Case study: Schools’ air conditioning

Scenarios are studied to ensure thermal comfort criteria in classrooms.

By Amarpreet Sethi, CEM, HBDP, BEMP, LEED AP, DLR Group, Seattle September 29, 2016

Many schools don’t have air conditioning in the Pacific Northwest; however, the trend has been changing due to increasing temperatures. In a typical school year, these higher temperatures are mostly experienced during 2 weeks in June and 2 weeks in September.

The design criteria of 75°F may not have been achievable year-round for an elementary school project in Everett, Wash., without active cooling, but the need for cooling was reduced by means of good passive design, optimum glazing percentage, external shading, and high-performance glazing. For this reason, it was necessary to quantify set thermal comfort criteria to be met, rather than a typical peak design setpoint. This discussion was an important one to have and understand for the owner, as it could lead to the complete elimination of cooling equipment, reduced ductwork in the space, reduced first cost, and the potential to decrease future maintenance and utility costs.

The conversation, in this case, was initiated by discussing the raw weather data without the complexities of internal loads, exterior-envelope specifications, and free-cooling potential by means of an air-side economizer or operable windows.

Figure 4 shows the occupied hours per year, within each temperature range, per the outside air conditions for the city of Everett. Operable windows were used when the outdoor temperature was below 70°F; cooling was achieved via a displacement ventilation system. All of the months are included due to summer school. The building owner also was also interested in using operable windows, for which a green-light system was put in place to indicate to the user when it is "good" to open the window (based on outside air (OA) conditions when 60°F<OA<75°F). When windows are open, the variable air volume (VAV) box is closed off, unless called by the carbon dioxide sensor. Minimum ventilation is provided mechanically unless windows are open.

Design day performance was analyzed for the three scenarios with 100% capacity, 70% capacity, and 50% capacity of peak airflow. Figure 5 shows the peak design day airflow capacity at 100%, 70%, and 50%. Figure 6 shows the annual airflow requirements for 100% capacity. This graph is indicative of the hours when full capacity is required, when 70% of the capacity meets the load, and what reducing to 50% capacity means.

The classrooms were modeled with controls where windows would open at 60°F<OA<75°F. The model was simulated to quantify the anticipated hours above the prescribed thermal comfort of 75°F, with and without a ceiling fan in operation. A no-cooling option showed the hours if no active cooling was provided year-round and windows were closed. These hours were not acceptable by the client, thus cooling was included and studied with reduced capacity. Figure 7 shows the hours inside the different classrooms in a pod without any active cooling (worst-case scenario).

Figure 8 shows hours above 75°F with operable windows used when outside air conditions can maintain a comfortable space temperature, with cooling capacity or airflow at 70% of peak capacity. Figure 9 illustrates this with cooling capacity or airflow at 50% of peak capacity.

The 70% capacity was incorporated into the design to allow for some reduction in supply airflow sizing, which had a trickle-down effect on the plant sizing. This school was designed with a central ground-source heat pump system, all within the budget of the project.

The building envelope included strategic exterior shading, optimized operable-window area, and contacts in operable windows to shut off the HVAC system when open. All scenarios were studied, with and without operable windows, to ensure the space would meet the thermal comfort criteria if windows had to remain closed due to acoustical reasons.

Amarpreet Sethi is a senior associate at DLR Group’s Seattle office. Her expertise is in building optimization and energy services. She was also a Consulting-Specifying Engineer 40 Under 40 winner in 2014.