I want to add a shallow shelf to my storage room for storing mason jars -- unfortunately I didn't consider "Will I be able to get this past a (semi-)horizontal ran of the water heater exhaust pipe?" before I built it in the garage. In case you're wondering the answer is "Not easily".

In order to get the shelf in I'd need to remove one or both sections of the water heater exhaust pipe. I realize that combustion gasses are bad juju to be leaking into your living space, so before I do that, I thought I'd ask -- can I just undo the sheet metal screws, remove the pipe, then when done put it back the way I found it, or are there seals/adhesives involved in this operation?

enter image description here

  • I'm curious what you're going to do with the plumbing against the wall...
    – FreeMan
    Commented Jan 16, 2023 at 17:50
  • @FreeMan The shelf is actually sized to fit between the pvc and shelf you see to the right (where the ironing board is peaking up), no plumbing collisions involved.
    – Sidney
    Commented Jan 16, 2023 at 18:59

3 Answers 3


Generally, no. They're just a friction-fit deal, usually held with screws.

Use caution to not deform the pipes. They should fit together accurately and snugly to seal against leakage. In your case I might detach the draft hood and disconnect at the wye to minimize hassle.

I might also reconfigure to raise the elbow that's above the water heater. You only need 1/4" per foot slope. You could gain some useful clearance to your shelves.

  • 2
    while mostly correct, it is misleading to say they provide seal against gases. They do not, that is why one uses sealing tape.
    – Traveler
    Commented Jan 16, 2023 at 22:56
  • In all the years I've been around such flues in my cold region I don't recall ever encountering sealed joints. Therefore, they do adequately seal against gases or we'd all be dead or seriously ill. Downvotes are inappropriate unless you can show me code requiring it.
    – isherwood
    Commented Jan 17, 2023 at 13:40
  • @isherwood I believe the tape sealing represents more of a belt and suspenders approach to minimize the chances of sloppy installation leading to a dangerous leak. More recent houses also may have reduced air changes per hour, magnifying the consequences of such a leak.
    – Armand
    Commented Jan 17, 2023 at 17:36

Usually they are just snug fit and held with screws.

But to provide seal against leaking exhaust gases and from the bad juju you can use the flue ducting tape.

high temp tape

  • 1
    Air duct tape is for air ducts, not for combustion flues. Combustible materials, including duct tape, must be kept well away from a flue. (I recognize the one you pictured is aluminum, but what about its adhesive - is that combustible?) There may exist high-temperature non-combustible tapes listed for flues, but.. I've looked at plenty of flues and can't recall ever seeing one with any kind of tape or even mastic over its joints.
    – Greg Hill
    Commented Jan 16, 2023 at 19:03
  • High-temp foil tape exists, but this answer should really include a caveat about combustibility if using the wrong variety. What is "the juju"?
    – isherwood
    Commented Jan 16, 2023 at 19:06
  • 1
    @GregHill Our gas water heater installed 2 years ago was installed with similar high temp aluminum flue tape covering each joint; previous setup had just the screws at each joint, so it might be a new thing.
    – Armand
    Commented Jan 17, 2023 at 8:14

Based on your photo, you may have some more significant issues than how to reassemble your vent piping. I'm concerned about your horizontal vs. vertical segment lengths, type of vent pipe, and clearance from combustibles.

A handy diagram from the inspectapedia.com website summarizes the rules:

enter image description here

It's not clear to me if your vent is single-wall or double-wall, and what appliance vent is coming in from the right. I'd verify with a knowledgeable person about whether that merge is appropriate for both vent systems.

Regarding the water heater venting, although having the first 12 inches straight up is apparently not explicitly in recent code, as a manufacturer's instructions specification it effectively becomes part of code. As @isherwood suggests, adding a vertical section there would also give you more headroom to the right.

Some further relevant venting rules from inspectapedia.com:

Routing Vent Pipe:

  • Horizontal distance: no more than 75% of total vertical height of the entire vent/chimney assembly
  • Horizontal slope: no less than 1/4" per foot upwards slope per foot of horizontal run.
  • A portion of the vent pipe (up to 75% of the total vertical height) can be horizontal, but the termination must be vertical. For the horizontal section, install without dips or sags with an upward slope of at least ¼ inch per foot. Install pipe avoiding unnecessary bends.
  • Pipe joints must be fastened by sheet metal screws or other approved means.

Vent Pipe Material and Allowed Penetrations:

  • The existing vent system must be UL listed Type B double wall or single wall metal vent pipe of either 3 inch or 4 inch diameter and installed according to the vent manufacturer’s instructions and the terms of its listing.

  • Local codes may be more restrictive and may not allow single wall vent pipe.

  • Single-wall vent pipe cannot be used for water heaters located in attics and may not pass through attic spaces, crawl spaces, or any confined or inaccessible location.

  • Single wall vent pipes cannot pass through a ceiling, floor, firewall, or wall. [An exception is made for non-combustible walls such as solid masonry or concrete.]

  • Power Vent Sharing - Common Venting: Do not share a power-vented-appliance flue: Do not common vent a natural-draft gas fueled water heater with any power vented appliance.

Vent Pipe Clearances to Combustibles:

  • Clearance to combustibles: for gas fired water heater vents: 6" minimum clearance from single-wall vent pipe or flue-vent connector to any combustible material.
  • Clearances for double-walled B-vent piping are given by the manufacturer. Typically a double-walled metal B-vent chimney section is stamped as requiring 1-inch clearance to combustibles.
  • If you cannot meet the 1-inch B-vent clearance you may be able to use a zero-clearance chimney material.

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