Single-Wall, Double-Wall and Class A Chimney Pipe

Connector or chimney: the question that sorts every pipe

Every venting system has two parts with different rules. The connector is the short run from the appliance's outlet to the start of the chimney. The chimney is the part that carries the gases up through the building and out above the roof.

Model code draws a hard line between them. The 2021 International Residential Code model text says a chimney connector or vent connector shall not pass through any floor or ceiling, and may pass through a wall only if it is listed for wall pass-through or goes through a device listed for that purpose. Factory-built chimneys, by contrast, must be listed and labeled and installed according to their maker's instructions, and those serving solid-fuel appliances in a home must meet the Type HT requirements of UL 103.

So the first question about any pipe is not how many walls it has. It is which of those two jobs the pipe is listed to do. Single-wall and double-wall stovepipe are both connectors. Class A pipe is a chimney.

Following the pipe from stove to sky

Picture a wood stove in a living room with a metal chimney going up through a bedroom closet and the attic. The smoke passes through four zones, and each one has a different kind of pipe.

1. In the room with the stoveConnector pipe, single-wall or double-wall, runs from the stove collar up to the ceiling. This is the only zone where stovepipe belongs.
2. At the ceilingThe connector ends at a listed support box or finishing collar that belongs to the chimney system. From here up, everything is chimney.
3. Through closets, floors and the atticListed chimney sections pass through firestop spacers at each ceiling level, with the air space the listing requires. DuraVent's DuraPlus instructions call for firestop radiation shields and insulation shields where the chimney passes through an attic.
4. Above the roofThe chimney's own flashing, storm collar and cap finish the run, and the outlet must clear the roof by the usual margins, 3 feet above its own roof penetration and 2 feet above any part of the building within 10 feet, which the DuraPlus instructions take from NFPA 211.

Swap a pipe from one zone into another and the system no longer matches what was tested. The wood stove venting guide covers the stove side of this layout; here the focus is the pipe itself.

What the walls of each pipe look like

Single-wall connector: hot surface, wide berth

Single-wall stovepipe is one layer of steel, usually painted black. Nothing slows the heat on its way to the outside surface, so it gives off a lot of radiant heat into the room. That is part of why people like it, and it is why it needs room around it.

The 2021 IRC's vent selection chart lists 18 inches as the minimum clearance for single-wall metal connectors serving oil and solid-fuel appliances, and the DuraPlus chimney instructions give the same 18 inches for single-wall stovepipe. The code also sets minimum steel thickness by diameter, 24 gauge for connectors from 6 to 10 inches, and asks for runs that are as short and straight as possible with a rise of at least 1/4 inch per foot.

DuraVent's chimney instructions put the most important limit plainly: never install single-wall stovepipe through a wall thimble. A wall or ceiling crossing is always made with listed chimney parts. Joint fastening and crimp direction come from your stove's manual; see thimbles and connector pipes.

Double-wall connector: closer to the wall, still a connector

Double-wall stovepipe has an inner pipe and an outer pipe separated by a gap. The outer skin runs cooler, so the pipe can sit closer to combustible surfaces. DuraVent's DVL double-wall connector, as one example, lists 6 inches to walls and 8 inches to ceilings, measured at right angles to the pipe, and says it can be used anywhere single-wall stovepipe is allowed.

The same instructions are just as clear about what it is not. They say not to enclose double-wall connector pipe or pass it through walls, ceilings, attics or closets. The second wall reduces clearance in the room; it does not turn the connector into a chimney. That is the most common mix-up with this product: "double-wall" describes construction, while "chimney" describes a listing and a job.

Double-wall connector also keeps the smoke a little warmer on its way to the chimney, which helps draft on long or cool runs. Whether it is worth it in a given room usually comes down to how much clearance the room can give.

Class A chimney: what the UL 103 HT label means

The pipe sold as "Class A" chimney is a factory-built chimney listed to UL 103. The standard's published scope covers chimneys for gas, liquid and solid-fuel residential appliances whose flue gases do not exceed 1000°F in normal use, and adds a limited-duration test at 1700°F or, at the maker's option, 2100°F. The 2100°F version is marked HT, and it is the one model code calls for on solid-fuel appliances in homes.

Brands get to that rating in different ways. DuraVent describes its DuraTech chimney as double-wall with a 1-inch ceramic blanket between a stainless inner wall and the outer wall, and its DuraPlus chimney as triple-wall, with an insulating blanket plus a ventilated air space. Both are listed for continuous 1000°F operation and three ten-minute chimney-fire tests at 2100°F, and both call for 2 inches of clearance to combustibles.

Two rules come with every listed chimney:

• One system, one brand. The DuraPlus instructions say a listed system takes 100% DuraPlus listed parts and warn against mixing in other products or improvised solutions.
• The air space is part of the design. The same instructions require at least 2 inches between the pipe and combustibles, plus insulation shields where the chimney passes through an attic, so loose attic insulation stays off the pipe.

Pipes that look similar but serve other fuels

Some pipes in a basement or attic look like chimney pipe but are listed only for gas, oil or pellets.

The look of a pipe tells you little. A shiny double-wall gas vent in an attic can resemble chimney pipe, yet it is built for the cooler, wetter exhaust of a gas appliance, not for wood smoke or a chimney fire. For gas appliances, see direct-vent, B-vent and vent-free; for pellets, pellet stove venting.

Reading what is already in your house

Factory-built chimney sections and many vent pipes carry printed labels or stamped markings naming the maker, model and listing. Labels are often visible on the section just above the ceiling support, in the attic, or on the outer wall near the roof. A piece with no visible label is not proof of anything; it just means the identification has to come from somewhere else, such as the install paperwork or the cap and support parts.

A few things are worth a closer look when you can see them safely from the floor or an attic hatch:

• Black stovepipe that disappears into a ceiling, a wall or a closet without a listed support box or thimble.
• Pipe of different colors, diameters or brands joined partway up the run.
• Insulation, boxes or framing touching a chimney pipe in an attic or chase.
• Bulging, warped or discolored sections, or joints that have pulled apart.
• Rust streaks on the ceiling around the chimney or at the support box.

Each of these justifies shutting the appliance down until someone has checked the run. They are clues, not a diagnosis. An inspection identifies each part, checks it against the system's listing and manual, and looks at clearances on the way up. If a carbon monoxide alarm goes off, nobody stays inside to investigate: leave, then call 911.

Questions homeowners ask

Sources

1. DuraVent: DVL Double-Wall Close Clearance Stovepipe installation instructions (PDF)
2. DuraVent: DuraPlus All-Fuel Chimney System installation instructions (PDF)
3. DuraVent: DuraTech 5"-8" chimney catalog (PDF)
4. UL Standards & Engagement: UL 103, Factory-Built Chimneys for Residential Type and Building Heating Appliances (scope summary)
5. 2021 International Residential Code, Chapter 18: Chimneys and Fireplaces (model code text as published in the 2021 Seattle Residential Code)
6. 2021 International Residential Code, Chapter 10: Chimneys and Fireplaces, Section R1005 factory-built chimneys (model code text as published in the 2021 Seattle Residential Code)

This guide is general information. It can't diagnose a specific chimney; an inspection can.