The cold-air ducts supplying heaters should be planned in a similar manner to that described for furnace heating. The air inlet should be on the north or west side of the building, but this of course is not always possible. The method of having a large trunk line or duct with inlets on two or more sides of the building should be carried out when possible. A cold-air room with large inlet windows, and ducts connecting with the heaters make a good arrangement for schoolhouse work. The inlet windows in this case should be provided with check valves to prevent any outward flow of air. A detail of this arrangement is shown in Fig. 13.

This consists of a boxing around the window, extending from the floor to the ceiling. The front is sloped as shown and is closed from the ceiling to a point below the bottom of the window. The remainder is open and covered with a wire netting of about 1/2 inch mesh; to this are fastened flaps or checks of gossamer cloth about 6 inches in width. These are hemmed on both edges and a stout wire is run through the upper hem which is fastened to the netting by means of small copper or soft iron wire. The checks allow the air to flow inward but close when there is any tendency for the current to reverse.

The area of the cold-air duct for any heater should be about three-fourths the total area of the warm air ducts leading from it.

A common rule for dwelling houses and similar work is to allow 1 1/2 square inches of area for each square foot of radiating surface. The inlet windows should be provided with some form of damper or slide, outside of which should be placed a wire grating, backed by a netting of about 3/8 inch mesh.