Draft & Airflow Control
Draft is the driving force that moves air into the burners and flue gas out through the stack. Getting draft right is a continuous balancing act — too little and the firebox goes positive pressure, which is dangerous; too much and you pull in excess air, losing efficiency. This page explains what creates draft, how it's controlled, and what happens when it goes wrong.
What creates draft
Draft is the difference in pressure between the inside of the firebox and the atmosphere outside. In a natural-draft heater, this pressure difference is created by the stack — a tall vertical column of hot flue gas that is less dense than the surrounding cold air. The density difference generates a buoyancy force that draws flue gas upward and creates negative pressure (suction) at the base of the stack and in the firebox.
The magnitude of natural draft depends on three factors: stack height (taller stacks create more draft), flue gas temperature (hotter gas is less dense and creates more buoyancy), and atmospheric conditions (cold days increase draft; hot, humid days reduce it). This means natural draft is not constant — it varies with firing rate, weather, and ambient temperature.
Draft systems — natural, forced, and induced
Stack buoyancy is the only driver. Air enters the burners through the air registers, drawn in by the negative pressure created by the hot rising flue gas column. A damper in or near the stack controls overall draft by varying the restriction to flue gas flow.
Operator controls: Stack damper (overall draft), individual burner air registers (air distribution per burner).
Sensitivity: Draft responds to ambient temperature changes and firing rate changes. Cold snaps increase draft; high ambient temperatures reduce it. Draft is highest when the heater is cold and firing rate is low (cold start); lowest on hot days at high firing rate.
Advantages
- Simple — no rotating equipment
- Reliable — no fan failures
- Low maintenance
Limitations
- Draft varies with conditions
- Limited turndown capability
- Less precise air control
A fan forces combustion air into a plenum (windbox) below or beside the burners. Air enters the burners under positive pressure. The stack still draws flue gas upward, but the primary airflow driving force is the fan, not buoyancy.
Operator controls: FD fan speed or inlet guide vanes (total airflow), individual air registers (distribution), stack damper (firebox pressure balance).
Sensitivity: More controllable and consistent than natural draft. But: forced draft heaters run with the firebox at near-zero or slight positive pressure — stack damper control is critical to maintain a slight negative pressure and prevent blowback.
Advantages
- Consistent air supply
- Better turndown range
- Supports O₂ trim control
Limitations
- Fan failure = forced shutdown
- More complex controls
- Positive pressure risk
A fan is placed in the flue gas path — typically between the convection section and the stack — and pulls flue gas through the heater. This actively increases the negative pressure in the firebox, drawing air through the burners.
Operator controls: ID fan speed (overall draft and airflow), burner air registers (distribution), dampers (balance).
Sensitivity: ID fans handle hot, corrosive flue gas — they are subject to erosion and fouling. Fan blade inspection and condition monitoring are important. Loss of ID fan on a heater that relies on it for draft is immediately serious — firebox can go positive within seconds.
Advantages
- Strong, controllable draft
- Good for high-resistance flue paths
- Supports low-NOx systems
Limitations
- Fan in hot gas — erosion risk
- Complex maintenance
- Draft loss on fan failure
The stack damper
The stack damper is the primary control for overall draft in a natural-draft heater. Opening the damper increases draft (more negative firebox pressure, more air drawn through burners). Closing it reduces draft. It sits in the stack or breeching and is operated either manually or by an automatic draft controller that maintains a set firebox pressure.
On forced-draft heaters, the stack damper serves a different role — it balances the firebox pressure between the positive-pressure air supply from the fan and the stack outlet. Incorrect damper position on a forced-draft heater can push the firebox positive, causing blowback at burner fronts and inspection ports.
