Quick answer: Class A chimney pipe is a factory-built, insulated metal chimney listed to UL 103 HT, the safety standard for chimneys serving residential and building heating appliances. It is the section of a wood stove system that passes through ceilings, walls, attics and roofs. Shasta Vent's Class A pipe has inner and outer walls of .020" type 304 stainless steel, ceramic insulation packed between them in a 1"-thick wall, and it needs only 2 inches of air-space clearance to combustibles.
What "Class A" actually means
"Class A" is the name the trade uses for a factory-built chimney that can safely vent solid fuel such as wood and coal. Building codes and inspectors usually describe it a different way: a factory-built chimney listed to UL 103. The HT (high temperature) part of the listing means the chimney also passed chimney-fire testing.
Shasta Vent All-Fuel HT Chimney is ETL listed by Intertek to the HT requirements of UL 103. It was tested at:
- 1,000°F continuous flue temperature
- 1,400°F for one hour
- 2,100°F three times for 10 minutes each, the kind of heat produced by a creosote fire
Plain stove pipe is not tested this way, which is why it can't be used where a chimney is required.
Class A chimney pipe vs. stove pipe
A wood stove setup uses two kinds of pipe:
- Stove pipe (connector pipe): runs from the stove's flue collar up to the ceiling or wall.
- Class A chimney pipe: takes over at the ceiling support box or wall thimble and carries the exhaust out of the building.
| Stove pipe (connector) | Class A chimney pipe | |
|---|---|---|
| Where it goes | Inside the room, stove to ceiling or wall | Through the ceiling, attic, wall and roof, and all the way to the cap |
| Construction | Single wall, or uninsulated double wall | Double wall with insulation packed between the walls |
| Can pass through a ceiling, wall or roof? | No | Yes. That's what it's designed for. |
| Clearance to combustibles | Large. Follow your stove manual and local code. | 2" air space (Shasta Vent All-Fuel HT) |
| Listing | Connector | Factory-built chimney, UL 103 HT |
For a deeper comparison, see Class A Chimney vs Single Wall Pipe.
Shasta Vent Class A specs at a glance
- Listing: ETL listed by Intertek to the HT requirements of UL 103
- Walls: .020" type 304 stainless steel, inner and outer
- Insulation: high-temperature ceramic blanket, packed solid between the walls
- Wall thickness: 1". The outside diameter is 2" larger than the flue (6" pipe is 8" OD; 8" pipe is 10" OD).
- Clearance to combustibles: 2" air space
- Diameters: 6" and 8"
- Lengths: 8", 19", 24", 39" and 48", plus an adjustable section (11"–20")
- Joints: twist-lock, with a locking band at every joint
- Fuels: wood, coal, oil and gas, for natural-draft stoves, fireplaces, furnaces and boilers
- Warranty: limited lifetime
Shop Class A chimney pipe in 6" and 8".
The parts of a Class A chimney system
A Class A chimney is a system, and the components are listed to work together. A typical install uses:
- Ceiling support box carries the chimney's weight at the ceiling and connects to the stove pipe. Used for through-the-ceiling installs.
- Wall thimble, tee with cleanout cap and tee support are used for through-the-wall installs.
- Chimney pipe sections twist-lock together and are secured with locking bands.
- Firestop radiation shield goes at each additional floor the chimney passes through.
- Attic insulation shield keeps insulation and debris 2" away from the pipe in the attic.
- Roof flashing and storm collar seal the roof penetration. Choose the flashing that matches your roof pitch.
- Chimney cap keeps out rain and animals and has a removable spark arrestor screen.
- Wall brackets and roof braces provide lateral support along the run and above the roof.
For most of these in one box, see the 6" Thru-The-Ceiling Kit or the 6" Thru-The-Wall Kit. Our guide to every chimney component explains what each part does.
When you need Class A chimney pipe
You need Class A pipe any time the exhaust from a wood stove, coal stove or other solid-fuel appliance has to:
- pass through a ceiling, floor, attic or roof
- exit through an exterior wall and run up the outside of the house
- run inside a chase enclosure
- replace a worn-out or damaged factory-built metal chimney
Cabins, shops and tiny homes follow the same rule. See Best Chimney Setup for Cabins, Tiny Homes, and Off-Grid Living.
How to choose the right size
Match the chimney to your appliance's flue outlet and follow the stove manufacturer's instructions. Never use a chimney with a smaller inside diameter than the stove's outlet unless the manufacturer specifically says to. Most residential wood stoves use 6" pipe, and larger stoves, furnaces and fireplaces often use 8". These two guides walk through the decision:
- 6 Inch vs 8 Inch Chimney Pipe: What's the Real Difference?
- What Size Chimney Pipe Do I Need for My Wood Stove?
Rules that apply to every Class A chimney
- Keep the 2" air space. The gap applies to every pipe section, elbow and tee. It must stay open air and can't be filled with insulation or anything else. See Chimney Pipe Clearances.
- Meet the 3-2-10 rule. The chimney must rise at least 3 ft above where it passes through the roof, and at least 2 ft higher than anything within 10 ft horizontally. See the 3-2-10 rule explained.
- Support it. Secure the chimney at least every 10 ft. If it extends more than 5 ft above the roof, brace it with an extended roof bracket.
- Limit offsets. Shasta Vent elbows come in 15° and 30°, with a maximum of four elbows (two offsets) per system. See How Many Elbows Can a Chimney Have?
- Don't cut the pipe. Use the adjustable section for odd lengths.
- Don't mix brands. Use only components from the same listed chimney system.
- Enclose it in living spaces. Where the chimney passes through closets, storage areas or rooms where people or stored items could touch it, enclose it. In cold climates, a chase around an exterior chimney helps draft and reduces creosote.
Frequently asked questions
Is Class A chimney pipe the same as triple-wall pipe?
Not necessarily. "Class A" refers to the listing, not the number of walls. Some Class A chimneys are double-wall with solid insulation, like Shasta Vent. Others are triple-wall and use moving air for cooling. Both can be UL 103 HT listed. See Insulated vs Air-Cooled Chimney Pipe.
Can Class A chimney pipe be used for oil or gas appliances?
Shasta Vent All-Fuel HT is listed for natural-draft stoves, fireplaces, furnaces and boilers burning wood, coal, oil or gas that call for a UL 103 or UL 103 HT chimney. Always confirm the venting requirements in your appliance's manual.
Can I use Class A pipe for a pellet stove?
Pellet stoves aren't among the listed applications for Shasta Vent All-Fuel HT. Follow your pellet stove manual's venting requirements. See our Pellet Stove Venting Guide.
Can Class A chimney pipe be cut to length?
No. Cutting a section breaks the insulated wall and the joint. Combine standard lengths with an adjustable section instead. Our guide How Much Chimney Pipe Do I Need? shows how to plan the run.
Why is it called "All-Fuel"?
The HT-rated listing covers solid, liquid and gas fuels. That's why Shasta Vent's chimney is called All-Fuel HT. See What Is All-Fuel Chimney Pipe? UL 103 HT Explained.
Ready to plan your chimney? Start with our chimney planning guide, or shop complete chimney kits.
Disclaimer: Shasta Vent blog articles share general information and common practices in the chimney and hearth industry. They are not installation instructions for your specific project, and they don't replace professional advice. Building codes, appliance requirements and site conditions vary. Before you install, modify or repair a chimney, follow your appliance manufacturer's instructions and the installation instructions that come with your chimney, check with your local building department, and consult a qualified professional such as a CSIA-certified chimney sweep, NFI-certified specialist or licensed installer.
