The Annual Fuel Utilization Efficiency (AFUE) guide is not a steady measure of efficiency, but instead attempts to represent the actual, season-long, average efficiency of equipment. It is a ratio of useful energy output to energy input, expressed as a percentage. For example, a 90% AFUE for a gas furnace means it outputs 90 BTUs of useful heating for every 100 BTUs of Natural Gas input, the rest is wasted. A higher AFUE means higher efficiency. In your new home you should install a boiler with the highest AFUE score possible.