236. All hot-air pipes which convey the warm air from the bonnet of the furnace to the several rooms, should be run as straight as possible, and must all pitch up toward their outlets, which are protected by registers or gratings.

The choice of material for hot-air flues should be governed by considerations of durability and cost. They are commonly made of I C or I X bright tin.

All hot-air pipes should be covered with non-conducting material, in order to preserve the temperature of the hot air as long as possible, and thus secure the greatest possible draft. The vertical pipes which are built into the walls and partitions are usually called stacks. These are made flat or oval, to suit the spaces through which they must pass. In wooden buildings, a clear space of not less than 1/2 inch should be provided all around the pipe. This space should be packed with proper non-conducting material, or else the pipe should be wrapped with at least two layers of asbestos paper and bound with wire. Stacks are sometimes made with double walls enclosing an air space between them; the air space is intended to prevent the escape of heat, and affords good protection against fire.

All hot-air pipes should be given an upward grade of not less than 1 inch per foot if practicable, and more if convenient. This grade is usually sufficient to overcome the friction of the air in the pipe and secure a reasonably good flow, provided that the temperature is not permitted to fall off during the transit.

237. Every leader should be provided with a damper at a point close to the furnace. The common butterfly damper is good enough for this purpose if carefully made and fitted.

238. Registers

Registers. This name is commonly used to designate the special opening through which air enters or leaves a room, and it is also applied to the combination of valves and grating which is employed to control the opening.

A register consists of a group of valve plates or louvres, which turn on pivots at each end and are operated simultaneously by means of a link connection and a lever. They are supported in an iron frame and are protected by a stout iron grating or grille, which is made strong enough to be walked upon without injury. Wall registers are constructed in substantially the same manner.

239. The ordinary method of setting a floor register is to place it over the top of a tin register box, which is connected to the hot-air pipe by means of a collar at the bottom. As the sole purpose of the box is to connect the pipe to the register, it should be so designed as to offer the least practicable resistance to the flow of hot air.

240. It is common practice to proportion the area of a hot-air pipe to the cubical contents of the rooms which they supply. The area of the pipe in square inches may be found by the following rule:

Rule 13

For the first floor, divide the volume of the room in cubic feet by 80 for rooms having only a moderate exposure, or by 25 to 20 for rooms having great exposure.

For second-floor rooms the divisor may range from 35 to 25, and for third-floor rooms, from 40 to 30.

241. A more accurate method is to proportion the area of the pipes to the cooling surfaces in the rooms. This may be done by the use of the following rule: