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Depth of fireplace opening 20 The hearth 21 The jambs 21 Fireplace back and sides 21 Supporting irons 21 Improving fireplace heating 22

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FUNCTION OF CHIMNEYS.

HE prime function of a chimney is to produce a draft that will cause sufficient combustion and carry off the resulting smoke; incidentally it assists ventilation. Many unsatisfactory heating plants and much excessive fuel consumption are due to improperly constructed chimneys, which are the rule rather than the exception. Although many of these are more inefficient than dangerous, yet reports of the National Board of Fire Underwriters[1] show that a larger number of fires are caused by defective chimney construction than by anything else. The annual loss resulting from such fires is greater than the fire loss from any other cause. Poor chimney construction is responsible for smoke pollution of the air, waste of fuel, and poor heating.

The most common faults in chimney construction are:

1. The use of unsuitable materials. Clay sewer pipe, hollow building blocks, or unprotected concrete should not be used.

2. Improper laying of brick. Brick should not be laid on edge and should be properly bonded. Lining should be used in all brick chimneys the walls of which are less than 8 inches thick. Lack of mortar, especially in the perpendicular joints, ruins many an otherwise good chimney.

3. Failure to support the chimney properly. It should never be carried on any timber construction of the building, and when it rests upon the ground sufficient masonry foundation should be provided to prevent settling.

4. Building inflammable material into the chimney or against it without proper insulation.

5. Failure to anchor the smoke pipe properly to the chimney.

6. Neglect of the connection between smoke pipe and flue or of the flue itself. The connection should be tight; rusted pipe should be replaced; the chimney should be kept clean and the joints in the brickwork properly pointed.

7. Lack of a tight flue. A flue free from leakage is unusual. Every flue should be tight enough to prevent escape of smoke when tested as described on page 14. A leaky flue is the most frequent cause of heating troubles, high fuel bills, and destructive fires.

8. Failure to maintain the full sectional area at the bend when a flue is offset.

9. Use of the main heating apparatus flue for water heater or other auxiliary equipment. The furnace or heater should have a separate flue.

10. Failure to provide a separate tight cleanout for each flue. Two or more otherwise good flues may be rendered inefficient if led into one cleanout, since air may be drawn from one into another and the draft in all affected.

11. Presence of deep pockets leading to cleanouts. They may cause eddying currents that are detrimental. Pockets should be only deep enough to permit installing a cast-iron cleanout frame and door just below the smoke pipe entrance. Deep pockets allow soot accumulation that may take fire.

THE CHIMNEY DRAFT.

The draft depends entirely upon the chimney flue. The better the flue the more satisfactory and efficient will be the operation of the entire heating apparatus. The strength or intensity of the draft is dependent mainly upon the tightness, size, and height of the chimney flue. The most common error in chimney construction is failure to distinguish between the size of flue necessary for free passage of the volume of smoke from a given amount of fuel and that which with proper height will produce the required draft. A chimney may be high enough, yet have an area too small to carry properly the volume of smoke. On the other hand, the size may be sufficient but the chimney too low to produce a draft strong enough to pull the air through the fire at a sufficiently rapid rate. Either fault or a combination of the two will result in unsatisfactory service.

Draft in a chimney flue is caused by the difference in weight between a volume of air on the outside and an equal volume of products of combustion from the fire on the inside. The higher the temperature of a given weight of air, the greater is its total volume and the lighter the weight of its unit volume. This produces a condition of unbalanced pressures at the base of the flue. The rising of the lighter gases within the chimney tends to equalize the pressures. So long as the fire burns this condition of unbalanced pressure persists, the result being draft.

This is the basic principle which governs chimney action and upon which the draft depends. The greater the difference between the temperature in the flue and that outside the greater the tendency toward equalization of pressure and hence the better the draft. In summer the draft of a chimney is not as good as in winter because the difference in temperature between the outside air and that of the gases in the flue is less.

SHAPES AND SIZES OF FLUES.

The most efficient chimney is one built perfectly