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New gas hot water heater is 5" taller than old (of course the same size would be too easy).Flex water and gas lines -no problem; but the class B ( double wall ) exhaust vent is ? Can it be cut? How? I need to remove 5". There is an 18 " straight run ( no joints) up into the attic, and straight up through the roof; very simple layout. At the bottom ,it has 2 adjustable elbows to connect to the heater, they can't reasonably be cut. It is near the side of the house so the attic clearance is about 18" at the vent so I don't want to work there if possible. The vent is nominally 3" , actual O.D. about 3.5 "

  • The bottom of those pipe sections is normally flared a little or has a screw clamp to attach to another piece. Look at what you have carefully. If you cut it, will you be able to attach it to the elbow any more? Might be better to just replace it with an adjustable section (commonly available at home stores). – JPhi1618 Mar 19 at 17:34
  • You may be able to push it up enough to get the clearance you need. Try gently turning it back and forth as you push up on it. – Alaska Man Mar 19 at 17:53
  • The adjustable elbows and the straight run are all just tight press fit, no screws or clamps. The ends are all single wall that goes to double wall in about an inch. – blacksmith37 Mar 19 at 18:08
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The short answer is no. B-vent cannot be cut.

  1. But there is hope. You don't have a joint in the straight run but you do have two 90s

This shows a B-C adapter with a C vent 90 Selkirk

C vent isn't actually a thing, it's just what people called regular venting.

Everything below the B-C adapter is fair game you could potentially remove the B vent 90s attach the B-C adapter to your straight run and then use C vent the rest of the way. C vent 90s are smaller and easier to work with. You can even trim up to the rivet to buy some space.

  1. Without a picture it's hard to say what will work but there are 12" flexible sections of B vent available.

    12" B vent flex enter image description here www.amazon.com

Unfortunately they are proprietary. You would have to match it. But 1 12" flex could buy enough space if you replaced the two 90s with it. (again pictures would help.)

  1. The last tactic in the arsenal is pushing it up through the thimble. This will require going on the roof.

    B-vent chimneyenter image description hereHart and cooley

Before you push it up (if necessary) you will need to make sure there are no screws going through the flashing into the B vent (you don't want to disturb your flashing) they might be hiding under the storm collar. The firestop (if equipped) may also have screws in it. That would pose a problem. They would also need to be removed. Once it's pushed up reattach the storm collar back just above the flashing.

Hopefully method number one works. You may need to do a combination of all three.

  • Thanks, I was afraid it should not be cut. I tried pulling it up on the roof but got less than an inch. Thanks for info. – blacksmith37 Mar 20 at 13:51

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