between the panel and the air. The amount of heat supplied by
radiation depends on the difference between the fourth powers of the absolute temperatures
of panel and occupants. Thus, as panel temperature is increased, persons
in the room receive a greater percentage of heat by radiation than by convection.
Inasmuch as high panel temperatures are uncomfortable, it is advisable to keep
floor panel temperatures about 85�F or lower and ceiling panel temperatures 100�F
or lower. The percentage of radiant heat supplied by a panel at 85�F is about 56%
and by one at 100�F about 70%.
Most advocates of panel heating claim that a lower than the usual design inside
temperature may be maintained because of the large radiant surface comforting the
individual; i.e., a dwelling normally maintained at 70�F may be kept at an air
temperature of about 65�F. The low air temperature makes possible a reduction in
heat losses through walls, glass, etc., and thus cuts down the heating load. However,
during periods when the heating controller is satisfied and the water circulation
stops, the radiant-heat source diminishes, creating an uncomfortable condition due
to the below-normal room air temperature. It is thus considered good practice to
design the system for standard room temperatures (Table 13.10) and the heating
plant for the total capacity required.
Design of Panel Heating. Panel output, Btu per hour per square foot, should be
estimated to determine panel-heating area required. Panel capacity is determined
by pipe spacing, water temperature, area of exposed walls and windows, infiltration
air, insulation value of structural and architectural material between coil and occupied
space, and insulation value of structural material preventing heat loss from
the reverse side and edge of the panel. It is best to leave design of panel heating
to a specialist.
(‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating,
and Air-Conditioning Engineers, 1791 Tully Circle, N.E., Atlanta, GA
30329.)
13.16 SNOW MELTING
Design of a snow-melting system for sidewalks, roads, parking areas, etc., involves
the determination of a design amount of snowfall, sizing and layout of piping, and
selection of heat exchanger and circulating medium. The pipe is placed under the
wearing surface, with enough cover to protect it against damage from traffic loads,
and a heated fluid is circulated through it.
If friction is too high in extensive runs, use parallel loops (Art. 13.15). All
precautions for drainage, fabrication, etc., hold for snow-melting panels as well as
interior heating panels.
Table 13.13 gives a design rate of snowfall in inches of water equivalent per
hour per square foot for various cities.