Offset Chimney

Chimney Elbows in an Offset Installation

If elbows are used in the fireplace system, they must not be used in combinations that will allow the chimney to be inclined more than 30 degrees from vertical. Best practice for optimum draft avoids use of elbows or limits the offset angle to 15 degrees. Each manufacturer's instructions specify the maximum number of elbows that may be used in the system as related to height. These instructions will also specify the maximum allowable length of an inclined portion of the system.

Elbows are designed to be used in pairs. The offset elbow, or bottom elbow, starts the inclined portion of the chimney. If the installation instructions so designate, this offset elbow can be installed either directly on top of the fireplace or farther up the chimney system. Some instructions may require a vertical section of chimney off the firebox before an offset elbow may be used.

The return elbow, or top elbow, returns, the inclined portion of the chimney back to a vertical position. Some manufacturers require the use of screws to join the elbows, but the inner flue wall should not be penetrated. In all cases, the return elbow will have support straps attached to it. These support straps must be nailed to a framing member of the structure to support the weight of the chimney above. When nailing the support straps to a framing member, the nails must be of the proper size and number as indicated by the instructions, and the support straps should be attached with all slack removed after they are in place, so that no ensuing sagging can take place. Make sure that all clearance requirements are maintained. Some manufacturers may offer offset firestops that allow an inclined portion of the chimney to pass through a floor or ceiling. Others may require that these penetrations be made by vertical chimney sections

When enclosing the elbows and the inclined portions of the chimney, enclosing materials should be installed vertically where possible to as to maintain the specified air space clearance to the chimney at the extremities of the offset. Try not to angle the enclosing materials to follow the inclined portion of the chimney. Doing so may result in the chimney sagging into the inclined enclosed section, or the enclosing materials may sag toward the chimney, thereby reducing necessary clearances.

Chimney stabilizers

Most fireplace manufacturers offer what is known as a chimney support stabilizer. This component is used in a vertical chimney at a specified height above the fireplace to support the weight of addition chimney. The support stabilizer may also be sued in the inclined portion of a chimney to assist to secure it in a fixed position. The stabilizer usually has attached support straps that must be securely fastened with no slack to structural framing members. The chimney sections can be quite heavy and, if not properly supported, separation of the joints may occur. As with every other part of the fireplace, follow the manufacturer's printed instructions to determine where stabilizers must be used, the minimum intervals necessary for their use, and how they should be installed.