233. The furnace should be supplied with fresh air by means of a duct, which has an inlet at some point above ground upon the windy side of the house. This inlet, or cold-air box, as it is often called, must always be upon that side of the house where the air pressure is highest-if it opens to the leeward side, the warm air in the furnace is very liable to be driven down through the cold-air flue and discharge out of doors. .

If strong winds are likely to blow from various directions, then the cold-air duct should have inlets at the several exposed sides of the building. Each inlet must be provided with a damper, or other closing device, so that all may be shut off except the one which faces the wind.

Cold-air inlet openings should always be made high enough above the ground to prevent surface water from entering them, and they should be covered with wire netting to keep out animals, etc.

The common method of constructing a cold-air duct in the ground, which consists in building two small walls of brick along the sides of a trench having an earth bottom, and covering the trench with flagstones or planking, is a very bad one, and should not be permitted, unless the bottom of the trench is made water-tight. The worst arrangement, however, is a similar duct having wooden sides and top.

The wood slowly rots and the ground air freely enters the duct.

234. Underground ducts should always be strictly watertight A very good method of constructing such a duct is to use a terra-cotta pipe of sufficient size, cementing each joint as carefully as for a sewage drain. Fig. 85 shows a cold-air duct made in this way, with openings at opposite sides of the building. Two methods are shown of arranging the inlets and dampers. At a, the shutter, or damper, is hinged at one edge to the window frame; and at b, an ordinary butterfly damper is employed. These dampers may be operated from the rooms above, if so desired, by the use of chains and pulleys.

Fig. 85.

The area of a cold-air duct should equal 2/3 or 3/4 of the aggregate areas of all the hot-air pipes.

235. Inside Cold-Air Ducts

Inside Cold-Air Ducts. In heating large rooms, the cold-air supply is sometimes taken through a register in the floor, as shown at b in Fig. 86. The air then circulates through the furnace and through the room continuously, without the introduction of any fresh air.

This method is useful only for heating the building, when ventilation is not required. It is used chiefly for warming a large space occupied by very few people. Or it may be used for heating a large auditorium before the audience arrives, but as soon as ventilation is required a damper in the inside supply pipe is closed and that in the outside supply pipe a is opened.

Fig. 86.

When this arrangement is used in private residence work, the inside supply register should be located in the front hall and near the front door, if possible.