Furnace and boiler efficiency is measured by ANNUAL FUEL UTILIZATION EFFICIENCY (AFUE). The higher the number, the more heat it can achieve. Since efficient furnaces generate fewer emissions, environmental considerations might also influence your decision. A furnace with a 90% AFUE might cost $1,000 more than a similar size unit with an 80% AFUE. Costs can be recouped through lower fuel bills over the life of the furnace. Upgrading a furnace or boiler from 56% to 90% efficiency in an average cold-climate house will save 1.5 tons of CO2 emissions each year if you heat with gas, or 2.5 tons if you heat with oil (Energy.gov)

Reliability is especially important because when a furnace failed, 77% needed significant work. A majority of those broke down completely, with nearly a third producing no heat for more than a day. For 40%, the repair cost $150 or more. (Consumer Reports)