Stove flue sizing
An independent reference on flue sizing, draught measurement and liner ratings, built from published standards, building regulations and the sizing rules manufacturers apply across their ranges.
Not a design service, not a product shop, and not a substitute for a site-specific assessment by a competent installer or building control body.
A sealed stove's chimney must be at least as big as the appliance outlet — roughly 100-130% of the outlet area.
Every stove leaves the factory with a flue outlet of a fixed diameter, and that outlet is the one measurement you may not go below. A chimney that necks down past the outlet throttles the appliance: smoke spills on lighting, the glass tars up and draught readings sag.
The working rule is that the flue run should carry 100-130% of the outlet's cross-sectional area, all the way from the stove collar to the terminal. A modest increase above the outlet is normal practice; a decrease is not.
Only after the outlet is satisfied does output in kilowatts come into play, because output is what pushes you from one standard diameter to the next.
For insulated twinwall, 125 mm covers about 5-8 kW and 150 mm covers about 8-12 kW; open fires run 175-200 mm.
Manufacturers converge on the same bands because flue gas volume tracks heat output. A 5 kW room heater and a 10 kW stove do not move the same amount of smoke, and the flue has to move all of it without throttling the fire.
Oversizing is not a free pass either. A very large flue on a small stove lets gases cool and slow before they reach the terminal, which is why the bands are bands rather than minimums.
Open fires sit outside this logic: with no sealed outlet to match, they are sized from the fireplace opening instead, and normally land on liners of 175-200 mm.
| Appliance and duty | Output or position | Flue size or limit | The check behind it |
|---|---|---|---|
| Sealed wood stove | 5-8 kW | 125 mm internal diameter | Draught 4-6 Pa at the outlet |
| Sealed wood stove | 8-12 kW | 150 mm internal diameter | Draught 4-6 Pa at the outlet |
| Open fire | opening area sets the size | 175-200 mm diameter | Flue area 12-20% of the opening |
| Single-wall stove pipe | 125 mm diameter | 375 mm clearance to combustibles | Clearance equals three times diameter |
| Insulated twinwall | 125-150 mm diameter | 50-65 mm clearance | Manufacturer's stated distance |
| Chimney terminal | more than 2.3 m from a boundary | at least 1 m above ground | UK Approved Document J |
| Chimney terminal | within 2.3 m of a boundary | at least 2.1 m above ground | UK Approved Document J |
| Liner for wood service | EN 1856-1 designation | T400 N1 | 400 °C continuous, 600 °C soot-fire peak, 20 Pa |
Draught is measured at the appliance outlet in pascals: 4-6 Pa minimum for a sealed stove, 5-10 Pa for an open fire.
A manometer or digital draught gauge connected at the appliance outlet tells you whether the chimney is actually pulling. The reading is taken with the fire established and the flue warm, not on a cold chimney at first light.
Below 4 Pa on a sealed stove the fire will be hard to light, prone to smoking back and lazy to respond to the air controls. An open fire wants a little more — 5-10 Pa — because it has no sealed door to shape the airflow.
If the number falls short, the cause is almost always one of five things: a flue too small for the outlet, a run with too many bends, a terminal sitting too low, a soot restriction, or a room too airtight to feed the fire. Each cause is worked through in the draught guide linked below.
The figures cited on this page trace to EN 1856-1, UK Approved Document J, Ecodesign requirements for solid-fuel stoves and standard sweeping practice.