ID#10142 made on
EQc7.2 - Thermal comfort - verification
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The project is an 80,000 sf K-12 school located in the pacific northwest. A majority of the building is being conditioned by "standard" mechanical means and can demonstrate compliance accor...
The project is an 80,000 sf K-12 school located in the pacific northwest. A majority of the building is being conditioned by "standard" mechanical means and can demonstrate compliance according to ASHRAE 55-2004 Figure 18.104.22.168., however there are portions of the building that will be thermally regulated by natural ventilation and with extensive radiant systems.
The purpose of this request is to determine whether the more passive, naturally ventilated portion of the building may be considered "naturally conditioned" with their thermal comfort criteria evaluated according to ASHRAE 55-2004 Figure 5.3 rather than being evaluated according to Figure 22.214.171.124.
The northwest climate is considered heating dominated with narrow diurnal temperature variations and as a consequence has limited cooling needs, especially during the school year. The need for cooling is isolated to only brief time periods at the beginning and end of the 9-month school year when potential thermal comfort concerns would arise. To mitigate thermal comfort concerns during these isolated times, the design incorporates occupant-controlled operable windows and ceiling fans to allow comfort control within the classrooms. In addition to the operable windows, the design also includes utilizing the thermal massing properties of the concrete floors to better modulate the release of cooling within the space(s). The cooling will be supplied by evaporatively-cooled water via hydronic loops in the floors during the nighttime periods when the building is unoccupied. The intent is pre-charge the thermal mass of the floors with enough cooling to offset the effects of internal heat gains and eliminate the need for active mechanical cooling during the day. Since the charging is taking place during the evenings, there will not be any ability for the occupants to directly call for additional cooling to be added to floor or space and the mass will in essence be allowed to release it cooling naturally trough out the day. Lastly, we are interpreting that this approach of pre-cooling the mass without the need of compressor-based mechanical cooling could be construed as a assisted passive approach, however it is unclear whether the USGBC will recognize these spaces as "naturally conditioned" for the purposes of IEQc7.1.
ASHRAE 55-2004, Section 5.3, states that "occupant-controlled naturally conditioned spaces are those spaces where the thermal conditions of the space are regulated primarily by the occupants through opening and closing of windows". We believe that the definition is still being satisfied with the mass pre-charging strategy described previously, since the regulation of the thermal conditions will still be controlled by the occupants through the opening and closing of windows.
Can the thermal comfort criteria in these rooms be evaluated according to "naturally conditioned" rather than "typical" criteria and if so, is modeling an acceptable means of documenting compliance?
The applicant is asking whether a space that meets the ASHRAE Standard 55-2004 definition of "naturally conditioned spaces, occupant controlled," but is also served by an in-floor radiant cooling system that is in turn served by an evaporatively-cooled water tower that will be utilized at night to pre-charge the high thermal mass concrete floors, may be evaluated according to ASHRAE Standard 55-2004 Section 5.3 - Optional Method for Determining Acceptable Thermal Conditions in Naturally Conditioned Spaces in lieu of the requirements of Section 5.2 Method for Determining Acceptable Thermal Conditions in Occupied Spaces.
Section 5.3 of Standard 55-2004 states, "there must be no mechanical cooling system for the space (e.g. refrigerated air cooling, radiant cooling, or desiccant cooling.)" in order to use the Optional Method for Determining Acceptable Thermal Conditions in Naturally Conditioned Spaces. If the radiant cooling system is operated during the day, the project would be considered mechanically cooled and not eligible to use the Section 5.3 method. However, if the system is operated as described in the Interpretation Request and the radiant cooling system is only used at night as a pre-cooling strategy then the intent and spirit of the conditions for using Section 5.3 are met.
1. For times when active mechanical cooling OR heating is used during occupied hours, Section 5.3 does not apply and Section 5.2 Method for Determining Acceptable Thermal Conditions in Occupied Spaces shall be used.
2. In the Cooling season when the night-time pre-cooling strategy is used, the minimum design operative temperatures in the morning shall be evaluated compared to the allowable minimum operative temperature based on Section 5.3 to ensure comfortable conditions throughout the day. Applicable internationally.
Related Addenda (Corrections & Interpretations)