Why Diameter Matters
The flue diameter determines how much combustion gas can leave the stove per second. Every stove has an optimal flue size, calculated by the manufacturer based on the stove's output (kW), combustion chamber volume, and flue gas temperature. Deviating from this — in either direction — causes problems.
Too small a diameter:
- Stove smokes back into the room
- Incomplete combustion
- Risk of carbon monoxide poisoning
Too large a diameter:
- Slow draft — flue gases cool too quickly
- Excessive condensation inside the pipe
- Tar and creosote deposits build up faster
- Higher risk of chimney fire
Step 1: Check Your Stove's Manual First
Before measuring anything, look at your stove's installation manual or data sheet. Every stove specifies a minimum flue outlet diameter and often a recommended flue height. This is your starting point — never go below this diameter.
Common stove outlet diameters are: 80mm, 100mm, 125mm, 150mm, 180mm and 200mm.
If you no longer have the manual, look for the model number on the data plate (usually on the back or underside of the stove) and search for the specification sheet online, or contact the manufacturer directly.
Step 2: Measure Your Existing Flue Outlet
If you are replacing an existing flue system, measure the internal diameter of the flue collar on top of your stove — not the outside diameter of the pipe.
How to measure:
- Use a metal ruler or tape measure
- Measure straight across the inside of the circular opening at its widest point
- This is your internal diameter in millimetres
- Round up to the nearest standard size if the measurement falls between sizes
Important: Always measure the internal diameter. Flue pipes are sold by their internal diameter — a "150mm flue pipe" has an internal opening of 150mm.
Step 3: Account for the Full Flue System
The diameter must remain consistent throughout the entire flue run — from stove outlet to chimney cap. Never reduce the diameter at any point along the route. A reducer fitting at the top of the system is acceptable in some situations, but only if explicitly permitted by your stove manufacturer.
If you are connecting to an existing masonry chimney, the relevant measurement is the internal diameter of the chimney flue, not the external dimensions of the chimney breast.
Step 4: Consider Flue Height
Diameter and height work together to create draft. A taller flue creates stronger natural draft. If your installation has a shorter than ideal flue height — for example in a bungalow or flat roof property — you may benefit from a slightly smaller diameter to compensate and maintain adequate draft velocity.
As a general rule:
- Minimum recommended flue height: 4 metres from stove outlet to chimney cap
- For pellet stoves: minimum 3 metres but check manufacturer specifications carefully as pellet stoves produce much cooler flue gases than wood-burning stoves
Step 5: Flexible Liner Inside an Existing Chimney
If you are inserting a flexible liner into an existing masonry chimney, the calculation is slightly different. The liner must fit inside the existing chimney flue with enough clearance to allow installation, while being large enough to serve the stove correctly.
Minimum liner diameter = stove outlet diameter
Never go smaller than the stove outlet diameter when choosing a liner. If the existing chimney is very large internally, a correctly sized liner will actually improve performance compared to using the unlined masonry flue directly.
Common Diameter Reference Table
| Stove Output | Typical Flue Diameter |
|---|---|
| Up to 5 kW | 80mm or 100mm |
| 5–10 kW | 100mm or 125mm |
| 10–15 kW | 125mm or 150mm |
| 15–20 kW | 150mm or 180mm |
| 20 kW+ | 180mm or 200mm |
These are general guidelines only. Always follow your stove manufacturer's specifications above all else.
Oval and Square Flue Outlets
Some stoves — particularly older models and certain continental European designs — have oval or rectangular flue outlets rather than round ones. In this case you will need a reducer adapter to convert to a standard round diameter. The correct round diameter to use is the one with the closest equivalent cross-sectional area.
Formula: Cross-sectional area of oval = π × (width/2) × (height/2)
If this is unclear, contact our specialists and we can advise on the correct adapter and diameter for your specific stove model.
Standards by Country
Flue diameter requirements are influenced by national building standards. Here is what applies in each of our markets.
