Learn & Understand

Following the Energy: Heat Balances and Sankey Diagrams

Heat loss is the energy that never reaches the process — and tracking exactly where it goes is the discipline of the heat balance, an accounting of every unit of energy in and out. Engineers even invented a striking way to picture it: the Sankey diagram, a flowing map that makes waste impossible to ignore.

The Energy Audit

A heat balance is an energy audit: it accounts for all the fuel energy entering a system and traces where every portion ends up — useful output, flue-gas loss, radiation, unburned fuel, and more. Because energy is conserved, the columns must balance, and any energy not delivered usefully is a loss with a specific address. The audit turns vague inefficiency into a itemized list.

The Big Losses

In most combustion systems, a few losses dominate. The largest is usually the heat carried away by hot flue gas up the stack; others include radiation and convection from hot surfaces, heat in ash or unburned fuel, and moisture losses. Naming and sizing each one shows where attention pays off — there is little point insulating a wall while a torrent of heat escapes up an untended chimney.

The Sankey Diagram

To make a heat balance vivid, the engineer Matthew Sankey popularized a diagram in which energy flows are drawn as bands whose width is proportional to their magnitude. A wide band of input splits into a narrower band of useful output and several branches of loss, each visibly sized. At a glance, a Sankey diagram shows how much energy is wasted and where — a picture worth many tables.

Loss as Opportunity

Every quantified loss is a target. Once a heat balance reveals that, say, a large fraction of energy leaves as hot flue gas, recovery becomes worthwhile — capturing that heat to preheat air or feed water. Measuring loss precisely, in kilowatts and percent, converts a vague sense that “the system wastes energy” into concrete, actionable engineering.

Quantifying the Loss

To find the heat lost from a known input and efficiency, use the Heat Loss Calculator. Establish the input with the Heat Input Calculator, and the efficiency behind it with the Boiler Efficiency Calculator.