Area, not diameter
The mistake that runs through every conversation about flue size is comparing diameters. Going from a six inch pipe to an eight inch pipe sounds like a third more. It is not: area goes as the square, so eight over six squared is 1.78, and the section is 78 percent larger. Going from a six inch outlet to an eight by twelve masonry flue is worse than it sounds again, because 96 square inches against 28.27 is 3.40 times.
| Section | Area | Against a 6 in outlet | Gas velocity |
|---|---|---|---|
| 6 in round | 28.3 sq in | 1.00x | 100% |
| 7 in round | 38.5 sq in | 1.36x | 73% |
| 8 in round | 50.3 sq in | 1.78x | 56% |
| 8 x 8 rectangular | 64.0 sq in | 2.26x | 44% |
| 8 x 12 rectangular | 96.0 sq in | 3.40x | 29.5% |
| 12 x 12 rectangular | 144.0 sq in | 5.09x | 19.6% |
The velocity column is the same volume of gas per unit time spread over a larger section. It is the reason area matters at all rather than being a free margin. The gas is carrying heat, and how fast it clears the chimney decides how much of that heat it still has when it reaches the top.
Why an oversized flue is not a safety margin
A flue only works because the gas in it is hotter and therefore lighter than the air outside. The chimney draft calculator shows the whole of that relationship. Anything that cools the gas on the way up cuts the draft, and an oversized section cools it two ways at once: the gas takes longer to get there, and there is more cold wall per unit of gas for it to lose heat to.
An 8 by 12 flue has 40 inches of wall perimeter around 96 square inches of area, which is 0.4167 inches of wall per square inch. A round flue of exactly the same area is 11.06 inches across with a perimeter of 34.73, or 0.3618 per square inch — 15.2 percent less wall for the same flow. That is the whole argument for a round section, and it is also why the corners of a rectangular tile are largely dead area with slow gas in them.
Then the second effect arrives. Unburned wood gas that stays gaseous all the way out of the chimney is a loss of efficiency. The same gas cooled below its condensation point on the way up is a deposit on the flue wall, and that deposit is the fuel for a chimney fire. An oversized flue does not make a fire more likely by being large; it does it by being cold, and it is cold because it is large.
Liner fit is geometry, and geometry is optimistic
A round liner has to pass the narrower of the two inside dimensions, so an 8 by 12 flue is an 8 inch problem, not a 12 inch one. Six inch liner plus half an inch of wrap on each side is seven inches across, which leaves an inch of the eight, or half an inch per side. On paper that is comfortable. In an eighty year old chimney it frequently is not, because the tiles are not aligned, because there is mortar squeezed into the section at every joint, and because a chimney that changes direction anywhere eats far more of the section at the offset than the drawing shows.
The void left around the liner is 67.7 square inches on those numbers, which is where an insulation wrap or a poured fill goes if one is used. Whether any of that is required, permitted or appropriate for a particular chimney and appliance is not something this page will tell you, because it is set by the liner listing and the code your jurisdiction adopted, and it is enforced by an inspector.
Before anything is connected
An existing chimney has to be inspected by a certified sweep before an appliance goes on it, and there is no arithmetic that substitutes for that. The things that decide the answer — an unlined flue, a cracked or displaced tile, a flue already serving something else, framing built against the chase, an old thimble buried in a wall — are invisible from the room and most of them are invisible from the roof as well. The hazards attached are chimney fires, carbon monoxide in living space, and concealed combustibles reaching ignition temperature over months without ever looking warm. This page names them and gives no method for looking at any of it.
Once the appliance is matched to the flue, the wood stove sizing calculator covers the other half of the deposit problem, which is an appliance too large for its room being run choked, and the firewood BTU calculator covers the third, which is wet fuel.
Questions people ask
My stove has a 6 inch outlet and the chimney is 8 x 12. Is that a problem?
This page will not answer that, because the answer depends on the appliance listing, the chimney height, whether the flue is inside or outside the building, and the code your jurisdiction adopted. What the arithmetic can tell you is the size of the mismatch: 96 square inches against 28.3 is 3.40 times the area, the gas moves at 29.5 percent of its connector velocity, and it is in the flue three and a half times as long. Take that to a certified sweep with the appliance instructions.
Why does a nominal 8 x 12 tile not have 96 square inches inside?
Because nominal sizes describe the outside. A clay flue tile sold as 8 by 12 has wall thickness on all four sides, so the actual inside section is smaller, and how much smaller depends on the maker and the standard it was made to. Use the inside figures from the manufacturer if you have them, or measure the inside of a spare tile. The default in this form is a round number for demonstration and should be replaced with your real one.
Does a round liner in a square flue really make a difference?
It changes three things at once. The section usually gets smaller, which raises the gas velocity. The shape gets rounder, which cuts the wall perimeter per unit of flow area by about 15 percent against an 8 by 12 rectangle. And if the liner is insulated the gas stays hotter over the height, which is the term that drives draft. Whether any particular chimney should be lined, and with what, is a question for the liner listing and a certified sweep, not for these three effects.
Can I connect two appliances to one flue to save the cost of a second chimney?
That is exactly the class of question this page refuses, and for good reason. What may share a flue, whether anything may, and under what conditions is set by the code your jurisdiction adopted and by both appliance listings, and getting it wrong produces spillage and carbon monoxide rather than a visible failure. The building department and a certified installer are the only places to ask.
What is the equivalent round diameter for and how is it worked out?
It is the diameter of a round pipe with the same cross-sectional area, so you can compare a rectangle to the pipe sizes everything else is sold in. Take four times the area, divide by pi, and take the square root: 96 square inches gives 11.06 inches. It is a comparison of area only. It says nothing about how the two shapes behave in flow, which is the point the wall-perimeter figure on the results is making.