This is the ratio of BTU output divided by BTU input on a particular boiler. This includes all the heat loss (e.g. radiation and convection losses) from a boiler and is typically used in a manufacturer's catalogue. Fuel-to-steam efficiency is a readily verifiable number and is often used in efficiency guarantees by various manufacturers. It is the ideal "Fire-rate" efficiency, which is tested for about 10 minutes with a stack analyzer.