NFPA 211 Combustible Wall Pass Thru Explained
January 15, 2024
Masonry chimneys are not always the most attractive thing to have inside a home in a living space. The easiest way to cover the unsightly bricks or blocks is to frame a wall with studs and drywall in front of the chimney. Framing a wall in front of your chimney can make it difficult if you would still like to use the chimney for a wood burning stove. Single and double wall black stove pipe is not designed to pass through a combustible wall. The pipe would get too hot and could cause the framed wall to combust.
Installing a U.L listed insulated chimney thimble is one option. However most of those are made for walls that are less than 14" from the face of the wall to the masonry chimney. If the wall thickness is larger than that, the NFPA 211 has laid out approved wall pass through systems that are safe to pass a single wall pipe through a combustible wall into a masonry chimney. Below are the 4 different methods for wall pass throughs explained in greater detail.
System A
Wall Pass Thru System A requires a minimum of 3.5" thick brick masonry wall that is framed into a combustible wall with a clay thimble. There needs to be at least a 12" clearance from the inside of the clay thimble to combustible material on all sides. The clay liners and thimbles need to have a minimum thickness of 5/8". A 2" clearance is required from any combustible material to the side of the brick chimney.
The clay thimble may extend into the living space. However, it may not extend beyond the inside wall of the vertical clay chimney.
Example: If you have a thimble that is 6" interior diameter, you will need to have a brick frame that measures. 30" x 30".
System B
Wall Pass Thru System B requires a solid pack U.L listed class A chimney section that is the same diameter as the chimney connector to be cemented into the brick masonry chimney with non water soluble refractory cement. The chimney section requires no less than 1" thick insulation. There must be at least a 9" clearance from the inner wall of the insulated chimney section to any combustible material.
The chimney pipe section must be flush with the inside wall of the vertical chimney flue not extending into the chimney flue beyond the inside wall. You must use the chimney pipe manufacturer's chimney adapter for connecting the connector pipe to the chimney section. Steel supports must be fastened to the wall and the outer wall of the insulated chimney section. The fasteners must not penetrate the inner wall of the chimney pipe length. A 2" clearance is required for any framing to the side of the brick chimney.
Example: If a 6" inner wall class A chimney pipe is being used, the framing dimensions for the pass thru must be 24" x 24".
System C
Wall Pass Thru System C requires a minimum 24 gauge thick chimney connector pipe that is cemented into the chimney thimble with non water soluble refractory cement. Surround the single wall connector pipe are two 1" sheet metal ventilated air channels. The ventilated air channels need to be separated with 1" non combustible spacers. 6" thick ceramic glass insulation will surround the two ventilated air channels shielding it from combustible material.
The insulation can be covered and the thimble supported with sheet steel. The supports are then to be fastened to the wall on all sides and should be sized to fit and hold the chimney section. The ventilated air channels must not be covered. Hardware used to secure the connector pipe must not penetrate the connector pipe.
Example: If a 6" connector pipe is used, the framing dimensions for the pass through will need to be 22" x 22".
System D
Wall Pass Thru System D requires a solid insulated U.L listed Factory Built chimney section that has an inside diameter that is 2" greater than the diameter of the connector pipe so that there is a 1" air gap between the inner wall of the factory built chimney section and the connector pipe. The factory built chimney section must have a minimum of 1" thick insulation. The outer wall of the factory built chimney section must maintain a 2" clearance from any combustible material.
The chimney connector pipe that is passing through the factory built chimney section must be a minimum of 24 gauge thick. Steel supports on both ends of the chimney section must be a minimum of 24 gauge thick. The supports should be secured with fasteners to the wall surface on all sides and fit to hold the factory built chimney section. The hardware used must not penetrate the connector pipe.
Example: If a 6" diameter connector pipe is used, the framing dimensions for the pass thru will need to be 14" x 14".
Note: The clearance stated in each system is the minimum clearance from combustible Material. It is perfectly safe if the clearances are greater.
Insulation material used as part of the wall pass through shall be of non combustible material and shall have a thermal conductivity of 1.0 BTU-in./hr-ft squared or less.
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