Size and efficiency are the most important factors to consider when shopping for a gas furnace, according to Consumer Reports. If a furnace is too large, it wastes energy by cycling on and off more than necessary.
A furnace that is too large can create unwanted noise if it causes air to flow through inappropriately sized ducts. A furnace's ability to convert gas into heating energy efficiently is also a significant consideration, particularly for those consumers concerned about environmental impact. More efficient furnaces tend to cost more up front, but the savings on energy bills over the life of the furnace often make up for the greater initial outlay.