High stack temperature: the failure signature
Stack temperature is the cheapest efficiency indicator a boiler has. Read on its own it is ambiguous, because it rises with firing rate. Read at a fixed firing rate and flue O2, a slow rise points to fouled heating surfaces, while a rise that comes with high O2 points to excess air. DOE’s steam tip sheets recommend exactly this trend.
Illustrative pattern · simulated, not plant data
Diagnose this with AI
Try Free Boiler Diagnostic Upload a trend export (CSV or Excel) or a screenshot of the trend around the event. InduQT lines up which signal moved first and compares it with these variants.
Where it shows up
- Stack or flue temperature high alarm (Fireye Nexus L59 where an O2 interface is fitted)
- Rising fuel use for the same steam output
- Visible smoke or soot at the stack
Signals to trend
| Signal | What it does in this failure |
|---|---|
| Stack (flue gas) temperature | The symptom. Compare at the same firing rate, not at different loads. |
| Firing rate | Use it to pick comparable periods. |
| Flue gas oxygen | High O2 with high stack temperature means excess air; normal O2 with rising stack temperature means heat transfer is falling. |
| Steam pressure | Higher operating pressure raises water temperature and therefore stack temperature; check it has not changed. |
| Steam flow | Steam output per unit of fuel falls as efficiency drops. |
Variants: what moves first, and what it points to
1Waterside scale
- Points to it
- Stack temperature creeps up over weeks or months at constant firing rate and O2.
- Check
- Trend stack temperature at a constant firing rate and excess O2, as DOE Tip Sheet #7 recommends; inspect tubes at shutdown and review water treatment.
2Fireside soot
- Points to it
- Rise follows a period of low O2, smoky combustion or oil firing.
- Check
- Inspect and clean fireside surfaces; correct the combustion setting that produced the soot.
3Excess air
- Points to it
- O2 higher than the commissioned value; stack temperature and fuel use up together.
- Check
- Have a combustion analysis done and the burner retuned. DOE Tip Sheet #4 gives the rule of thumb of about 1% efficiency for each 15% reduction in excess air or 40°F reduction in stack temperature.
4Step change after maintenance
- Points to it
- Stack temperature jumped after work on the boiler, at the same firing rate and O2.
- Check
- Confirm baffles, turbulators, seals and any economizer bypass were restored as designed.
How to read the trend
- Pick comparable pointsSelect periods at the same firing rate and steam pressure, months apart.
- Check O2 firstIf O2 rose, the cause is combustion air; if O2 is steady, look at heat transfer.
- Look at the shapeSlow creep suggests fouling; a step change suggests a mechanical or tuning change.
- Plan the checkCombustion analysis now; tube inspection at the next shutdown.
Evidence to collect
- Stack temperature, firing rate and O2 trend over months
- Last combustion analysis report
- Water treatment records
- Maintenance history
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.
Related
- Components involved: Boiler tubes and heating surfaces, Flue oxygen probe and stack sensor, Air damper, actuator and linkage
- Flame loss while firing: the failure signature
- Combustion airflow faults: the failure signature
- Feedwater and drum level instability: the failure signature
- Steam pressure drop: the failure signature