Clause C.2.5

[Brad.B.]

Hi All,

C.2.5
If vent connectors are combined prior to entering the common vent, the maximum common vent capacity listed in the common venting tables shall be reduced by 10%, the equivalent of one 90 deg elbow (0.90 x maximum common vent capacity).  See Figure C.9.  The horizontal length of the common vent connector manifold (L in Figure C.9) should not exceed  1-1/2 ft (457mm) for each inch (25.4mm) of common vent connector manifold diameter.

Can someone explain the application of the above?  I have looked at Figure C.9 but still don't get it. 

[Porcupinepuffer]

I don't believe the first sentence actually pertains to the diagram very well. Maybe it's a mistake. If you had two appliances combining prior to entering the common vent, there would be no need to have any common vent connector manifold. Only the second sentence makes sense to the diagram.

[Admin]

It's saying the common vent can run horizontally and it's length is determined by the diameter of the common vent.  If the common vent is 6" we multiply it by 1.5 feet and are allowed 9' of horizontal common vent to the Tee fitting.

If you look at page 233 of the B149 handbook you will see they call the horizontal common vent the vent connector manifold.

[Brad.B.]

Thanks all.  I'm in a co-op week now (actually 2 weeks this time), and the last class week we did venting.  It was a slow start for me but the light turned on eventually.  And I do remember one practice question needing to multiply the diameter by 1.5ft. to get max horizontal run, but I don't remember applying the first part of that clause, reducing by 10%.  Before I make my module summary/cheat sheet I'm going to need to be sure of this clause. 

It could be a mistake, not impossible, since that flashlight clause I saw has the wrong division number and group letter (still haven't heard back from them on that).

Since it doesn't sound like neither of you are absolutely sure either, I'm going to stare at it more, with your input in mind, and see if it comes to me, and/or wait for next class week see if the teacher is 100% about it.  I'm finding this is like the fire alarm codes in that there are some clauses that are interpreted differently, and maybe even the CSA/ULC themselves aren't sure LOL.

[Admin]

You can download some Common Vent Tables - Here.

They have some built in reductions to make things easier.