P2. Night Ventilation with Building Thermal Mass > Outcomes

• The VSAT screening tool was used to evaluate include a small office building, a sit-down restaurant, a retail store, a school class wing, a school auditorium, a school gymnasium, and a school library. The night ventilation precooling algorithm uses a set of rules to enable precooling. Whenever the ambient temperature drops below the zone temperature, the ambient air can be used to precool the zone and reduce cooling loads during the next day. However, the next day savings associated with operating the ventilation system at night should be sufficient to offset the cost of operating the fan. In addition, the ambient humidity should be low enough to avoid increased latent loads during the next day and the ambient temperature should be high enough so as to avoid additional heating requirements after occupancy.

• A simplified version of the control algorithm operated correctly within two field sites. The first field site, located at the Field Diagnostic Services, Inc. (FDSI) headquarters near Philadelphia, PA, was used primarily for initial debugging of the implementation. The second field site is a Walgreens located in Rialto, CA. The Walgreens site employs five rooftop units (RTUs) each with a controller that provides ventilation precooling under the appropriate conditions.

• The electrical energy savings varied between 0 and 8%. The electrical demand cost savings associated with night ventilation varied between about 0 and 28%, whereas the total electrical cost savings ranged from about 0 to 17%. For the default building and system parameters, the savings in compressor energy ranged from about 0 to 53% depending on the location and building type. However, the savings in total air conditioning electrical energy usage are much smaller than the compressor savings due to an increase in fan energy.

• Generally, the greatest percentage compressor and total cost savings occur in coastal climates with relatively mild ambient temperatures. However, the savings are also significant in hot inland climates having larger total loads.
  
• The savings are considerably smaller for the restaurant than for the other buildings. Compared to the other buildings, the restaurant has less thermal mass, a longer occupancy schedule, and greater ventilation requirements.
  
• Even greater savings would be possible if efficient, variable-speed fan motors were employed.
  
• For areas with high utility rates or demand charges, the energy savings would be higher.

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