05-09 Branch vent

Question: 05-13Code Section: 7Date:OBC 2006 Reference:

Question: If a 2″ branch vent is receiving 12-1 1/2” vents other than water closets and only 5 water closets, would the 2′” branch vent need to be increased in size if an additional 2 – 1 1/4″ were added in lieu of the 6th water closet?  

Interpretation: (On. Reg. 403/97) Presently, Table does not permit or allow you to assign a value to a water closet vent.  Back in trade school, we were taught that if your scenario did not fit quite right in the combinations, you simply increased the size and went to the next highest size. However, students today want to know why should we increase what is the value associated with a water closet vent?

Back in Dec 1999, a similar question was posed to the Code Technical Committee.  It was determined that the 2 inch branch vent was large enough to handle the load and we did propose a code amendment that an additional Note be provided to the Table in order to cover most combinations by establishing a value for a WC vent.

The Ministry did state in the 1999 response that 1 WC vent of 1-1/2 inch is equal to 2 – 1 1/2 inch vents and that it would be printed in the next code change.  So now we are in 2005 and we are still having problems with Table because our code has not been amended yet.  Except for Danny and myself, no other member of the present committee knew of the 1999 answer and yet they came up with the same answer.

We would all accept replacing the missing WC vent in the above scenario with 2 1-¼ inch vents with a caution to verify with the Ministry if your interpolation of the Table is accurate.