Annual fuel utilization efficiency (AFUE) is a way of measuring how much heat a furnace or boiler can make from the fuel it burns. The higher the AFUE number, the more efficient the furnace is at turning fuel into heat for your home.
Think of it like making a cake - if you have a recipe that uses all the cake mix and makes a perfect cake, that's like having a high AFUE furnace. But if some of the mix goes to waste and the cake turns out lumpy or burnt, that's like having a low AFUE furnace.
AFUE is always shown as a percentage, with the maximum possible being 100%. So if your furnace has an AFUE of 80%, it means 80% of the fuel it burns turns into heat, while the other 20% is lost as waste.
When you're shopping for a furnace or boiler, it's important to look for one with a high AFUE rating so you can save money on your energy bills and reduce your carbon footprint.