Important things to consider when purchasing a gas furnace are the size and insulation status of the space the gas furnace is intended to heat, as well as costs and availability of gas versus fuel oil in the area. The cost of electricity also needs consideration when purchasing a furnace.
Gas furnaces have the potential to be highly efficient methods of heating a home, with 95 percent average fuel usage efficiency in newer models. However, the size of the furnace required and its effective efficiency depend greatly on the size of the space to be heated; a gas furnace that is undersized for the space it heats runs more than a properly sized furnace, reducing overall efficiency and comfort while increasing costs. Roughly 20 to 30 British thermal units per square foot are required to heat a home in moderate climes.
The efficiency of a gas furnace is also affected greatly by the insulation present in the structure to be heated. Improving insulation for a home can allow for a much smaller furnace to heat the home sufficiently, and the costs of insulation can be lower than those for a larger furnace and the fuel to run it.
Finally, prospective gas furnace buyers should consider whether or not fuel oil or electrical heat are more economical choices. Though fuel oil furnaces are less efficient than most gas furnaces, regional variations in fuel prices sometimes mean that fuel oil or electric heat are more cost effective in the long run than a gas furnace.