Masonry chimneys must be built on a concrete footing, properly proportioned to support the load. Because a chimney may contain more than one flue, the minimum dimensions depend on the number of flues, and their arrangement and size. The wall thickness of a masonry chimney should not be less than 3 in. (75 mm) of solid masonry units.

The flue Is a vertical shaft through which smoke and gases are carried to open air. A single flue may serve one or more appliances located on the same floor, for example, a furnace and water heater. In this case, both connections to the flue should be located one above the other to ensure a good draft. As well, installing a certified chimney liner is recommended. The size of the flue, and the placement of multiple connections, depends on the capacities of the appliances connected to it. A fireplace must always have a separate flue.

The flue lining for masonry fireplaces usually consists of rectangular glazed clay pipe in sections, which are about 24 in. (600 mm) long and installed when the surrounding masonry is being placed. Care should be taken to set the linings close and flush on top of each other with full mortar beds. If more than one flue is used in a chimney, flues should be separated from each other by at least 3 in. (75 mm) of solid masonry or concrete, or 3 1/2 in. (90 mm) of fire brick where fire-brick liners are used (Fig. 114). The linings usually start about 8 in. (200 mm) below the flue pipe connection and extend 2 to 4 in. (50 to 100 mm) above the chimney cap.

The top of masonry chimneys should be capped to keep water away from masonry joints. Concrete is generally used for this purpose. The top of the cap should be sloped away from the flue lining and extend beyond the chimney wall at least 1 in. (25 mm) to form a drip edge.

Most factory-built metal chimneys are fabricated in sections and connected during installation. They are comparatively light in weight, and can be supported by special anchors that are attached to the floor joists when the chimney is erected. Two precautions to be observed in the use of a factory-built chimney are:

• make sure the model has been tested and approved by the Underwriters' Laboratories of Canada; and

• ensure that it is installed in strict accordance with the manufacturer's instructions and the conditions of approval set out by the Underwriters' Laboratories of Canada.

The chimney flue should be carried high enough above the roof to avoid downdrafts caused by wind turbulence. The height should never be less than 3 ft. (900 mm) above the

Installation of the flue linings.

3" (75 mm) minimum solid masonry between linings

highest point where the chimney intersects the roof and should extend at least 2 ft. (600 mm) above the ridge or any other roof surface within 10 ft. (3 m) of the chimney (Fig. 115).

A metal cleanout opening and door shall be provided near the bottom of the flue so that soot can easily be removed from the chimney.

Chimneys may be used to vent gas-burning equipment provided the lining complies with gas appliance installation codes. Alternatively, the equipment may be vented through special gas vents approved for this purpose.

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