Efficiency

A high efficiency furnace or boiler will heat your home with less fuel and low emissions, thereby cutting the cost of fuel as well as maintenance. The estimated efficiency of a heating unit can usually be found on the label, listed as the AFUE (Annual Fuel Use Efficiency). This number can give you an idea of where to start with efficiency, but is unfortunately fairly inaccurate. Due to not accounting for "Idle Loss", systems with identical AFUE ratings can differ in performance by as much as 25%.

There are also several more industry-specific efficiency numbers that are used by manufacturers, such as ASME testing (in the US) and the European EN 303-5 testing standard for wood boilers. These can offer a bit more accurate comparison, but they have their own issues. In fact, the ASME test is being retooled this year for better accuracy, which may cause a lot of efficiency numbers to change.

When comparing different makes and model of furnaces, the ASME efficiency value is the most consistent. However, know that it is not an iron-clad comparison and real-world values may vary by a wide margin.