The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.3. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.
Exception: Whole-house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.
Dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.
Exception: This section shall not apply to listed and labeled condensing (ductless) clothes dryers.
The maximum length of the exhaust duct shall be 35 feet (10 668 mm) from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.5.1. The maximum length of the exhaust duct does not include the transition duct.
TABLE M1502.4.5.1
DRYER EXHAUST DUCT FITTING EQUIVALENT LENGTH
| DRYER EXHAUST DUCT FITTING TYPE | EQUIVALENT LENGTH |
| 4 inch radius mitered 45 degree elbow | 2 feet 6 inches |
| 4 inch radius mitered 90 degree elbow | 5 feet |
| 6 inch radius smooth 45 degree elbow | 1 foot |
| 6 inch radius smooth 90 degree elbow | 1 foot 9 inches |
| 8 inch radius smooth 45 degree elbow | 1 foot |
| 8 inch radius smooth 90 degree elbow | 1 foot 7 inches |
| 10 inch radius smooth 45 degree elbow | 9 inches |
| 10 inch radius smooth 90 degree elbow | 1 foot 6 inches |
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 degree = 0.0175 rad.
Where space for a clothes dryer is provided, an exhaust duct system shall be installed. Where the clothes dryer is not installed at the time of occupancy the exhaust duct shall be capped or plugged in the space in which it originates and identified and marked “future use.”
Exception: Where a listed condensing clothes dryer is installed prior to occupancy of the structure.
Range hoods shall discharge to the outdoors through a duct. The duct serving the hood shall have a smooth interior surface, shall be air tight, shall be equipped with a back-draft damper and shall be independent of all other exhaust systems. Ducts serving range hoods shall not terminate in an attic or crawl space or areas inside the building.
Exception: Where installed in accordance with the manufacturer’s instructions, and where mechanical or natural ventilation is otherwise provided, listed and labeled ductless range hoods shall not be required to discharge to the outdoors.
Ducts serving range hoods shall be constructed of galvanized steel, stainless steel or copper.
Exception: Ducts for domestic kitchen cooking appliances equipped with down-draft exhaust systems shall be permitted to be constructed of schedule 40 PVC pipe and fittings provided that the installation complies with all of the following:
- The duct is installed under a concrete slab poured on grade.
- The underfloor trench in which the duct is installed is completely backfilled with sand or gravel.
- The PVC duct extends not more than 1 inch (25 mm) above the indoor concrete floor surface.
- The PVC duct extends not more than 8 inches above grade outside of the building.
- The PVC ducts are solvent cemented.
Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or naturally provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not less than one damper. Each damper shall be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. Dampers shall be accessible for inspection, service, repair and replacement without removing permanent construction or any other ducts not connected to the damper being inspected, serviced, repaired or replaced.
Exception: In a single-family dwelling, make-up air is not required for range hood exhaust systems capable of exhausting:
- Four hundred cubic feet per minute or less; or
- More than 400 cubic feet per minute but no more than 800 cubic feet per minute if there are no gravity vent appliances within the conditioned living space of the structure.
The length of exhaust and supply ducts used with ventilating equipment shall not exceed the lengths determined in accordance with Table M1506.2.
Exception: Duct length shall not be limited where the duct system complies with the manufacturer’s design criteria or where the flow rate of the installed ventilating equipment is verified by the installer or approved third party using a flow hood, flow grid or other airflow measuring device.
TABLE M1506.2
DUCT LENGTH
| DUCT TYPE | FLEX DUCT | SMOOTH-WALL DUCT | ||||||||||||||
| Fan airflow rating (CFM @ 0.25 inch wca) | 50 | 80 | 100 | 125 | 150 | 200 | 250 | 300 | 50 | 80 | 100 | 125 | 150 | 200 | 250 | 300 |
| Diameterb (inches) | Maximum lengthc, d, e (feet) | |||||||||||||||
| 3 | X | X | X | X | X | X | X | X | 5 | X | X | X | X | X | X | X |
| 4 | 56 | 4 | X | X | X | X | X | X | 114 | 31 | 10 | X | X | X | X | X |
| 5 | NL | 81 | 42 | 16 | 2 | X | X | X | NL | 152 | 91 | 51 | 28 | 4 | X | X |
| 6 | NL | NL | 158 | 91 | 55 | 18 | 1 | X | NL | NL | NL | 168 | 112 | 53 | 25 | 9 |
| 7 | NL | NL | NL | NL | 161 | 78 | 40 | 19 | NL | NL | NL | NL | NL | 148 | 88 | 54 |
| 8 and above | NL | NL | NL | NL | NL | 189 | 111 | 69 | NL | NL | NL | NL | NL | NL | 198 | 133 |
For SI: 1 foot = 304.8 mm.
- Fan airflow rating shall be in acordance with ANSI/AMCA 210-ANSI/ASHRAE 51.
- For noncircular ducts, calculate the diameter as four times the cross-sectional area divided by the perimeter.
- This table assumes that elbows are not used. Fifteen feet of allowable duct length shall be deducted for each elbow installed in the duct run.
- NL = no limit on duct length of this size.
- X = not allowed. Any length of duct of this size with assumed turns and fittings will exceed the rated pressure drop.
The wholehouse mechanical ventilation system shall provide outdoor air at a continuous rate of not less than that determined in accordance with Table M1507.3.3(1).
Exception: The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25-percent of each 4-hour segment and the ventilation rate prescribed in Table M1507.3.3(1) is multiplied by the factor determined in accordance with Table M1507.3.3(2).
TABLE M1507.3.3(1)
CONTINUOUS WHOLE-HOUSE MECHANICAL VENTILATION SYSTEM AIRFLOW RATE REQUIREMENTS
| DWELLING UNIT FLOOR AREA (square feet) | NUMBER OF BEDROOMS | ||||
| 0 – 1 | 2 – 3 | 4 – 5 | 6 – 7 | > 7 | |
| Airflow in CFM | |||||
| < 1,500 | 30 | 45 | 60 | 75 | 90 |
| 1,501 – 3,000 | 45 | 60 | 75 | 90 | 105 |
| 3,001 – 4,500 | 60 | 75 | 90 | 105 | 120 |
| 4,501 – 6,000 | 75 | 90 | 105 | 120 | 135 |
| 6,001 – 7,500 | 90 | 105 | 120 | 135 | 150 |
| > 7,500 | 105 | 120 | 135 | 150 | 165 |
For SI: 1 square foot = 0.0929 m2, 1 cubic foot per minute = 0.0004719 m3/s.
TABLE M1507.3.3(2)
INTERMITTENT WHOLE-HOUSE MECHANICAL VENTILATION RATE FACTORSa, b
| RUN-TIME PERCENTAGE IN EACH 4-HOUR SEGMENT | 25% | 33% | 50% | 66% | 75% | 100% |
| Factora | 4 | 3 | 2 | 1.5 | 1.3 | 1.0 |
- For ventilation system run time values between those given, the factors are permitted to be determined by interpolation.
- Extrapolation beyond the table is prohibited.
Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.4.
TABLE M1507.4
MINIMUM REQUIRED LOCAL EXHAUST RATES FOR ONE- AND TWO-FAMILY DWELLINGS
| AREA TO BE EXHAUSTED | EXHAUST RATES |
| Kitchens | 100 cfm intermittent or 25 cfm continuous |
| Bathrooms-Toilet Rooms | Mechanical exhaust capacity of 50 cfm intermittent or 20 cfm continuous |
For SI: 1 cubic foot per minute = 0.0004719 m3/s.