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Passive discomfort as a result of surface temperatures As the daily outdoor average temperatures rise, passive strategies, like night ventilation, become ineffective and heat is driven inward, raising the temperature of the building's mass. Some or all of the inside surfaces have heated up to above the upper limits of the comfort envelope (above 78.8F or 26C) and the air in the space warms to the new higher average surface temperature. If no active cooling is supplied, interior conditions rise to well beyond the comfort envelope. The animation below shows how the comfort variables and building envelope change during a daily cycle when outdoor conditions rise to 101 F with no conditioning. |
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Comfort Control
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Active control of comfort by controlling indoor air temperature (Air Conditioning) Compare the natural conditioning of the space described above to what happens when we need to go to the active mode with "air" conditioning. To counteract the heat gain, we fill the space with cooler air. This cool air intercepts the incoming heat; it is warmed, and returns to the conditioner to be re-cooled. So, we occupy a box filled with cool air, colder than it would be if it didn't have to remove heat from the interior surfaces.
With this form of conditioning,
the temperature of one or more surfaces (or panels) is/are controlled.
For either heating or cooling, these panels can only deal with sensible
heat and not humidity control. A separate system would be necessary for
ventilation and humidity control (if needed). With radiant panels, usually
in the floor or the ceiling, the majority of the heat (over 50%) is transferred
by radiant exchange. Both heating and cooling can be accomplished by low
energy means (assisted passive) as in the Skytherm system developed by
Harold Hay and demonstrated at various locations. |
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In this 1979 house, solar heat is gathered in the roof's water bags during the day and conducts through the metal ceiling, providing a warm radiant panel in the spaces below. Movable insulation panels above the water bags control the amount of heat retained. For summer cooling, the water bags are exposed to the cold night sky to reject heat and covered during the day. The cool ceiling absorbs heat from the occupied space.
For fully active radiant systems, we are most frequently seeing advances coming from Western Europe where higher energy cost spur more rapid development than in the United States. The ventilation and dehumidification systems are decoupled from the radiant system, providing economies of energy and superior control of both the sensible and latent loads. The heating and/or cooling loads of the radiant panels are served by pumps rather than less efficient fans used for conventional air systems. The smaller quantities of air require smaller fans and ductwork. The resulting system provides quieter operation and improved indoor air quality. Occupants seem to prefer the feeling of comfort obtained with a radiant system over that of the typical convective (or all-air) system. It may be more like that feeling of comfort we have in the spring and fall (71F - 75F) when surfaces determine the air temperature. Design of building for passive comfort The design of the passive systems in a building should attempt to maintain conditions in the comfort envelope as long as possible. The longer these conditions can be maintained passively during the year, the better the passive design. Extending this period is usually best accomplished with the use of thermal mass.
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