Interpretation No. 5080 – Combination Waste and Vent System Requirements
Manual Reference Section: 5
Applicable Code: BC Plumbing Code (1998), Part 7, Section 7.5
Status: Approved by VIPCC
Revision Note: This interpretation was revised twice (No. 5080-2 and No. 5080-3) to address questions arising from field application of the original ruling.
Background
The Plumbing Officials’ Association of British Columbia issued Interpretation No. 5080 to clarify the conditions under which a combination waste and vent system could be installed in place of a conventional vented drainage arrangement. The original ruling (225 words) established the basic parameters. Two subsequent revisions expanded the guidance in response to inspector enquiries about fixture eligibility, pipe sizing criteria, and the relationship between combination systems and the standard venting provisions of Section 7.5.
Subject Matter
The enquiry asked whether a combination waste and vent configuration was permitted for floor drains and similar fixtures in commercial and institutional buildings, and what restrictions applied to the types of fixtures that could be connected to such a system. Supplementary questions raised in the revision process addressed the minimum pipe diameter for horizontal sections functioning as both drain and vent, and whether a combination system could serve fixtures on more than one floor level.
Ruling
The Association confirmed that combination waste and vent systems were permitted under Section 7.5 of the Code, subject to specific limitations. The system was restricted to floor drains, standpipe connections, and similar fixtures that received only indirect waste or clear-water discharge. Fixtures producing solid waste, including water closets and urinals, were not eligible for connection to a combination waste and vent arrangement.
Horizontal piping serving as both waste carrier and vent passage was required to be sized at least two pipe diameters larger than the minimum drain size that would otherwise apply to the connected fixtures. This oversizing ensured that the upper portion of the pipe cross-section remained open to air circulation while the lower portion carried the waste flow.
Revised Provisions (No. 5080-2 and No. 5080-3)
The first revision (No. 5080-2) clarified that combination waste and vent systems were limited to a single storey of construction. Fixtures located on upper floors could not discharge into a combination system serving the storey below, as the vertical drop would compromise the self-venting function of the oversized horizontal piping.
The second revision (No. 5080-3) addressed the connection point where a combination system joined the conventional vented building drain. At this junction, a properly sized vent was required to rise from the connection fitting to open air, providing pressure relief for the combined system and preventing trap seal loss in the connected fixtures. The vent diameter at this point was to be determined by the total fixture unit load of the combination system, using the vent sizing tables in Section 7.5.
Scope of Application
This interpretation applied to new plumbing installations in commercial, institutional, and industrial buildings where the fixture types and building configuration met the specified criteria. Residential applications were generally outside the intended scope, given that household plumbing fixtures typically included water closets requiring conventional venting. The ruling did not alter requirements for wet venting, circuit venting, or individual fixture vents addressed in other subsections of Section 7.5.