DODGING [CHIMNEY] DRAFT PROBLEMS

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[2] Chimney too big: Chimneys should have no more than twice the cross-sectional area of the flue outlet on the appliance. Thus a stove with a six-inch-diameter collar shouldn't have more than an eight-inch-diameter chimney. Excess chimney area results in cooling of the flue gases and reduction of the draft, particularly in uninsulated exterior masonry chimneys. See SOLUTIONS F and H.

[3] Chimney too short: Draft is proportional to the square root of the height of the chimney, assuming that other factors don't interfere. Thus, doubling chimney height increases draft 41%. In practice, chimney height is more important in limiting the effects of wind patterns produced by the building and nearby obstacles than it is in producing draft by stack effect. The National Fire Protection Association's (NFPA) guidelines for chimney height (which are indicated in Fig. 1) should be considered minimums. See SOLUTION E.

[4] Chimney blocked: The primary causes of chimney blockage are creosote (in the chimney itself or in the spark screen of the cap), dampers that have come adrift, birds' nests, and fallen parts of masonry chimneys. See SOLUTIONS A, F, and H.

[5] Chimney has leaks: Any leak in the lower half of a chimney will reduce overall draft. In practice, joints in stovepipe connectors are not major sources of loss of draft. Look for other solid-fuel appliances connected to the same chimney, open cleanout doors, unplugged connectors for other stoves, or serious deterioration. See SOLUTIONS B, F, and H.

[6] House prevents adequate airflow up chimney: If your house is very small and well sealed, it may be too tight to allow adequate combustion airflow. (Most airtight stoves need between 10 and 50 cubic feet per minute.) More often, though, the lack of adequate airflow is caused either by other appliances (or fans) exhausting air from the building, or by stack effect: If your house has more openings for air leakage near its top than its bottom (this isn't usually the case but is more likely in multistory dwellings), and if the chimney has marginal draft to begin with because of excessive cooling (see PROBLEM 7), the natural flow may actually be downward in the chimney. See SOLUTIONS C and D.

[7] Flue gas temperature too low: Up to certain limits (which you're not likely to exceed), chimney draft increases as the difference in temperature between the gases in the chimney and the outside air increases. However, almost half-of the chimney's capacity is available with only a 35°F temperature difference, so you're not likely to encounter problems unless you have an exterior masonry chimney (which loses a lot of heat and cools the gases) and operate at low heat outputs. The new breed of highly efficient woodburners (see MOTHER N0. 95) is more likely to suffer from this problem, because it extracts so much heat from the gases before they enter the chimney. See SOLUTIONS F and H.

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