The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California
How To Design And Install Plumbing
Sizing the House Drain. To determine the proper size of a house drain its classification must be known. If it is a drain of the combination type, which is quite commonly installed, the roof area is the deciding factor in determining the proper size. Should the sewer be one of the sanitary variety, the fixture discharge is the governing factor. Because of the complexity of the problem, the responsibility of sizing the house drain is not generally assumed by the plumber. However, the sizing of the house drain is simplified by means of charts formulated for use in plumbing codes. Engineering data compiled over a long period, and the experience of installations giving satisfactory service, are the bases for these charts. State and local codes govern the house drain installation to a large extent.
It is impractical, even though the code and fixture discharge rating would permit, to install a vitrified pipe of 4-inch diameter. A 4-inch tile is impractical because it does not always run true to size, and it is difficult to lay without forming cocked and poor cement joints which may cause stoppage. It is also subject to breakage, due to settling and backfill of earth into the open trench. For this reason a clay drain should at no time be constructed of pipe less than 6 inches in diameter.
When iron pipe is used there is a somewhat better condition. The 4-inch pipe is generally full size in diameter and can be laid as satisfactorily as the 6-inch pipe. Codes and fixture load govern the installation of this size of pipe, and if these requirements come within the service possibilities of a 4-inch pipe, the installation is served efficiently.
Many mechanics believe that if they make the drain a little larger than is necessary they will increase its efficiency. However, this is not the case. Scouring action is not secured by increasing the size of the drain. The solids are carried along the bottom of the pipe and, because the water flow within the larger pipe is shallow and slow, they become separated from the water and remain in the drain. The result of this may be stoppage of the drain branch, and often the entire house drain is affected.
A drain, therefore, should be of proper size with a flow of about 50 per cent of the pipe diameter. This assures a scouring action and overcomes many house drain difficulties. On the other hand, a drain too small in size is overtaxed by flow and is apt to produce siphonage, back-pressure, and basement flooding.
