How Is It Different From Conventional "Radiation"?
may be transferred by conduction (warmth transferred by objects
that are touching each other), or convection (heat that is carried
from a warmer to a cooler body by an intermediate fluid such as
air) or by radiation. Most conventional tube and fin "radiators"
are in fact, convectors. They heat a space by warming the air
that passes by the heated fins. This air, now warmer than the
surrounding air, rises due to buoyancy. This is known as free
convection. Where the air is moved past the fins by a blower or
fan it is called forced convection. In either case the idea is
the same, to compensate for perimeter heat losses by circulating
warm air within the space.
contrast, between 75 and 90 per cent of the energy from a radiant
panel is propagated by thermal radiation. Thermal radiation is
an electromagnetic radiation propagated due to a difference in
temperature. Therefore, the heat is transferred to all bodies
"seen" by the panel that are at a lower absolute temperature than
the panel itself.
Why Are The Panels In the Ceiling?
is often asked "Why put the panels in the ceiling? Heat rises,
doesn't it?" The answer is no, heat does not rise. Warm air rises.
The panels are most effective in the ceiling because from there,
they are best able to "see" the objects in the room. Lights are
placed in the ceiling for maximum effect. This is because from
above, it is not obstructed by room objects and its light can
be most effectively cast on the surroundings. Light is a type
of electromagnetic radiation which has a shorter wave length than
thermal radiation. Therefore, as with a light, the optimal location
for a radiant panel is the ceiling.
Another reason for placing the panels in the ceiling is so that
higher surface temperatures can be used. The rate of radiant heat
transfer from the panel is governed by the Stefan-Boltzmann Law,
q = S Ts4 , which demonstrates that the radiative heat transfer
from a body increases dramatically as its temperature is increased.
Radiant ceiling panels are typically operated at 79-85°C (175-185°F).
On the other hand, were the radiant panels mounted on the walls
or near the floor, safety code dictates that the panels could
not be operated at high temperatures. Thus by placing the panels
in the ceiling, the panels can be made to produce more output
per area by simply increasing the water temperature.
Is this a Proven Technology?
panel technology is not new to the HVAC (Heating, Ventilating
and Air Conditioning) industry. It is a mature, proven technology
that has been in use in Europe for over 100 years. Over the last
35 years, the Canadian and United States HVAC industry has employed
radiant panels in many commercial and institutional settings.
In fact, since the much publicized outbreak of Legionnaire's disease
(spread by contaminated central air systems), radiant heating
systems have been the choice for U.S. Veteran's Administration
Hospitals and, in the last 10 years, over 80% of the new hospitals
constructed in the provinces of Western Canada have employed radiant
panel ceiling heating systems.
What About Cold Feet...Warm Head?
feet? Hardly. Tests have shown that the floors in a room heated
by a radiant panel ceiling exhibit temperatures 1 to 2°C (3
to 4°F) above the ambient air temperature and actually provide
a source of re-radiated heat themselves. In fact, where down drafts
from cold walls or glazing present design challenges with respect
to occupant comfort, radiant panels provide a solution. The ceiling
panels warm the wall or window surfaces by direct transfer of
radiant energy, significantly increasing the temperature of each.
It has been found that even under extreme cold wall conditions
(27°F, -2.7°C), the air velocities are non-draft in nature
(less than 50 fpm or 0.25 m/s).
radiant panel surface temperatures are 170 to 185°F (about
82°C). This can cause the directional mean radiant temperature
(DMRT) to be 16 to 20°C (30 to 40°F) above the ambient
air temperature. By contrast, on a sunny day, the DMRT outside
is 30 to 40°C (50 to 70°F) higher than the ambient air
temperature. So, if you enjoy being out in the sunshine, you will
enjoy the indoor warmth of a radiant panel ceiling.