This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The best method of connecting traps, and their actual value under all conditions, are matters upon which there is much difference of opinion. Cities also vary in their requirements to a greater or less extent, so that it will be possible to show in a general way only the various principles involved and to illustrate what is considered good practice, in the average case, at the present time.
In addition to the traps at the fixtures a main or running trap is placed in the main soil pipe outside of all the connections; this is sometimes placed in a manhole just outside the building, but more commonly in the cellar before passing through the wall; the former method is much to be preferred, as the trap may be cleaned without admitting gases or odors to the house. The running trap has been shown in Fig. 29, and is provided with a removable cap for cleaning.
The agencies which tend to destroy the water seal of traps are siphonage, evaporation, back pressure, capillary action, leakage and accumulation of sediment.