Consider a building in Denver with a 90,000-cfm single-duct CV AHU. The building is occupied 60 hours per week with 600 people. The indoor temperature is always maintained at 72°F. Fuel cost is $4.50/MMBtu. Electricity cost is $0.10/kWh. The fan pressure differential inside the AHU is 5.0 in. The fan is operating 80 hours per week.

a. A temperature measurement indicated that when the outdoor air is at 50°F, the mixed air is at 70°F, and the return air is at 75°F. Determine the heating fuel savings by reducing the outdoor air to the minimum ventilation requirements (i.e., 20 cfm/person). Outdoor air is provided only during the occupied period.

b. If the system is converted into a VAV system, determine the energy and cost savings due to the fan. The average load on the fan is 60 percent of the peak.

For Denver, the average outdoor dry-bulb (DB) temperatures are 35.2°F for winter and 77.9°F for summer. The length of the heating season is 29.4 weeks, whereas the length of cooling season is 22.6 weeks.