Right size, safe exhaust
We match flue sizes to your stove outlet and manual, ensuring safe and efficient heating.
What flue diameter do you need?
Published on 24 August 2026
Written by Dries Nijs3 min read
Written by Dries Nijs3 min read
What flue diameter do you need?
The correct flue diameter depends directly on your appliance connection and manual. Pellet stoves usually require 80 mm, while wood burning stoves typically need 130 to 150 mm. Sizing too narrow or too wide harms performance.
The size on the appliance outlet is leading, have the diameter calculated if in doubt.
Choosing the correct flue diameter depends on the spigot connection and user manual of your stove, not guess work. Check the flue outlet on your appliance first and use that size as your starting point. Pellet stoves usually run on 80 mm, sometimes 100 mm. Wood burning stoves generally require 130 to 150 mm or more.
A flue that is too narrow restricts airflow, increasing the risk of soot buildup and smoke spillage. A flue that is too wide cools flue gases down too quickly, weakening the draft and causing condensation. Sizing is not a minor detail, it dictates whether your stove burns safely and efficiently.
2>Pellet stove diameter vs wood stove diameter
These two stove types have completely different requirements. A pellet stove actively pushes exhaust gases out via a fan, whereas a wood stove relies entirely on natural draft. This explains the significant difference in common pipe diameters.
Always match the diameter of the appliance outlet and manual, as that spec is mandatory.
Compliant with standards
All installations follow official building regulations and standards, giving you full peace of mind.
Calculated, not guessed
For long or complex runs we calculate draft and diameter to prevent condensation and backdraft issues.
An undersized flue creates excessive resistance, preventing exhaust gases from escaping smoothly. The result is sluggish airflow, faster creosote accumulation, and smoke spilling back into the room. A stove starving for air will also burn fuel inefficiently.
An oversized flue creates the opposite issue. Exhaust gases spread out over a larger surface area, cool down, and lose velocity. This drops the chimney draft and causes water vapour to condense, leading to corrosion and moisture damage over time.
The stove connection is decisive because manufacturers test appliances with a specific flue size. Deviating from this invalidates optimal operation and often voids the warranty. The diameter is part of the certified unit, not an arbitrary choice.
How to determine the right flue size
Check the data plate and manual to note the exact flue outlet diameter.
Use that dimension as the minimum size for the complete flue system from stove to terminal.
Verify CE markings and specified values against installation guidelines.
Account for overall flue height and offset bends, as these affect draw.
For long or complex runs, have the flue size and chimney draft calculated by a specialist.
Following these steps prevents the most common mistake: choosing a flue size based on an existing chimney or an old stove. Your new stove's specifications are always decisive, regardless of the previous opening size.
Signs your flue diameter is incorrect
Poor draw, smoke backdrafting, or difficulty reaching operating temperature.
Excessive condensation or damp spots around the flue pipe.
Heavy soot deposits, dirty glass, or rapid creosote buildup.
Reducers used to downsize the outlet without technical justification.
Uncertainty regarding the specification or a missing user manual.
Reducing or increasing flue size: when is it allowed?
Reducing means fitting a flue pipe smaller than the stove outlet, which is strictly prohibited in almost all circumstances. Increasing to a larger diameter is sometimes permitted, but only if explicitly authorized in the appliance manual. Incorrect size transitions disrupt draft and ventilation.
Note the difference between the complete flue system and the stove pipe. This guide focuses on flue system diameter, whereas the connecting pipe between stove and wall is sized separately. For details on that segment, read what stove pipe diameter you need.
Only use an increaser or reducer if recommended by the manufacturer or supported by calculations. Step-up adaptors from 80 mm to 100 mm without proper design can cause unpredictable draft. Always seek professional advice before installing adaptors.
If you are unsure which flue system fits your appliance, check the layout per stove type. Learn which system suits a pellet stove flue installation and which works for a wood stove flue pipe.
Flue size regulations and purchasing guide
Choosing the correct flue diameter also means complying with building regulations. In Belgium, NBN standards apply to flue installations, while UK and European building regulations mandate specific minimum sizes and clearances. The manual and CE marking remain mandatory, but local building rules must also be followed.
When purchasing, consider insulation specifications and overall height, as these directly influence flue performance. Taller chimneys or systems with multiple elbows may require calculated adjustments to maintain stable draft.
For help choosing wall types and insulation, read about single wall vs twin wall flue systems. To make sure you do not miss anything, use our flue pipe buying checklist.
If you are unsure about flue size or installation design, have it checked before buying. Request a free quote or advice today to ensure you buy the correct diameter with confidence.
Need help checking or installing your flue?
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Frequently asked questions
A pellet stove typically requires an 80 mm flue, or 100 mm for larger output models. Check your stove outlet and manual, which are always decisive.
Reductions are generally not allowed, as smaller pipes restrict exhaust airflow and draft. Match your stove outlet diameter and only increase if allowed by the manufacturer.
Start with the stove outlet size. For long flue runs or multiple bends, have a professional calculate the draft requirements, considering height and airflow resistance.
An oversized flue cools exhaust gases too quickly, reducing chimney draft and causing condensation. This leads to moisture stains and creosote accumulation.
