the manufacturer of the brand we use says in their
instructions don't put ANY screws into their nice shiny metal B-vent.
Obviously, this makes supporting the flues a bit trickier.
An old-timer in our outfit says the mfg is just covering
their butts...that if one uses short enough screws to
NEVER pierce the liner a few screws are ok. He claims, beyond
the obvious problem of a hole being a potential flue gas leak, that
when a too-long screw is in contact with both the liner and the
outer shell it sets up galvanic corrosion and the liner
can erode pretty quickly into powder.
Thoughts, anyone? How do you'uns support flues when they can't go straight up?
Our local codes require us to screw the B-Vent, but ONLY with 3/8" screws. They say it won't pierce the inner liner, but I've seen it happen on some pieces.
I never really have the need for anything like that. We screw the single wall pipe to the bvent, sometimes throw a couple screws in the chimney top. But for supporting we typically let the ventfast do that for us.
The whole screws in B-Vent has to do with the fire rating. The air space between the two walls provides for clearance to combustible materials. The joints make contact(inner wall with outer wall) so there is minimal fire rating so screws have little affect. One manufacturer told me that if you install his B-vent the #@%&*!@ way it won't come apart but srcews are okay. Screws cannot be used to seal holes in B-vent when we do combsution testing. This does transfer heat from inside to outside and messes up the fire rating.
to support offset flues in attics i use a couple long 1x3 angles. overlap the angles about 4 inches and use 2 screws to hold angles together. make 2 snips on the 1 inch bend side 4 inches apart, then bend into an a frame. wrap a frame around flue and squeeze then secure to truss. ill use 1 on the horizontal part and another where exits the roof. very sturdy installation. the roofer can try to push that flue around and wont be able to.
Once you've pierced the inner liner, you loose the insulative properties created by the double wall. This is a Dewar's Flask or thermos bottle effect. How well would your thermos be at keeping your coffee warm with a hole in the outer wall?
Also, once the insulation is lost, you lose fire rating/ clearance as Jim Discussed. It reverts back to a single wall vent connector, which carries a 6" clearance. Without the insulation, you lose heat, which hinders draft. The cooler flue liner also contributes to condensation. If it ever went to court that a CO exposure resulted from a leak in B-vent caused by corrosion because someone put a screw into it, you'd be toast. Have there been cases like this? Who knows? Most companies keep a tight lip on litigation.
Use the mfrs. supplied supports or recognized field expedient adjuncts such as plumbers straps or drives with stove bolts.
I used to put 3 screws in each joint. One time I looked up a water heater vent I had just screwed together. The screws didn't penetrate the liner, they just pushed it in. This left a big gap where the gases could get up outside the liner. This would probably cause the pipe to corrode quickly. If a joint feels too loose a little hammer work will fix it.
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