Boilers, furnaces and stoves: Science and limits

Why a near-perfect boiler still wastes most of the flame

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

Boilers, furnaces, kilns and stoves burn fuel to release its chemical energy as hot gas, then pass that heat into water, steam, a product or a cooking pot. Heat made up almost half of the world's final energy use and 37% of energy-related CO2 emissions in 2024 (IEA).

The conversion, step by step

Combustion
Chemical (gas, oil, coal, LPG, wood) → Heat in hot flue gas · Near complete in a well-tuned burner; the DOE benchmark assumes about 3% oxygen left in the flue gas³Heat transfer
Hot flue gas → Steam, hot water, hot air or the product (metal, glass, clinker, food) · Industrial boilers with economisers about 86 to 90% (DOE benchmarks by fuel, higher heating value basis); condensing home boilers 90 to 98.5% AFUE; open fires far lower³⁴Distribution
Steam or hot water → Heat at the point of use · Losses through pipes, steam traps, walls and openings; 36% of US process heating energy is lost overall²End use
Delivered heat → Drying, melting, cooking, space heating · Cooking: LPG stoves about 42 to 70% (World Bank estimates); open fires and the simplest stoves sit in the lowest ISO lab tier (below 10%)⁶¹³

The science

Burning joins carbon and hydrogen in the fuel with oxygen, releasing heat and producing CO₂ and water vapour. The hot gas transfers heat by radiation, which dominates in hot furnaces, and by convection. Losses come from heat leaving up the flue, unburnt fuel, and heat escaping through walls; above about 540 C (1,000 F) radiation losses from equipment become significant. A key detail is the water vapour in the flue: it carries latent heat. A condensing boiler cools the exhaust below its dew point so the vapour turns to liquid and gives up that heat, which is why it beats older boilers. For natural gas the gap between higher and lower heating value (with and without that latent heat) is about 10% (calculated from methane's 55.5 and 50.0 MJ/kg).

Limits set by science

Limit	Value	Why it exists	Best today
First-law (energy) limit	Close to 100% of the fuel's higher heating value if flue gas is cooled to room temperature and all vapour condensed⁴	Heat is the easiest energy to produce: nothing in physics stops a boiler capturing nearly all the released heat.	Condensing boilers reach up to 98.5% AFUE (annual fuel utilisation efficiency).
Second-law (exergy) limit	Useful work content of heat = 1 - T0/T. Hot water at 60 C (333 K) with surroundings at 10 C (283 K): about 15%. Steam at about 240 C (513 K): about 45% (calculated)⁴¹²	A flame is far hotter than the water it heats, so using it to make 60 C water throws away most of its ability to do work. A heat pump with a COP of 3 delivers the same heat from about a third as much energy input, as electricity (calculated).	A 95% efficient boiler making 60 C water has an exergy efficiency of only about 14% (calculated).
Flue gas dew point and corrosion	Condensate from gas is acidic⁴	Condensing requires corrosion-resistant heat exchangers and may need a new vent or relined chimney; non-condensing boilers keep exhaust hot to avoid this.	Mid-efficiency non-condensing units: 80 to 83%.
High-temperature losses	About 60% of unrecovered US industrial waste heat is below about 230 C (450 F)²	Metal smelting and melting furnaces run at about 430 to 1,650 C (800 to 3,000 F); hot products and flue gas carry heat away that is hard to reuse.	Recuperators, regenerative burners and economisers recover part of it.

Sources

Quadrennial Technology Review 2015, Chapter 6 Technology Assessment 6I: Process Heating US Department of Energy, 2015
Benchmark the Fuel Cost of Steam Generation, Steam Tip Sheet #15 (DOE/GO-102012-3391) US Department of Energy, Advanced Manufacturing Office, 2012
Furnaces and Boilers US Department of Energy, Energy Saver, 2025
Design and experimental investigation of an LPG injera baking stove for sustainable energy use in Ethiopia (Abdella, Alemu and Dino), Discover Energy 4, 32 (quotes World Bank stove efficiency estimates) Springer Nature, 2024
Heat Pumps Step Up: What the IEA's Heat Pump Monitor 2026 Means for Heating, Industry and Energy Security IEA Heat Pumping Technologies TCP, 2026
Voluntary Performance Targets for cookstoves (ISO 2018 tiers) Clean Cooking Alliance, 2018