ASHRAE Journal- September 2008

Vertical Air Temperature Difference (K) ﬂoor covering) and system (type of pipes, distance between pipes, water ﬂow rate) may limit the cooling capacity. Therefore, the design of a ﬂoor system for highest cooling capacity may be different than for heating. This article describes a ﬂoor cooling system that includes such considerations as thermal comfort of the occupants, which design parameters will inﬂuence the cooling capacity and how the system should be controlled. Examples of applications are presented. Thermal Comfort and Floor Cooling 3.0 2.5 2.0 1.5 1.0 0.5 0.0 10 8 6 4 Temperature Difference Room Floor (K) 2 Sedentary Standing The thermal environment may be Figure 1: Vertical air temperature difference (K) between ankle and head (0.1 m and 1.1 m described by the following parameters: for sedentary, 0.1 m and 1.7 m for standing), depending on the temperature difference (K) thermal insulation of the occupants between the room operative temperature and the surface temperature of a cooled ﬂoor. clothing, activity level of the occupants, air temperature, mean radiant temperature, air velocity, and has the same cooling effect as lowering the air temperature humidity.4,5 The combined inﬂuence can be described by the by 2°C (4°F). In most standards,1,6 the upper limit for the operative temperaPMV-PPD index.4,5 To provide thermal comfort, it is also necessary to account for local thermal discomfort, which may ture in summer is 26°C (79°F) for spaces with mainly sedentary be caused by radiant temperature asymmetry, draft, vertical occupants (1.2 met) and summer clothing (0.5 clo). air temperature differences, and too warm or too cool ﬂoor temperatures. Floor Surface Temperature As mentioned earlier, discomfort may occur at too-low or Operative Temperature too-high ﬂoor temperatures. In most national and international The two main parameters for providing acceptable thermal standards,4,5 a ﬂoor temperature interval of 18°C/19°C to 29°C conditions in a space, which may be significantly influenced (64°F/66°F to 84°F) is recommended for rooms occupied with by the heating/cooling system, are the air temperature and sedentary and/or standing people wearing normal shoes. the mean radiant temperature. The combined influence of For sedentary persons, a lower limit of 20°C (68°F) for ﬂoor these two temperatures is expressed as the operative tem- cooling is normally used. perature. For low air velocities (<0.2 m/s [0.66 fps]), the operative temperature can be approximated with the simple Vertical Air Temperature Difference average of air and mean radiant temperature. This means It is usually recommended to limit the air temperature differthat the air temperature and the mean radiant temperature ence between ankles (0.1 m [4 in.] level) and head (1.1 m [4 ft] are equally important for the level of thermal comfort in level) to 3 K for sedentary persons.4,5 Because most of the heat a space. exchange between a cooled ﬂoor and the space is by radiation, For a radiant cooling system, an important factor is the angle the air at the ankle level will not be cooled too much due to the factor between the occupants and the radiant heat source or low convective heat exchange. The vertical air temperature difsinks.6 This factor depends on the distance between a person ferences with ﬂoor cooling will not cause any discomfort. This and the surface and the area of the surface. This means a ﬂoor has been conﬁrmed in an experimental study.6 In this study the normally has the highest angle factor of all surfaces (walls, vertical temperature differences are less than 0.5 K, which is ceiling, windows, etc.) in a space to the occupants. For a person similar to the vertical temperature difference 0.4 K without ﬂoor positioned at the center of a 6 m by 6 m (20 ft by 20 ft) ﬂoor cooling. Another experimental study,7 shows somewhat higher the angle factor is 0.40 for sedentary. For a 12 m by 12 m (39 values (Figure 1) but still within acceptable comfort limits. ft by 39 ft) ﬂoor, the corresponding angle factors are 0.46. This should be placed in relation to the angle factor for a half Humidity room, 0.5. If the ﬂoor surface temperature is decreased by 5°C A limiting factor for the ﬂoor temperature and the cooling (9°F) and all other surface temperatures are assumed to be capacity is the dew-point temperature in the space. Some unchanged, then the mean radiant temperature will decrease standards5 recommend a limit for the relative humidity in a by 2°C (4°F). The impact on an occupant is expressed by the space to 60% or 70% rh, which at an air temperature of 26°C operative temperature, which will decrease by 1°C (2°F). (79°F) corresponds to a dew point between 17°C and 20°C Put another way, a 5°C (9°F) lower ﬂoor surface temperature (63°F and 68°F). Others4 recommend an absolute humidity September 2008 ASHRAE Journal 17

Table of Contents Feed for the Digital Edition of ASHRAE Journal- September 2008

ASHRAE Journal- September 2008 Section: Contents Contents Section: Commentary Options for Sustainability Section: Industry News The Silk Route for Energy Solar Thermal Is Unrealized Opportunity Industry Groups Sue City of Albuquerque Section: Letters Letters Section: Meetings and Shows Meetings and Shows Section: Feature Articles Article-Radiant Floor Cooling Systems Article-HVAC Design for Sustainable Lab Article-Mixed Mode Ventilation Article-Single- Design Considerations For Active Chilled Beams Article-Acoustic Design In Green Buildings Article-Teams, Contracts & BIM Section: Building Sciences Some Old Lessons Distilled Section: Washington Report Energy in Federal Buildings Section: Products HVAC&R Product Showplace Section: Emerging Technologies Toplighting & Lighting Controls For Commercial Buildings Section: Special Products Fans & Blowers Section: Classified Ads Classified Ads Section: Advertising Index Advertising Index

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