Section 675IAC15-2-87. Circulating air system

   (a) Supply systems shall be as follows:

(1) Supply ducts and any dampers contained therein shall be made from galvanized steel, tinplated steel, or aluminum, or shall be listed Class 0, Class 1, Class 2 air ducts. Class 2 air ducts shall be located at least three feet (3ʹ) from the furnace bonnet or plenum. A duct system integral with the structure shall be of durable construction that can be demonstrated to be equally resistant to fire and deterioration. Ducts constructed from sheet metal shall be in accordance with the following Table No. 4B.

TABLE NO. 4B
Minimum Metal Thickness for Ducts*
	Diameter or Width
Duct Type	14 inches or less	over 14 inches
Round	0.013 in.	0.016 in.
Enclosed Rectangular	0.013 in.	0.016 in.
Exposed Rectangular	0.016 in.	0.019 in.
*When "nominal" thickness is specified, 0.003 inch shall be added to these "minimum" metal thicknesses.

(2) Sizing of Ducts. Ducts shall be designed so that when a labeled forced-air furnace is installed and operated continually at its normal input rating in the mobile structure, with all registers in full open position, the static pressure measured in the duct plenum shall not exceed that shown in Table No. 4B or exceed that shown on the label of the appliance. When an air-cooler coil is installed between the furnace and the duct plenum, the total static pressure between the furnace and the coil shall not exceed that shown on the label of the furnace. The minimum dimension of any branch duct shall be at least one and one-half inches (1 1/2"), and of any main duct, two and one-half inches (2 1/2").

(3) Airtightness of Supply Duct Systems. A supply duct system shall be considered substantially airtight when the static pressure in the duct system, with all registers sealed and with the furnace air circulator at high speed, is at least eighty percent (80%) of the static pressure measured in the furnace casing, with its outlets sealed and the furnace air circulator operating at high speed. For the purpose of this subdivision and (b), pressures shall be measured with a water manometer or equivalent device calibrated to read in increments not greater than one-tenth inch (1/10") water column.

TABLE NO. 5B
Maximum Allowable Static Pressures in Supply Duct Systems
	External Static Pressure Inches Water Column Measured at the Furnace Outlet
Input to Forced-Air Furnace, Btu/hr	Temperature of Outlet Air Determined by Function of Limit Control
	Above 165° F	165° F or Less
55,000 and under	0.10	0.20
Over 55,000 to 80,000	0.10	0.24
Over 80,000 to 100,000	0.15	0.30

(4) Expandable or Multiple Mobile Structure Connections. An expandable or multiple structure may have ducts of the heating system installed in the various units. The points of connection must be so designed and constructed that when the mobile structure is fully expandable or coupled, the resulting duct joint will conform to the requirements of this section.

(5) Air supply ducts that are not located within the heated side of the mobile structure insulation having an R factor of at least four (4) shall be insulated.

(6) Supply ducts within the mobile structure but not within insulation described in (a)(5) shall be insulated with rigid insulation having a thermal insulation (R) not less than 2.5 or flexible insulation having a thermal insulation (R) not less than three (3) with a continuous vapor barrier having a perm rating of not more than 1.0.

(7) Supply ducts exposed directly to outside air, such as under chassis crossover ducts, shall be insulated with material having a thermal insulation (R) of not less than 4.0 with continuous vapor barrier having a perm rating of not more than 1.0.

(8) Aluminum foil used as a vapor barrier shall be at least two (2) mils in thickness.

  (b) Return air systems shall be as follows:

(1) Return Air Openings. Provisions shall be made to permit the return of circulating air from all rooms and living spaces, except toilet room(s) and kitchen, to the circulating air supply inlet of the furnace.

(2) Duct Material. Return ducts and any diverting dampers contained therein shall be in accordance with the following:

(A) Portions of return ducts directly above the heating surfaces, or closer than two (2) feet from the outer jacket or casing of the furnace shall be constructed of metal in accordance with Table No. 4B of this section or shall be listed Class 0 or Class 1 air ducts.

(B) Return ducts, except as required by (a) above, shall be constructed of one inch (1") (nominal) wood boards (flame spread classification of not more than 200), other suitable material no more flammable than one inch (1") board or in accordance with (a).

(C) The interior of combustible ducts shall be lined with noncombustible material at points where there might be danger from incandescent particles dropped through the register or furnace such as directly under floor registers and the bottom of vertical ducts or directly under furances having a bottom return.

(3) Sizing. The cross-sectional areas of the return air duct shall not be less than two (2) square inches for each one thousand (1,000) Btu/hour input rating of the appliance. Dampers shall not be placed in any return air duct, except that a diverting damper may be placed in a combination fresh air intake and return air duct so arranged that the required cross-sectional area will not be reduced at all possible positions of the damper.

(4) Permanent Unclosable Opening. Areas not served by return air ducts or closed off from the return opening of the furnace by doors, sliding partitions or other means shall be provided with permanent unclosable openings in the doors or separating partitions to allow circulated air to return to the furnace. Such openings may be grilled or louvered. The net free area of each opening shall be not less than one (1) square inch for every five (5) square feet of total living area closed off from the furnace by the door or partition serviced by that opening. Undercutting doors connecting the closed-off space may be used as a means of providing return air area. However, in the event that doors are undercut, they shall be undercut a minimum of two inches (2") and not more than two and one-half inches (2 1/2"), and no more than one-half [sic.] (1/2") of the free air area so provided shall be counted as return air area.

  (c) Joints and Seams. Joints and seams of ducts shall be securely fastened and made substantially airtight. Slip joints shall have a lap of at least one inch (1") and shall be individually fastened. Tape or caulking compound may be used for sealing mechanically secure joints. Where used, tape or caulking compound shall not be subject to deterioration under long exposures to temperatures up to 200° F. and to conditions of high humidity, excessive moisture, or mildew.

  (d) Supports. Ducts shall be securely supported.

  (e) Registers or Grills. Fittings connecting the registers or grills to the ducts system shall be constructed of metal or material which complies with the requirements of Class 1 or 2 ducts as stated in the Indiana Mechanical Code (675 IAC 18-1 [675 IAC 18-1 was repealed filed Oct 28, 1988, 3:00 p.m.: 12 IR 588, eff Jan 2, 1989. See 675 IAC 18-1.3.]). Registers or grills shall be constructed of metal or conform with the Indiana Mechanical Code (675 IAC 18-1 [675 IAC 18-1 was repealed filed Oct 28, 1988, 3:00 p.m.: 12 IR 588, eff Jan 2, 1989. See 675 IAC 18-1.3.]).

  Floor registers or grills shall resist without structural failure a two hundred (200) pound concentrated load on a two inch (2") diameter disc applied to the most critical area of the exposed face of the register or grill. For this test the register or grill is to be at a temperature of not less than 165° F. and is to be supported in accordance with the manufacturer's instructions. (Fire Prevention and Building Safety Commission; 675 IAC 15-2-87; filed Mar 25, 1986, 1:44 pm: 9 IR 2012, eff Jun 15, 1986; errata, 9 IR 2931; readopted filed Nov 29, 2001, 12:21 p.m.: 25 IR 1306; readopted filed Sep 21, 2007, 9:20 a.m.: 20071010-IR-675070388RFA; readopted filed Oct 10, 2007, 9:16 a.m: 20071031-IR-675070388RFA; readopted filed Aug 4, 2011, 8:35 a.m.: 20110831-IR-675110254RFA)