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Assumptions for ERV System Financial Applicability Map

Indoor Air Quality Design Tools for Schools

The Energy Recovery Ventilation (ERV) System Financial Applicability Map was developed for the assumptions listed below.

  1. ERV system installed with a packaged HVAC system, serving a block of eight classrooms in an elementary school.
     
  2. 30 occupants per classroom (29 students and 1 teacher), yielding 240 total occupants.
     
  3. Outdoor air ventilation rate of 15 cfm per occupant (in accordance with ASHRAE 62-1999), with a total outdoor air flow rate of 3,600 cfm. Outdoor air ventilation scheduled Monday through Friday from 6 A.M. to 5 P.M. during the school year. School year is from Sept. 1 through June 10, with a two-week winter break and a one-week spring break. Longer operating schedules will generally improve ERV system financial performance.
     
  4. ERV Systems
     
  5. HVAC System

  1. Costs
     
    • ERV added costs of $2 per cfm of ERV rated flow.
    • Cooling equipment avoided costs of $1,050 per ton.
    • Building exhaust fan avoided costs of $0.35 per cfm of outdoor ventilation air.
    • Electricity utility costs of $0.0726 per kWh.
    • Heating fuel utility costs of $0.72 per therm.
    • Note: Many locations may have different local utility rates, which can significantly affect ERV financial payback.
       
  2. School Building Shell
     
    • Single-story, ground-contact wing containing eight classrooms. Each classroom is 40-feet by 30-feet (floor area of 1,200 ft2), with one exterior wall.
    • Wall construction is 4-inch face brick and 8-inch lightweight concrete block, with 6-inch insulation and gypsum wall board.
    • Window construction is double-pane, clear, insulating with 6 mm air gap. Windows comprise 30% of the exterior wall area.
    • Roof construction is 4-inch insulation over 2-inch heavyweight concrete deck with suspended acoustic tile ceiling.
    • Floor construction is uncarpeted slab-on-grade, with insulation.
    • Internal lighting and receptacle loads of 2 Watts/ft2 and 0.3 Watts/ft2, respectively.
    • Infiltration is continuous at 0.1 air changes/hour.
       
  3. Marginal Payback Locations

    There is a pocket of Southwestern U.S. locations on the map that shows reasonable ERV financial performance. This area includes southern Arizona, southern Nevada, and part of southern California. For these cases the financial payback of ERV systems is largely dependent on the ERV system added costs, and the cost savings associated the reduction in design-day cooling load and downsizing of the cooling system. For the given set of assumptions, the net annual energy savings of ERV systems are relatively low for some of these Southwestern locations.

Locations where alternate cooling season design conditions were specified

Location Design Indoor
Temperature, oF

 Design Indoor
Relative Humidity, %
Amarillo TX 76 42
Bakersfield CA 76 42
Boulder CO 76 26
Burns OR 76 37
Cedar City UT 76 25
Cheyenne WY 76 25
Colorado Springs CO 76 27
Daggett CA 76 32
Fresno CA 76 45
Las Vegas NV 76 28
Lubbock TX 76 45
Medford OR 76 39
Midland TX 76 42
Phoenix AZ 76 39
Pueblo CO 76 30
Redmond OR 76 30
Reno NV 76 26
Sacramento CA 76 42
Salt Lake City UT 76 30
Scottsbluff NE 76 38
Tucson AZ 76 29
Alaskan Locations: During the summer/cooling season, most Alaskan locations had a design day
outdoor temperature less than the design indoor temperature. A design day avoided cooling load of
zero was assumed for ERV systems in these locations.

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