Boiler Stack Temperature Too High: Causes & Fixes

What Should the Stack Temperature Be?

The stack (exhaust) temperature is one of the most important indicators of boiler efficiency. The ideal stack temperature depends on the boiler type and fuel, but as a rule of thumb:

Causes of High Stack Temperature

• Fouled boiler tubes (most common): Scale on the water side or soot on the fire side of the tubes acts as insulation, reducing heat transfer. The heat that should be absorbed by the water instead exits with the flue gas, raising the stack temperature. Fix: clean the tubes — mechanically or chemically.
• Excess combustion air: Too much air means more flue gas mass flowing through the boiler. The extra air absorbs heat and carries it up the stack. Fix: tune the burner air-to-fuel ratio. Target 3–5% O₂ in flue gas for gas, 5–8% for oil.
• Defective or missing economiser: An economiser recovers heat from the flue gas to preheat feedwater. If the economiser is fouled, bypassed, or missing, the stack temperature rises. Fix: inspect, clean, or repair the economiser.
• Burner problems: A burner firing at high excess air, or one with poor fuel atomisation (oil), produces hotter flue gas. Fix: perform a combustion analysis and tune the burner.
• Low feedwater temperature: If the feedwater is cold (no deaerator or economiser), the temperature differential across the boiler is larger, but the stack temperature also rises because less heat is absorbed per kg of steam. Fix: preheat feedwater with an economiser or deaerator.
• Operating above design pressure: Higher steam pressure means higher saturation temperature, which means the flue gas must be hotter to transfer heat. This is normal but should be verified against the boiler's design conditions.
• Flue gas bypass: If the baffles inside the boiler are damaged, flue gas can bypass the heat transfer surfaces (a 'short circuit'), exiting at a higher temperature. Fix: inspect and repair internal baffles.

Diagnostic Steps

1. Record the baseline: Measure the stack temperature when the boiler is clean and operating at normal load. This is your baseline for future comparison.
2. Perform a combustion analysis: Use a flue gas analyser to measure O₂, CO₂, and CO. If O₂ is above 5% (gas) or 8% (oil), excess air is the problem — tune the burner.
3. Check for soot: If CO readings are high or the flame is smoky, soot is forming on the fire-side tubes. Open the boiler and inspect — clean if soot is present.
4. Check water-side scale: If the fire-side is clean but the stack temperature is still high, scale may be on the water side. Inspect through the manhole or handhole. Clean chemically if scale is present.
5. Check the economiser: Measure temperature before and after the economiser. If there's little or no temperature drop, the economiser is fouled or bypassed.

Prevention

• Trend the stack temperature daily — a rising trend indicates gradual fouling. Clean before the temperature exceeds the action level.
• Maintain proper water treatment — scale prevention is far cheaper than tube cleaning.
• Tune the burner annually (or more frequently for oil-fired boilers) — combustion drift causes excess air and high stack temperatures.
• Install a flue gas analyser for continuous monitoring — O₂ and temperature trends provide early warning of efficiency loss.
• Clean fire-side tubes annually (oil-fired boilers may need more frequent cleaning due to soot).
• Install an economiser if the boiler doesn't have one — it's one of the best efficiency investments available.