It is very important that the connection between a furnace and the chimney be properly and safely implemented.
Note: These plans and instructions are general in nature. Always check with your local building department for information on specific building codes in your jurisdiction.
The proper connection is performed by using a cylindrical clay section of pipe, or a clay collar as it normally termed, installed in the foundation of the chimney wall and serves as a connection between the furnace smoke pipe and the inside of the chimney foundation.
Once inside the chimney foundation structure the smoke pipe becomes a flue that continues up through the fireplace into the chimney and provides exhaust for furnace smoke and gases. The furnace flue is built in sections simultaneously with the masonry walls. The procedure is similar to that of a fireplace flue. It is important to keep the collar flush with the inside of the chimney foundation walls, as shown in Figure 1. A projecting collar could block the passage of smoke and gases into the flue and therefore to the outside, as shown in Figure 2.
Figure 1 - Correct installation of furance collar into chimney flue
Figure 2 - Incorrect installation of furance collar into chimney flue
The furnace flue too must be supported at the foundations. This is done by filling the cavity underneath it with concrete blocks.
Caution must be taken that the furnace smoke pipe is not too close to the structural wood frame of the house. Minimum clearances are usually specified by local building codes. If the code does not include minimum clearances, make sure that the smoke pipe is not less than 18″ and preferably 24″ away from the ceiling joists and 6″ away from any other woodwork. For additional protection install a metal shield between the woodwork and the smoke pipe as shown in Figure 1.