Optimising stack damper and furnace draught
Stack damper and draught optimisation is the tuning of dampers and draught control so a furnace or boiler runs at the correct, slightly negative pressure with the minimum air drawn through it. It stops the excess tramp air and over-draughting that carry heat up the stack and unbalance combustion.
What it is
Draught is the pressure difference that pulls combustion gases through a furnace and up the stack, set by stack height, fans and dampers. Optimisation means controlling that draught to hold furnace pressure at its target — typically just below atmospheric — so the unit takes in only the combustion air it needs, not uncontrolled tramp air leaking through openings. Damper positions and any induced or forced fans are tuned together to achieve it.
Why it is done
Too much draught pulls excess cold air through the firing zone and out of the stack, carrying away heat and raising flue-gas losses, while under-draught lets combustion products spill and burn incompletely. Loose dampers and open seals admit tramp air that the burner never accounts for, skewing the oxygen reading and wasting the heat used to warm that air. Setting draught correctly recovers stack losses, steadies combustion and reduces the fan power spent moving unnecessary gas.
How it is done
Furnace pressure is measured and a target set, then the draught control loop — damper or variable-speed fan — is tuned to hold it steadily across the firing range. Air in-leakage points are surveyed and sealed so the measured draught reflects real combustion air. Damper linkages and characterisation are checked so the control has authority without hunting, and the relationship between draught, flue-gas oxygen and stack temperature is reviewed together rather than in isolation. The settings are then confirmed at low, mid and high fire.
- Measure furnace pressure
- Set draught target
- Tune control loop
- Seal tramp-air leaks
- Check damper authority
- Confirm across firing range
What to watch for
Tuning draught at one load leaves it wrong elsewhere, since the balance shifts with firing rate. Chasing a low flue-gas oxygen figure by closing dampers without sealing tramp-air leaks simply moves the leak air elsewhere and risks a positive-pressure furnace that spills hot gas. Sticky or poorly characterised dampers cause the control to hunt, swinging draught and combustion together.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
How to Improve Boiler Efficiency: A Practical Guide · Flue Gas Loss · Boiler Efficiency · Control Loop
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