Boiler Energy Losses
Where does the energy go? Learn the major sources of boiler energy loss, from stack losses and blowdown to radiation and incomplete combustion, and how to quantify each.
The Energy Balance
A boiler converts fuel energy into steam energy. The difference is lost. A typical industrial boiler achieves 80 to 85% efficiency, meaning 15 to 20% of fuel energy is wasted. Understanding where these losses occur is the first step to reducing them.
The major loss categories are: stack losses, blowdown losses, radiation losses, incomplete combustion, and unreturned condensate.
Stack Losses (10 to 20%)
The largest single loss. Hot flue gases carry energy up the stack and into the atmosphere. Two factors determine stack loss: stack temperature and excess air.
Higher stack temperature = more energy wasted. Higher excess air = more gas to heat and more energy carried away. Minimising both is key to efficiency.
| Stack Temperature | Approximate Loss |
|---|---|
| 150 degrees C | ~6 to 8% |
| 200 degrees C | ~10 to 12% |
| 250 degrees C | ~14 to 16% |
| 300 degrees C | ~18 to 20% |
Blowdown Losses (1 to 5%)
Blowdown removes hot boiler water to control TDS. The energy in the blowdown water is lost unless recovered. A boiler blowing down 5% of its feedwater at 10 bar loses about 3% of fuel input energy.
Reduce by: improving feedwater quality, using conductivity-controlled continuous blowdown, and installing blowdown heat recovery (flash vessel plus heat exchanger).
Radiation and Convection Losses (1 to 5%)
Heat radiates from the boiler casing, steam pipes, valves, and fittings. A well-insulated boiler loses about 1 to 2% at full load. At low load, the percentage increases because radiation losses are constant while steam output decreases.
Uninsulated pipes and valves add significantly. An uninsulated 100mm steam pipe loses about 150 W per metre. Over 100 metres, that is 15 kW, or roughly $8,000 per year in wasted fuel.
Incomplete Combustion (0 to 5%)
If combustion air is insufficient, fuel doesn't burn completely. Carbon monoxide (CO) in the flue gas indicates incomplete combustion. Even 0.1% CO in flue gas represents significant fuel waste.
Monitor CO in flue gas alongside O2. If CO rises while O2 is at target, the burner may need cleaning, adjustment, or the fuel/air mixing may be poor.
Unreturned Condensate (0 to 10%)
Condensate that is dumped to drain instead of returned wastes the sensible heat it contains. Dumping 1,000 kg/hr of condensate at 90 degrees C wastes about 314 MJ/hr compared to using makeup water at 15 degrees C.
This is often overlooked because it's not a direct boiler loss. But it increases fuel consumption because the boiler must heat cold makeup water to boiling point.
Loss summary
Stack losses: 10 to 20%. Blowdown: 1 to 5%. Radiation: 1 to 5%. Incomplete combustion: 0 to 5%. Condensate loss: 0 to 10%. Total losses: 15 to 35%. Target efficiency: 85%+. Use a boiler efficiency calculator to benchmark your performance.
Tip: Start by measuring your stack temperature and excess O2. These two numbers tell you most of what you need to know about your boiler efficiency. If stack temp exceeds 200 degrees C or O2 exceeds 5%, there is room for improvement.