Section SPS 323.08 - Ductwork(1) DUCT USE. Ducts designed for the transmission of air shall be used for no other purpose.(2) INTERIOR DUCTS. All interior ducts shall be constructed in accordance with the following: (a)Supply and return air ducts. Supply and return air ducts shall comply with this paragraph except that ducts attached to appliances may be constructed of materials specified in the appliance listing.1. Kitchen exhaust ducts and ducts for air exceeding 250° F shall be constructed of sheet metal or lined with sheet metal or constructed of other noncombustible noncorrugated materials.2. Ducts connected to furnaces shall be constructed of sheet metal for at least 6 feet from the furnace.3. Spaces formed by unlined wood joists, studs or wood I-joists with solid webs may be used as return air ducts. Spaces used as return air ducts shall be cut off from all remaining unused portions of the space by tight-fitting stops of sheet metal or of wood joist material. Bridging shall be removed from the joist space.(b)Under-floor plenums. An under-floor space may be used as a plenum in a single dwelling unit in accordance with this section. 1. The use of the under-floor space shall be limited to buildings not more than 2 stories in height. Except for the floor immediately above the under-floor plenum, supply ducts shall be provided extending from the plenum to registers or other floor levels.2. The under-floor spaces shall not be used for storage, shall be cleaned of all loose scrap material and shall be tightly and substantially enclosed.3. The enclosing material of the under-floor space, including the side wall insulation and vapor barriers, shall not be more flammable than one-inch (nominal) wood boards (flame spread classification of 200).4. Access shall be through an opening in the floor which shall be 18 inches by 24 inches.5. The furnace supplying warm air to the under-floor space shall be equipped with an automatic control which will start the air circulating fan when the air in the furnace bonnet reaches a temperature not higher than 150° F. Such control shall be one that cannot be set higher than 150° F.6. The furnace supplying warm air to the under-floor space shall be equipped with an approved temperature limit control that will limit outlet air temperature to 200° F.7. A noncombustible receptacle shall be placed below each floor opening into the air chamber. The receptacle shall be securely suspended from the floor members and shall be not more than 18 inches below the floor opening. The area of the receptacle shall extend 3 inches beyond the opening on all sides. The perimeter of the receptacle shall have a vertical lip at least one inch high at the open sides if it is at the level of the bottom of the joist, or 3 inches high if the receptacle is suspended.8. Floor registers shall be designed for easy removal to permit access for cleaning the receptacles.9. Exterior walls and interior stud partitions shall be firestopped at the floor.10. Each wall register shall be connected to the air chamber by a register box or boot.11. A duct conforming to par. (a) shall extend from the furnace supply outlet at least 6 inches below combustible framing.12. The entire ground surface and enclosing exterior walls of the under-floor space shall be covered with a vapor barrier having a vapor permeability rating of one perm or less and a flame spread rating of 200 or less.13. Fuel gas lines may not be located within the under-floor space.14. A smoke detector shall be placed in the under-floor space. The alarm and low-battery signal of the smoke detector shall be audible in the occupied areas of the dwelling, when actuated.15. The exterior walls of the under-floor spaces shall be insulated in accordance with subch. IV of ch. SPS 322. The insulation may not be omitted under the provisions of s. SPS 322.21 or subch. VII of ch. SPS 322. Note: Subch. VII of ch. SPS 322 was repealed.
16. Electrical wiring installed in the plenum shall be in conformance with the Wisconsin Administrative Electrical Code Volume 2. (3) EXTERIOR DUCTS. (a)General. Except as provided in par. (b), ducts, which are located in garages, storage attics and similar spaces susceptible to physical damage, shall be constructed of galvanized steel or corrosion-resistive metal.(b)Exception. Plastic may be used for bath fan or air-to-air heat exchanger exhaust runs located in spaces outside the dwelling.(4) UNDERGROUND DUCTS. Ducts, plenums and fittings constructed of metal encased in concrete or ceramic, or other approved materials, may be installed in the ground. Encasement of underground supply air ducts shall be moistureproof. Note: See s. SPS 322.42 for insulation requirements for underground ducts.
