Wood Stove Flue Guide: Sizing, Flue Pipe and When to Reline
"Flue" is one of those words that gets used for four different things, which is exactly why people end up buying the wrong part. Before you can answer what size flue do I need, it helps to be precise about what the flue actually is.
Flue, chimney, liner, stove pipe: four different things
The chimney is the structure — the masonry stack or the insulated metal chimney running up through the house.
The flue is the passage inside it. It is the void the smoke travels up, not the material around it. When someone says "my chimney has two flues," they mean one structure carrying two separate passages — often a fireplace and a furnace.
The flue liner is what forms that passage. In an older masonry chimney it is usually clay tile. In a modern installation it is a stainless steel liner dropped down the inside of the chimney. For a line-by-line budget, see our cost to install a wood stove breakdown.
The stove pipe — properly the connector — is the black pipe between the back or top of the stove and the point where it enters the chimney. It is not chimney and never runs through a wall or ceiling on its own.
Almost every venting mistake we see traces back to blurring those four. Stove pipe is not rated to pass through combustibles. A clay-tile flue is not a substitute for a liner sized to your appliance. And a chimney that is structurally fine can still have a flue that is completely wrong for the stove you want to put in it.
Fireplace flue vs stove flue
An open masonry fireplace flue is built enormous on purpose. It has to carry the smoke from a large open fire that is drawing huge volumes of room air. A modern EPA-certified stove burns a fraction of that volume, far more completely, and sends much less heat up the chimney.
Drop a modern stove onto a fireplace-sized flue without relining and you get a flue that never gets hot. Exhaust cools on the way up, condenses on the walls, and lays down creosote. Draft is weak and inconsistent. Smoke rolls out of the door on a cold start.
This is the single most common reason an insert or a stove installed into an existing fireplace performs badly — the appliance is fine, the flue is four times too big for it.
Why flue size decides whether the stove drafts
Draft comes from the difference in density between hot flue gas and outside air. A flue sized to the appliance keeps that gas hot and moving fast. Too large and the gas slows, cools, and drops its moisture and tar on the way up. Too small and you choke the appliance and risk spillage.
Height matters too. Both directions of error get worse in a short chimney and better in a tall one, which is why the same stove can behave perfectly in one house and sulk in another.
The rule that matters: match the flue to the appliance's flue collar, not to the chimney opening. Your stove's installation manual states the required diameter. That number is not a suggestion — it is part of what the appliance was tested and listed with.
What size flue your stove needs
| Appliance type | Flue size | What the vent system is |
|---|---|---|
| Wood stoves (Drolet, Englander) | 6" | Stove pipe to a listed chimney, or a 6" stainless liner in masonry |
| Wood inserts | 6" | 6" stainless liner run to the top of the chimney |
| ComfortBilt and Englander pellet stoves | 3" | Sealed pellet vent — not wood-stove pipe |
| Drolet ECO-55 | 3" | Sealed pellet vent |
| Drolet ECO-65R | 4" | Sealed pellet vent, or a 4" stainless liner in masonry |
Pellet appliances are a genuinely different system. They run a pressurized, sealed vent at low temperature, and every joint has to be gastight. You cannot vent a pellet stove with wood-stove connector pipe, and you cannot mix vent brands within a run.
Relining a masonry flue
If you are putting a stove or insert into an existing masonry chimney, assume you are relining. A flexible stainless liner sized to the appliance runs from the stove connection to the top of the chimney, where a top plate and cap finish it.
Measure the full height from the appliance connection to the top of the crown before you order, and leave yourself margin. Trimming a liner is easy; splicing one is not. Our Vortex kits come in 25 ft and 35 ft lengths, and our Forever Flex kits in 15 to 40 ft:
- 6" liner kit, 25 ft ($499) and 35 ft ($579) — for wood stoves
- 6" insert liner kit, 25 ft ($449) and 35 ft ($559) — configured for wood inserts
- 4" liner kit, 25 ft ($399) and 35 ft ($519) — for a 4" appliance such as the ECO-65R venting into masonry
- Forever Flex 6" tee kit (from $620.99) and 6" appliance connector kit (from $618.99) — 316Ti, for wood, oil or gas, with 4", 5", 5.5", 7" and 8" sizes on the chimney liners page
Every kit includes the liner, top plate, rain cap and connector hardware. The Vortex kits are 304-grade stainless; the Forever Flex kits are 316Ti, which also suits a flue that has served an oil or gas appliance. Browse the full range in stainless steel chimney liner kits — No Sales Tax in any state and free shipping on all of them.
Insulating the liner is worth it in an exterior chimney or a cold climate. An insulated liner comes up to temperature faster, holds it better, and drops far less creosote over a season. The Forever Flex 6" pre-insulated tee kit and pre-insulated appliance connector kit arrive with the insulation already on. For the full sizing and insulation detail, see our chimney liner guide.
Connecting the stove to the flue
The connector runs from the appliance to the chimney. Single-wall pipe is cheaper and radiates more heat into the room but needs greater clearance; double-wall runs closer to combustibles and keeps flue gas hotter, which helps draft in a marginal setup.
Keep the connector as short and as straight as the layout allows. Every elbow costs you draft. Maintain a rise toward the chimney rather than a level or falling run, and never route connector pipe through a wall or ceiling — that is what a listed wall pass-thru or ceiling support is for. Which of those you need comes down to whether you take the vent out through the wall or straight up through the roof.
Build your parts list: no existing chimney? Our free chimney kit calculator works out the Class A chimney and stove pipe for your ceiling height and roof pitch, prices every part, and adds the whole kit to your cart. No Sales Tax in any state.
Browse single-wall stove pipe, double-wall connector pipe, complete wood stove pipe kits and pellet vent pipe and fittings.
Flue dampers, caps and terminations
A cap keeps rain, snow and animals out of the flue and stops the liner corroding from the inside. On a wood system the cap sits above the top plate; on a pellet system the termination is either a jet cap through the wall or a vertical cap above the roof.
Note one thing pellet owners get wrong: never install a damper in a pellet stove's exhaust. The system is pressurized by the exhaust blower and a damper interferes with it. Wood stoves are different — a pipe section with damper gives you control over burn rate on an over-drafting chimney.
When to call someone
Get a chimney sweep or inspector involved before you buy if the chimney is unlined, if there are cracks in the clay tiles, if the flue has been serving an oil or gas appliance, or if you do not know when it was last swept. A level 2 inspection with a camera costs far less than discovering a problem after the stove is in.
Getting the parts
We carry the full venting line for the stoves we sell — chimney liner kits, connector pipe and Class A chimney, pellet vent, supports, flashing and terminations. No sales tax in any state, so the price you see is the price you pay, which on a full reline and vent system is real money against a local supplier.
If you are not sure which parts your layout needs, work from your stove's installation manual: it specifies the flue diameter, the maximum equivalent vent length, and the termination clearances that will decide where the vent can exit. Or email info@stovesource.com or text 1-888-866-8206 with your chimney height and what you are venting, and we will spec it in five minutes.
Cleaning the flue: sweep it at least once a season. Use a wire chimney brush on masonry and clay tile and a poly brush on a stainless liner, with enough chimney brush rods to reach the top. See all chimney brushes and sweep rods.