Limit | Constant Volume | Variable Volume | |
Option 1: Fan system motor nameplate hp | Allowable nameplate motor hp | hp ≤ CFMS × 0.0011 | hp ≤ CFMS × 0.0015 |
Option 2: Fan system bhp | Allowable fan system bhp | bhp ≤ CFMS × 0.00094 + A | bhp ≤ CFMS× 0.0013 + A |
For SI: | 1 cfm = 0.471 L/s. 1 bhp = 735.5 W, 1 hp = 745.5 W. | |
Where: | ||
CFMS | = | The maximum design supply airflow rate to conditioned spaces served by the system in cubic feet per minute. |
hp | = | The maximum combined motor nameplate horsepower. |
bhp | = | The maximum combined fan brake horsepower. |
A | = | Sum of [PD × CFMD/4131] |
Where: | ||
PD | = | Each applicable pressure drop adjustment from Table C403.2.10.1(2) in. w.c. |
CFMD | = | The design airflow through each applicable device from Table C403.2.10.1(2) in cubic feet per minute. |
Device | Adjustment |
Credits | |
Fully ducted return and/or exhaust air systems | 0.5 inch w.c. (2.15 inches w.c. for laboratory and vivarium systems) |
Return and/or exhaust air flow control devices | 0.5 inch w.c. |
Exhaust filters, scrubbers, or other exhaust treatment | The pressure drop of device calculated at fan system design condition |
Particulate filtration credit: MERV 9 - 12 | 0.5 inch w.c. |
Particulate filtration credit: MERV 13 - 15 | 0.9 inch w.c. |
Particulate filtration credit: MERV 16 and greater and electronically enhanced filters | Pressure drop calculated at 2x clean filter pressure drop at fan system design condition |
Carbon and other gas-phase air cleaners | Clean filter pressure drop at fan system design condition |
Biosafety cabinet | Pressure drop of device at fan system design condition |
Energy recovery device, other than coil runaround loop | (2.2 × energy recovery effectiveness) – 0.5 inch w.c. for each airstream |
Coil runaround loop | 0.6 inch w.c. for each airstream |
Evaporative humidifier/cooler in series with another cooling coil | Pressure drop of device at fan system design conditions |
Sound attenuation section (fans serving spaces with design background noise goals below NC35) | 0.15 inch w.c. |
Exhaust system serving fume hoods | 0.35 inch w.c. |
Laboratory and vivarium exhaust systems in high-rise buildings | 0.25 inch w.c./100 feet of vertical duct exceeding 75 feet |
Deductions | |
Systems without central cooling device | -0.6 inch w.c |
Systems without central heating device | -0.3 inch w.c. |
Systems with central electric resistance heat | -0.2 inch w.c. |
For SI: 1 inch w.c.= 249 Pa, 1 inch = 25.4 mm. | |
w.c. = water column. |
Fan Location | Air Flow Rate Minimum (cfm) | Minimum Efficacy (cfm/watt) | Air Flow Rate Maximum (cfm) |
Exhaust fan: Bathroom, utility room, whole house | 10 | 1.4 cfm/watt | < 90 |
Exhaust fan: Bathroom, utility room, whole house | 90 | 2.8 cfm/watt | Any |
Cooling System Type | Fan Motor Size | Mechanical Cooling Capacity |
DX cooling | Any | ≥ 65,000 Btu/h |
Chilled water and evaporative cooling | ≥ 5 hp | Any |
≥ 1/4 hp | Any |