IAQ Design Tools for Schools
The Energy Recovery Ventilation (ERV) System Financial Applicability Map was developed for the assumptions listed below.
- ERV system installed with a packaged HVAC system, serving a block of eight classrooms in an elementary school.
- 30 occupants per classroom (29 students and 1 teacher), yielding 240 total occupants.
- 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.
- ERV Systems
- ERV system rated flow of 3,600 cfm, sized to match the application.
- Sensible energy recovery effectiveness of 75% at rated flow.
- Latent energy recovery effectiveness of 75% at rated flow (for total recovery ERV systems).
- Added pressure losses due to ERV heat exchanger of 0.75-inches water column at rated flow.
- Equipped with a bypass feature to avoid excess energy recovery (enthalpy-controlled).
- Exhaust-only frost control specified for locations requiring frost control.
- HVAC System
- Constant-air-volume packaged system, factory-provided with integral ERV system.
- Cooling Seasonal Energy Efficiency Ratio (SEER) of 8.5.
- Heating Seasonal Efficiency of 75%.
- Cooling season indoor design conditions of 76oF and 50% Relative Humidity
- (with some exceptions, e.g., arid locations where lower indoor relative humidities were specified to avoid a latent-recovery cooling penalty for total-recovery ERV systems).
- For the locations where alternate cooling season design conditions were specified, see section and table below.
- Cooling season outdoor design conditions for avoided cooling load calculations based on design dry bulb temperature and mean coincident wet bulb temperature.
- Heating season indoor design set point of 71oF.
- 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.
- 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.
- 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 (%)|
|Cedar City UT||76||25|
|Colorado Springs CO||76||27|
|Las Vegas NV||76||28|
|Salt Lake City UT||76||30|
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.