Canadian, Berlin, Ont. writes:

"In a small house the plumbing fixtures usually consist of a bath and closet upstairs and a kitchen sink down stairs. The traps of the bath and sink are usually required to be vented, but is there any valid reason why the closet trap should be vented? Even if the bath waste were connected with the lead bend under the closet, which is not now considered good practice, the discharge from the bath would hardly cause the closet trap to syphon, and of course the discharge from the sink below cannot affect the water in the closet trap when the soil pipe is continued through the roof. A vent to the closet trap would hardly be required to prevent it syphoning itself dry while being discharged; that would scarcely be possible with a 4 inch trap and a 1½ - inch supply, so that I am at a loss to know why, here in Canada, we have not in the market a single porcelain washout closet without a horn for the trap vent."

[There are undoubtedly cases where fixtures are few in number, and their relative position and particular connections to the soil such as to make back-ventilation of traps an unnecessary precaution, but the cases where back venting can be omitted with safety are so rare and the uncertainty of how the traps may be affected is so great that the rule for the greatest safety to the greatest number must govern, and back-venting be considered a factor of safety well worth introducing: into the work. It is like all generalizations not applicable or requisite for certain isolated cases. It has, however, value for air circulation and ventilation aside from the mere protection of the trap against syphonage, but in the case cited these considerations do not appear to apply.

Referring to your immediate inquiry, the form of closet and its relation to soil pipe and the method of venting the soil pipe at the roof, would have some influence on the question of syphonage. We have seen the seal of hopper water-closet traps affected by a strong wind blowing across the open end of the soil pipe. The actual seal of water in the trap is about 1½ or 2 inches, representing a pressure of 1.15 ounces per square inch. The atmospheric pressure is 2.40 ounces per square inch, and any influence increasing the pressure 1.15 ounces on one side of the trap or reducing it 1.15 ounces on the other would unseal the trap. The conditions are so delicately balanced that special circumstances of construction would outweigh theoretical considerations. A practical test repeated under various conditions of atmosphere, coincident discharge of fixtures, etc , would best answer your question for the particular case named It has been found experimentally and in a vast field of practice that back-venting properly done practically insures the seal of the trap, and as a matter of insurance the outlay incidental to it seems to be well invested against the hazard.]