by Jerry Peck - Codeman on Mon Feb 16, 2009 1:05 am
Hi James,
I will answer based on the IRC not knowing where you are located, and presuming this is a dwelling which qualifies for being constructed under the IRC and not the IBC (although under the IBC the IMC and IFGC would apply and they are basically, if not exactly, the same as the IRC in this matter).
The first problem I see is actually accomplishing what you are describing.
I will start off calculating the size the correct way with two ducts in one stud space with separate openings for each duct, then end with calculating the wrong way with one duct in one stud space with two openings for that one stud space duct
First, with the minimum opening (and hence duct size) of 100 square inches and 3 inches minimum dimension, the opening's and duct's other dimension would be 100 square inches / 3 inches = 33-1/3 inches, or an opening and duct of 3 inches by 33-1/3 inches. That would not fit withing 16 inch on center studs or 24 inch on center studs.
To accommodate a 24 inch on center spacing, which leaves 21-1/2 inches between the studs, then allowing 1/2 inch for being able to fit the duct in the stud space, that gives a dimension of 21 inches, making the size 100 square inches / 21 inches = 4.76 inches.
Fitting two duct in a single stud space would require the use of 4.76 inches + 4.76 inches = 9.52 inches, then allowing 1/2 inch for being able to fit the ducts in the stud space, the studs would need to be 10 inches minimum, or 2 x 12 studs for 11-1/4 inches.
If you used 2 x 12 studs, then, yes, the two ducts could be configured to fit into the same stud space and would be allowed.
Even one combustion air duct in one stud space would require the use of 2 x 6 studs (5-1/4 inches) spaced 24 inches on center.
From the 2006 IRC. (underlining and bold are mine)
-> M1703.2 Two openings or ducts. Outside combustion air shall be supplied through openings or ducts, as illustrated in Figures M1703.2(1), M1703.2(2), M1703.2(3) and M1703.2(4). One opening shall be within 12 inches (305 mm) of the top of the enclosure, and one within 12 inches (305 mm) of the bottom of the enclosure. Openings are permitted to connect to spaces directly communicating with the outdoors, such as ventilated crawl spaces or ventilated attic spaces. The same duct or opening shall not serve both combustion air openings. The duct serving the upper opening shall be level or extend upward from the appliance space.
- -> 1703.2.1 Size of openings. Where directly communicating with the outdoors, or where communicating with the outdoors by means of vertical ducts, each opening shall have a free area of at least 1 square inch per 4,000 Btu/per hour (550 mm2/kW) of total input rating of all appliances in the space. Where horizontal ducts are used, each opening shall have a free area of at least 1 square inch per 2,000 Btu/per hour (1100 mm2/kW) of total input of all appliances in the space. Ducts shall be of the same minimum cross-sectional area as the required free area of the openings to which they connect. The minimum cross-sectional dimension of rectangular air ducts shall be 3 inches (76 mm).
The above is presuming you are intending to use the required separate ducts.
If you were instead planning on simply using one stud space (still 2 x 6 studs (5-1/4 inches) spaced 24 inches on center to meet minimum dimension for one duct, and then wanting to put two openings in that single stud space acting as one duct, with one opening high and one opening low, the answer is no, that is not allowed.
One reason this is not allowed is you are only providing, at most, one-half of the required combustion air.
Another reason this is not allowed is you are required to get the combustion air for each from outdoors, and in this case the combustion air would be combined, thus you would be getting the combustion air from inside the same confined space.
Codeman
Jerry Peck - CodeMan
AskCodeMan.com
Construction Litigation Consultant - Retired
Construction and Code Consultant - Semi Retired