(5) DUCT CONSTRUCTION. Ductwork shall be constructed and installed in accordance with any one of the appropriate following standards: (a) ASHRAE Handbook HVAC Systems and Equipment.(b) SMACNA, Residential Comfort System Installation Standards Manual(c) SMACNA, HVAC Duct Construction Standards-Metal and Flexible.(d) SMACNA Fibrous Glass Duct Construction Standards.(e) ASHRAE HVAC Applications Handbook.(f) NAIMA Fibrous Glass Duct Construction Standards.(6) THICKNESS. Sheet metal ducts shall conform to the minimum thicknesses listed in Table 323.08-A.(7) DUCT SUPPORT. Rigid metal ductwork shall be supported in accordance with Table 323.08-B. TABLE 323.08-A
DUCT CONSTRUCTION MINIMUM SHEET METAL GAUGES
Minimum thickness galvanized sheet gauge | Minimum thickness aluminum B & S gauge |
Metal gauges (duct not enclosed in partitions) |
Round ducts |
Diameter, inches |
Less than 12 | 30 | 26 |
12-14 | 28 | 26 |
15-18 | 26 | 24 |
Over 18 | 24 Rectangular Ducts | 22 |
Width, inches |
Less than 14 | 28 | 24 |
14-24 | 26 | 22 |
25-30 | 24 | 22 |
Over 30 | 22 | 20 |
Metal gau | ges (ducts enclosed in | partition) |
Width, inches |
14 or less | 30 | 26 |
Over 14 | 28 | 24 |
TABLE 323.08-B
DUCT SUPPORT FOR RIGID DUCTS - 16 GAGE MAXIMUM THICKNESS
Maximum | Hanger Type, Size and |
Duct Type | Size | Duct Position | Spacing1 |
Circular | 10" diam. | Vertical | Strap - one 18 ga. galv. steel X 2" @ 12 o.c. |
Horizontal | Strap - one 22 ga. galv. |
steel X 1" @ 12' o.c. Rod - one 1/4 @ 12' o.c. Wire - one 12 ga. @ 12' o.c. |
18" diam. | Vertical | Strap - one 16 ga. galv. steel X 2" @ 12 o.c. |
Horizontal | Strap - one 22 ga. galv. steel X 1" @ 12' o.c. Rod - one 1/4 @ 12' o.c. Wire - one 8 ga. @ 12' o.c; or Wire - two 12 ga. @ 12' o.c. |
Rectangular | 60" perim. | Vertical | Strap - one 18 ga. galv. steel X 2" @ 12 o.c. |
Horizontal | Strap - pair2 22 ga. galv. steel X 1" @ 10' o.c. Rods or Wires - pair2 10 ga. @ 10' o.c; or Rods or Wires - pair2 12 ga. @ 5' o.c. |
144" perim. | Vertical | Strap- one 16 ga. galv. steel X 2 ' @ 12' o.c. |
Horizontal | Strap - pair2 18 ga. galv. steel X 1" @ 10' o.c; or Strap - pair2 20 ga. galv. steel X 1" @ 8 o.c; or Strap - pair2 22 ga. galv. steel X 1" @ 5 o.c. Rods or Wires - pair2 3/8" @ 10' o.c; or Rods or Wires - pair2 1/4" @ 8' o.c. |
1 These hangers are the minimum required to support the weight of the duct off of the joist, stud or similar structure. The band, wire or strap cradling the duct shall not cause any deformation of the duct.
2 "Pair" means that there are 2 vertical legs. One continuous strap can form both vertical legs.
Note: This table does not prohibit nailing for duct support.
(8) JOINTS AND SEAMS. All joints and seams shall be securely fastened or locked. Round pipe slip joints shall be lapped at least one inch.(9) VIBRATION CONTROL. When used, vibration isolation connectors shall be installed at the joint between the duct and fan or heating equipment. Vibration isolation connectors shall not be used where the air temperature is in excess of 250° F.(10) AIR PASSAGEWAYS OF ENVELOPE DWELLINGS. The air passageways of envelope type dwellings shall comply with this subsection. (a) No heating equipment shall be placed in the air passageways.(b) Wood exposed to the air passageways shall be of at least 2 inches nominal thickness.(c) Finishes and insulation exposed to the air passageway shall have a flame spread rating of 25 or less and a smoke development rating of 50 or less.(d) A vapor barrier shall be installed on the warm side of insulation which forms a part of the thermal envelope of the dwelling. In the roof-ceiling air passageway, a vapor barrier for the insulation of the ceiling may be omitted if heated air is circulated on both sides of the ceiling insulation. The insulation on the roof side of the air passageway shall be provided with a vapor barrier on the warm side of the insulation. Any vapor barrier exposed to circulating air shall have a flame spread rating of 25 or less and a smoke development rating of 50 or less.Wis. Admin. Code Department of Safety and Professional Services SPS 323.08
Cr. Register, November, 1979, No. 287, eff. 6-1-80; cr. (2) (b) and (10), Register, January, 1989, No. 397, eff. 2-1-89; r. and recr. (2) (a), am. (3) and (6), Register, March, 1992, No. 435, eff. 4-1-92; am. (2) (a) 3., (2) (b) 15., (3) (a), and (4), Register, November, 1995, No. 479, eff. 12-1-95; r. and recr. (2) (b) 15. and (5), Register, January, 1999, No. 517, eff. 2-1-99; CR 08-043: r. and recr. (7) and Table 23.08-B Register March 2009 No. 639, eff. 4-1-09; correction in (2) (b) 15. made under s. 13.92(4) (b) 7, Stats., Register March 2009 No. 639; CR 09-104: am. (4) Register December 2010 No. 660, eff. 1-1-11; correction in (2) (b) 15., (6), (7) made under s. 13.92(4) (b) 7, Stats., Register December 2011 No. 672.