Stack damper — what can go wrong
| Fault | Symptom | Consequence | Response |
|---|---|---|---|
| Damper stuck closed | Draft reduces to near-zero or goes positive despite attempts to open; flue gas temperature rises | Firebox goes positive — blowback risk at burner fronts. Reduced airflow, increasing CO. In extreme cases, overtemperature of convection section. | Reduce firing rate immediately. Attempt manual operation. If firebox goes positive, emergency shutdown may be required. |
| Damper stuck open | Excessive draft — cannot reduce air below a minimum; cold ambient temps worsen it | Too much excess air — efficiency loss and potential flame instability at low loads. Closing air registers at burners is the only mitigation. | Operate at elevated excess air. Reduce via burner air registers. Schedule damper repair at next opportunity. |
| Damper falls partially off seat | Draft reading erratic; sudden increase in O₂ | Loss of draft control. Unexpected airflow increase. Can cause lean flame conditions. | Investigate immediately. Confirm damper position visually if safe to do so. Adjust burner air registers to compensate. |
| Damper drive failure (auto control) | Draft controller output changing but draft unresponsive | Loss of automatic draft control — draft will vary with firing rate and ambient conditions uncontrolled. | Switch to manual control. Monitor draft locally. Notify instrument technician. |
Measuring and monitoring draft
Draft is measured in millimetres of water column (mmH₂O) or Pascals. The measuring points that matter most to the operator are:
| Location | Typical reading | High (more negative) | Low / positive |
|---|---|---|---|
| Firebox floor / burner level | −2 to −5 mmH₂O | Excessive draft — too much air being drawn through burners. Stack damper may be too open. | Positive or near-zero — risk of blowback. Investigate immediately. |
| Bridgewall (radiant exit) | −1 to −3 mmH₂O | Higher than expected for firing rate — convection section may be clear; stack damper too open. | Lower than expected — possible convection section fouling or blockage restricting flue gas flow. |
| Stack base (above convection section) | −3 to −10 mmH₂O | High draft available — useful for cold starts. Watch for excessive air ingress through any openings. | Reduced draft — check for stack blockage, cold ambient conditions, or low stack temperature. |
Draft upsets — recognition and response
Draft is rarely a slow-moving variable. Changes in firing rate, ambient conditions, or damper position affect it quickly. The following upsets are the ones operators encounter most frequently.
Loss of draft — firebox going positive
The most serious draft upset. A fired heater firebox that goes positive pressure will push hot combustion gases and flame out through any opening — burner fronts, peepholes, inspection doors. This is a burn hazard to personnel and can ignite any fuel gas leak in the vicinity.
Causes: stack damper closes unexpectedly; convection section plugged with soot; ID fan trips on a fan-draft heater; sudden very high firing rate; cold stack on startup before draft is established.
Response: Reduce firing rate immediately. Open stack damper if accessible and safe. If firebox remains positive and the condition cannot be corrected quickly, initiate controlled shutdown. Do not approach peepholes or inspection openings while firebox pressure is positive or unknown.
Excessive draft
Excessive draft draws more air than needed through the burners, increasing excess air beyond the optimal range. Effects include efficiency loss, flame instability (lean flame, risk of lifting), and increased NOx at high temperatures. This is more common in cold weather, during low-load operation, and immediately after startup when the stack is cold but rapidly heating.
Response: Close stack damper partially. If automatic draft control is fitted, lower the setpoint. Closing individual air registers is also an option but changes the air distribution between burners rather than the total draft — address at the damper first.
Draft variation with firing rate
In a natural-draft heater, draft increases as firing rate increases (hotter flue gas → more buoyancy) and decreases as firing rate is reduced. This means that when firing rate is reduced during a turndown, both fuel and air reduce together — but the relationship is not perfectly linear. Monitor O₂ after significant firing rate changes and re-trim the stack damper or air registers as necessary.
Practical draft management during operations
| Observation | Diagnosis | Adjustment |
|---|---|---|
| Stack O₂ high, draft reading high, flame lean / short | Excessive draft or air register too open | Close stack damper slightly. Then trim individual air registers if needed. Verify O₂ stabilises in target range. |
| Stack O₂ low, CO rising, flame lazy / long | Insufficient air — could be insufficient draft or registers too closed | Open air registers first (fastest response). If O₂ still low after register adjustment, check draft — open stack damper if draft is the limiting factor. |
| Draft readings drop after firing rate increase | Normal — convection section absorbing more heat as firing increases, initially reducing flue gas temperature before it stabilises | Allow 10–15 minutes to stabilise. Re-check draft and O₂ and adjust damper as needed. This is normal transient behaviour. |
| Firebox draft lower than bridgewall draft | Abnormal — suggests air infiltration into the convection section between the two measurement points, or a blocked firebox tap | Investigate for air ingress at convection section access doors or casing cracks. Verify tap is clear. |
| Draft varies erratically, not following firing rate changes | Stack damper control instability; wind effects on stack; convection section partial blockage creating variable restriction | Switch to manual damper control to stabilise. Investigate for blockage if problem persists. |