This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Sub-section A of By daw 65 states that the second opening to the drain shall be obtained by carrying up, at the head of the drain if possible, a ventilating pipe or shaft in such a position as effectually to prevent any escape of foul air into the building, and of the same sectional area as the drain itself. And Subsection 4 prescribes that no bends or angles shall be used except where unavoidable. There is a proviso in Sub-section 5 that where the situation of the soil-pipe from any water-closet admits of its being used as a ventilator, and all other things comply with the requirements previously referred to, such soil-pipe may be utilized as the sole drain-ventilator. As, however, opinions are very strongly divided as to whether the soil-pipe should be disconnected at its foot, it is proposed to discuss the question in the next chapter. It will be noticed that the ventilating shaft must be of the same sectional area as the drain itself. This requirement may by some people be regarded as inconvenient, because it has hitherto been the custom to provide large drains, such as 9-inch pipes even to single houses, and no doubt a 9-ineh pipe-shaft at the side of a house would have an unsightly appearance.A 9 inch drain it, however, quite unnecessary for the purpose of house-drainage, aan while a 6-inch pipe is commonly adopted, a 4-inch pipe is often adequate, and there is no hardship or inconvenience in providing a ventilator for cither of these sizes. The importance of having a ventilating shaft of the same size as the drain teems to be very imperfectly appreciated, if one may judge from the provision for ventilation so frequently seen. It is very common to see a 2-inch pipe leading off the T-piece of the soil-pipe and carried up to the eaves of the roof, while the soil-pipe is probably connected with a 6-inch branch drain joined to a 9-inch main. It must he remembered that the areas of circles increase as the square of their diameters. The sectional area of a 2-inch pipe is 3.141 square inch< and of a 9-inch pipe 63.617, the area of the 2-inch pipe being only about 1/20 of the area of the 9-inch pipe, and it would require, therefore, twenty 2-inch pip to provide sufficient ventilation for a 9-inch pipe. Fig. 337 shows the relative proportions of 2-inch, 4-inch, 6-inch, and 9-inch pip Another common method of ventilating drains is by connection with the rainwater pipes. The evils of this system have already been pointed out Sub-section 4 of the By-law prohibits the formation of needless bends and angles in ventilating pipes. According to Knight's annotated edition of the Model By-laws, it has been estimated that a single right-angle in an air-pipe impedes the passage of air along it to the extent of about fifty per cent. The bends, therefore, which are so frequently seen in ventilating- pipes, defeat the object for which a ventilator is provided.
Independent ventilating pipes should be of strong cast-iron, securely jointed, and it is desirable to make provision for collecting the rust, which would greatly diminish the air-passage, and would very likely in course of time completely intercept it. There are numerous inventions for overcoming this difficulty, and a very simple one, designed by Mr. Fred. Lynde, A.M.I.C.E., is shown in Fig. 338.
Fig. 337 - Comparetive Areas of Circles.
Fig. 338 - "Loco" Rust-chamber at Foot of Ventilation -pipe.
There is a great variety of opinion as to the efficacy of cowls fixed on the head of vent-pipes to induce or accelerate the currents of air. Much will depend upon the position of the terminal, which, if under the shelter of overhanging walls, will be influenced by the rebounding air and the eddies caused by such a position. It is verv questionable whether even the best of these cowls is of any real value. Mr. Hellyer has made a number of most exhaustive tests, which have led him to adopt something to prevent a down-draught, such as the mushroom cap. Where a cap is not used, the head of the ventilator should be finished with a galvanized wire balloon,which serves to prevent access by birds.
It is not necessary to form the manholes for house-drains so large as those made on public sewers, but they should be always sufficiently large to enable a workman to turn round freely, with whatever tools he may require in clearing away accumulations, etc. Where manholes are required on drains sufficiently shallow that a man lying on the surface can get access to the drain, 3 feet x 2 feet 3 inches will be large enough, but where drains are deeper, it is advisable not to have them less than 4 feet 6 inches x 2 feet 3 inches, as sometimes two men may be occupied in the manhole. Manholes of rectangular shape are sufficiently strong, and are more readily constructed than circular ones. Figs. 339 and 340 show plan and sections of ordinary manholes. Of course, the shape must be altered to meet special circumstances, as at bends and junctions in drains; and where a number of branches are formed in the bottom of the manhole, the length must probably be increased. It is not necessary to carry the manhole of the full dimensions up to the surface of the ground, where the depth exceeds 8 feet In these cases a chamber should be formed at one end of the manhole by turning an arch from side to side, or by covering it with a strong stone "landing", and carrying a shaft at the other end of the manhole up to the surface to afford access. This shaft should lie of the same dimensions as the clear opening of the entrance cover The height of the chamber, from the invert to the soffit of the arch or under side of the landing, should be at least 6 feet, so as to allow the man working in the manhole to stand erect.
Fig. 339 - Vertical section* of Deep Manbote.
Fig. 340 - Section and Plan of Shallow Manbote.