A chimney should extend at least 3 feet above flat roofs and 2 feet
above the ridge of peak roofs. Where chimneys cannot be built
high enough above the ridge to prevent trouble from eddies caused by
wind being deflected from the roof, a hood may be provided with the
open ends parallel to the ridge. Eddies which force air down the
flues may be caused by building the chimney too near trees (fig. 5, _B_)
or a higher structure (fig. 6).
[1] Farmers' Bulletin 1698, Heating the Farm Home, contains information
on estimating the size of the heating plant needed for houses of
different sizes and for determining grate areas.
[Illustration: Figure 2.--Heavy masonry chimneys of this type are still
being built in rural areas. The thick walls, with unlined flues, are in
good condition after 75 years of continuous use.]
Table 1.--_Sizes of flue linings and heights of chimneys recommended for
flat-grate furnaces burning soft coal_[A]
Nominal size of flue lining
--------------------------------------------------- Height of
Grate Round (inside chimney top
area diameter) Rectangular (outside above grate
(Sq. at elevation dimensions) at elevation at elevation
ft.) indicated indicated indicated
- ---------------- -------------------------------------- --------------
SL 2K 4K 6K Sea Level 2,000 ft 4,000 ft 6,000 ft SL 2K 4K 6K
- ---------------- -------------------------------------- --------------
In In In In In In In In Ft Ft Ft Ft
1 8 8 8 10 8½ by 8½ 8½ by 8½ 8½ by 8½ 8½ by 13 22 26 32 36
2 10 10 10 10 8½ by 13 8½ by 13 8½ by 13 8½ by 13 24 29 35 41
3 10 10 12 12 8½ by 13 8½ by 13 13 by 13 13 by 13 26 33 41 49
4 12 12 12 12 13 by 13 13 by 13 13 by 13 13 by 13 30 37 45 49
5 12 12 15 15 13 by 13 13 by 13 13 by 18 18 by 18 32 37 43 52
6 15 18 18 18 18 by 18 18 by 18 20 by 20 20 by 20 30 37 47 56
7 18 18 18 18 20 by 20 20 by 20 20 by 20 20 by 20 32 41 49 64
8 18 18 18 18 20 by 20 20 by 20 20 by 20 20 by 20 35 42 56 10
[A] If anthracite is to be burned the area of the flue cross section may
be reduced about 25 percent.
The ratings given are based on comparatively smooth lined flues with no
offsets greater than 30° with the vertical.
The smallest sizes of fuels require excessive drafts and may necessitate
taller chimneys.
Flue heights and sizes are based upon approximately the several altitudes
indicated; it is sufficiently accurate to use the column giving the
altitude nearest to that of the particular problem.
When 2 or 3 appliances are connected to the same flue their total grate
area may be reduced 15 percent. The method of determining the proper flue
size for an altitude of 2,000 feet, when 1 appliance with a grate area of
3 square feet and another with an area of 1.5 square feet are attached to
the same flue, is shown by the following example:
Add the 2 grate areas, 3 + 1.5 = 4.5 square feet. Reduce this total
area by 15 percent. Thus, 4.5 - 0.68 = 3.8 square feet is the required
area. Use the nearest whole number, 4. From the table it is seen that
for a grate area of 4 square feet at an elevation of 2,000 feet either
a 12-inch (inside diameter) round flue or a 13- by 13-inch (outside
dimensions) rectangular flue 37 feet high is required.
[Illustration: Figure 3.--This tall chimney produced good draft for
the kitchen range, but the flue was too small for a furnace. When the
house was remodeled, its appearance was greatly improved by building the
chimney inside.]
[Illustration: Figure 4.--Short chimneys are frequently provided for low
bungalows, for architectural reasons. This flue is ample in size but not
high enough for use with a stove. The stone masonry has been laid to
harmonize with the rustic surroundings.]
Frequently metal-pipe extensions are provided to increase the height of a
flue on account of the low cost and ease of installation, but these must
be securely anchored against wind and have the same area as the flue.
Metal extensions are likely to rust in a short time. They are available
with a metal cowl or top that turns with the wind to prevent air blowing
down the flue. Terra-cotta chimney pots or extensions are more durable
and attractive.
A chimney located entirely inside a building has better draft because the
masonry retains heat longer when protected from cold outside air.
[Illustration: Figure 5.--Two pleasingly designed fireplace chimneys that
fit into their surroundings. _A_, This chimney stands in the clear and
should provide a good draft; _B_, a chimney under overhanging trees is
likely to backdraft. Contrast the appearance of these two chimneys with
that of figure 3.]
[Illustration: Figure 6.--Several extensions were necessary before this
chimney would draw properly on account of the wind deflected from the
nearby wall.]
[Illustration: Figure 7.--_A_, A good foundation extending below the soil
affected by frost. This chimney is well protected from ground moisture
by the concrete carried above the surface; _B_, an insecure foundation.
Supporting a chimney in this manner is a dangerous practice.]