This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The cold-air duct may be of earthenware drain pipe, if it can be obtained large enough for the job, failing which galvanised sheet iron may be used ; or, in large jobs, a brickwork culvert is made, cemented smooth inside and with access for cleaning. This can come under the basement floor or by practically any convenient route, but it should have its opening on that side of the building which will cause it to be least affected by wind pressure. Particular care must be taken to draw the fresh air from a sweet source (not from a stable-yard, as was once seen). The outer opening must be grated, and lined with wire gauze, to keep out insects and debris, but as this retards the inflow of air this opening should be of double the area of the duct. It may be thought desirable to have two-cold-air ducts, one on each of two opposite walls, and fitted with valves so that either may be used according to the direction of the wind, but this involves more attention than can generally be given. In certain cases the air may need filtering, in which case a frame covered with muslin or cheese-cloth may be used, but this requires the opening to be of so large an area (owing to the flow of air being so retarded by the cloth) that it is never resorted to unless absolutely necessary. The cloth, too, has to be frequently changed, probably once a week in manufacturing districts. A different plan is adopted with large Plenum works to both filter and moisten the air, as is referred to later.
Provision must be made to moisten the air (as already explained), and this is effected by placing a pan of water in the heating chamber. The pan can be in communication with a small cistern outside, fed either by hand or a ball valve, and a hygrometer, must be used to test and decide the amount of water that the pan should carry.
The warm-air delivery ducts may be of earthenware drain pipes, securely jointed to the ground floors, with either brick-built or sheet-metal vertical ducts to upper floors. All joints in basement pipes must be soundly made, or there will be a proportion of basement air delivered into the rooms. Horizontal ducts should be given a slight rise like hot-water pipes.
The grated openings of warm-air ducts, where they come in rooms, should be on the side walls and not in the floors. All delivery openings should be larger than the ducts connected with them, so as to reduce the velocity (not volume) of discharge. Added to this, the opening must be made still larger to allow for the fact that the grating - a good open grating - has but two-thirds its area clear space, the remainder being metal. The delivery gratings should therefore be of a total area equal to double that of the warm-air duct, a duct of 50 square inches having a square-shaped trumpet mouth, to receive and be fitted with a 10 by 10, or 12 by 8-inch grating.
In ordinary living rooms there usually exist chimneys, and these are relied on to act the necessary part of extract ventilating shafts, a part they usually perform sufficiently and well ; but where chimneys do not exist or are not capable of dealing with the volume of air, extract flues or shafts are provided as already mentioned. These are best if brick built, as explained.
Proportions of Parts of a warm-air heating apparatus. These figures are for places occupied by a reasonable number of people. For schools, concert halls, and the like the calculations must be at per person, based on the figures previously given.
Stoves or heaters : about 8 feet of low-temperature surface per 1000 cubic feet of space to be served.
Cold-air delivery ducts, three-fourths to seven-eighths the area of all the warm-air ducts. The inlet gratings, lined with wire gauze, to be double this area.
Warm-air delivery ducts 50 square inches area per thousand cubic feet of ground-floor space to be served ; 40 square inches for first floors, 32 square inches for second floors. Gratings or registers to these to be of double area, including the metal part of the grating.
Extract flues or shafts, from ground floors 32 square inches area for each thousand feet of space in the room ; first floor, 40 square inches ; second floor, 50 square inches ; gratings or registers to these one-half larger, measuring over the metal part of the grating.
In regard to the positions of the inlet and outlet gratings or registers, considerable controversy exists, engineers having quite different opinions as to the best place to locate these. A long list could be made of the points, good or bad, that are possible, particularly as to whether both should be above or below the breathing line, or one above and one below, which should be above or below, and what positions they should occupy in relation to one another. Of course, the idea aimed at is to get the air movement to occur in every part of the room as uniformly as possible, and to leave none moving too sluggishly or stagnant. The broad outcome of this, when dealing with warmed air, is that the greatest favour is given to putting both gratings on the same wall, one low and one high (not less than 7 feet above floor line), the wall to be an inner wall and facing an outer wall. There is no doubt about this being best, and from experiments made, and the experience gained by several engineers, there is little hesitation felt in saying that all parts of the room get their share of air movement by this arrangement. Fig. 73 illustrates this, showing the air movement noticed in experiments made.
An argument used in opposing the arrangement just shown is that the air movement brings down the vitiated air, products of respiration, etc., past the breathing line. The reply to this is that for these to come down they must first go up, and this they never do. On the other hand, the descending current does not flow past soiled or doubtful clothing of the occupants of the room, and, most importantly, it does not carry up any dust from the floor to the breathing line. The products from burning gas have to come downwards, but as a rule this is a minor item, and quite commonly has no existence.