The required size of the warm-air pipe to any given room depends upon the heat loss from the room and the volume of warm air required to offset this loss. Each cubic foot of air warmed from zero to 140 degrees brings into a room 2.2 B. T. U. We have already seen that in zero weather with the air entering the registers at 140 degrees, only one-half of the heat contained in the air is available for offsetting the losses by radiation and conduction, so that only 1.1 B. T. U. in each cubic foot of entering air, can be utilized for warming purposes. Therefore if we divide the computed heat loss in B. T. U. from a room, by 1.1 it will give the number of cubic feet of air at 140 degrees necessary to warm the room in zero weather.

Warm Air Pipes 100037

Fig. 31.

As the outside temperature becomes colder the quantity of heat brought in per cubic foot of air increases, but the proportion available for warming purposes becomes less at nearly the same rate, so that for all practical purposes we may use the figure 1.1 for all usual conditions. In calculating the size of pipe required, we may assume maximum velocities of 280 and 400 feet per minute for rooms on the first and second floors respectively. Knowing the number of cubic feet of air per minute to be delivered, we can divide it by the velocity, which will give us the required area of the pipe in square feet.

Round pipes of tin or galvanized iron are used for this purpose. The following table will be found useful in determining the required diameters of pipe in inches.

TABLE VI.

DIA. OF PIPE IN INCHES.

AREA IN SQ. INCHES.

AREA IN SQUARE FEET.

6

28

.196

7

38

.267

8

50

.349

9

64

.442

10

79

.545

11

95

.660

12

113

.785

13

133

.922

14

154

1.07

15

177

1.23

16

201

1.40

Example. - The heat loss from a room on the second floor is 22,000 B. T. U., per hour. What diameter of warm air pipe will be required?

22,000/ 1.1 = 20,000 = cubic feet of air required per hour. 20,000/ 60 = 333 per minute. Assuming a velocity of 400 feet per minute we have 333 - 400 = .832 square feet, which is the area of pipe required. Referring to table VI. we find this comes between a 12 and 13-inch pipe and the larger size would probably be chosen.

Example. - A first floor room has a computed loss of 33,000 B. T. U. per hour when it is 10° below zero. The air for warming is to enter through two pipes of equal size, and at a temperature of 140 degrees. What will be the required diameter of the pipes?

There will be needed 33,000/ 1.1 = 30,000 cubic feet of air per hour, or 500 cubic feet per minute. At a velocity of 280 feet per minute, the pipe area must be 500/ 280 = 1.786. This is conveniently obtained by two 13-inch pipes.

Warm Air Pipes 100038

Fig. 32

Warm Air Pipes 100039

Fig. 33.

Since long horizontal runs of pipe increase the resistance and loss of heat, they should not in general be over 15 feet in length. This applies especially to pipes leading to rooms on the first floor or to those on the cold side of the house. Pipes of excessive length should be increased in size because of the added resistance.

Fig.. 32 and 33 show common methods of running the pipes in the basement. The first gives the best results and should be used where the basement is of sufficient height to allow it. A damper should be placed in each pipe near the furnace for regulating the flow of air to the different rooms or for shutting them off entirely when desired.

While round pipe risers give the best results, it is not always possible to provide a sufficient space for them, and flat or oval pipes are substituted. When vertical pipes must be placed in single partitions, much better results will be obtained if the studding can be made 5 or 6 inches deep instead of 4 as is usually done. Flues should never in any case be made less than 3 1/2 inches in depth. Each room should be heated by a separate pipe. In some cases however, it is allowable to run a single riser to heat two unimportant rooms on an upper floor. A clear space of at least 1/2 inch should be left between the risers and studs and the latter should be carefully tinned, and the space between them on both sides covered with tin, asbestos or wire lath.

The following table gives the capacity of oval pipes. A 6-inch pipe ovaled to 5 means that a 6-inch pipe has been flattened out to a thickness of 5 inches and column 2 gives the resulting area.

TABLE VII.

DIMENSION OF PIPE.

AREA IN SQUARE INCHES.

6 ovaled to 5

27

7 " " 4

31

7 " " 3 1/2

29

7 " " 6

38

8 " "5

43

9 " " 4

45

10 " " 3 1/2

46

9 " " 6

57

9 " " 5

51

11 " " 4

58

12 " " 3 1/2

55

10 " "6

67

11 " "5

67

14 " "4

76

15 " "31

73

12 " " 6

85

12 " "5

75

19 " " 4

96

20 " ." 3 1/2

100

Having determined the size of round pipe required, an equivalent oval pipe can be selected from the table to suit the space available.

Registers

The registers which control the supply of warm air to the rooms, generally have a net area equal to two-thirds of their gross area. The net area should be from 10 to 20 per cent greater than the area of the pipe connected with it. It is common practice to use registers having the short dimension equal to, and the long dimension about one-half greater than the diameter of the pipe. This would give the following standard sizes for different diameters of pipe.

TABLE VIII.

DIAMETER OF PIPE.

SIZE OF REGISTER.

6

6 X 10

7

7 X 10

8

8 X 12

9

9 X 14

10

10 x 15

11

11 X 16

12

12 X 17

13

14 X 20

14

14 X 22

15

15 X 22

16

16 X 24

Combination Systems

A combination system for heating by hot air and hot water consists of an ordinary furnace with some form of surface for heating water, placed either in contact with the fire or suspended above it. Fig. 34 shows a common arrangement where part of the heating surface forms a portion of the lining to the fire pot and the remainder is above the fire.

Care must be taken to properly proportion the work to be done by the air and the water, else one will operate at the expense of the other. One square foot of heating surface in contact with the fire is capable of supplying from 40 to 50 square feet of radiating surface, and one square foot suspended over the fire will supply from 15 to 25 square feet of radiation.

Care And Management

The following general rules apply to the management of all hard coal furnaces.

The fire should be thoroughly shaken once or twice daily in cold weather. It is well to keep the fire pot heaping full at all times. In this way a more even temperature may be maintained, less attention required and no more coal burned than when the pot is only partly filled. In mild weather the mistake is frequently made of carrying a thin fire, which requires frequent attention and is likely to die out. Instead, to diminish the temperature in the house, keep the fire pot full and allow ashes to accumulate on the grate (not under it) by shaking less frequently or less vigorously. The ashes will hold the heat and render it an easy matter to maintain and control the fire. When feeding coal on a low fire, open the drafts and neither rake nor shake the fire till the fresh coal becomes ignited. The air supply to the fire is of the greatest importance. An insufficient amount results in incomplete combustion and a great loss of heat. To secure proper combustion the fire should be controlled principally by means of the ash pit, through the ash pit door or slide.