When purchasing a new furnace there are several things you should consider.
First ensure you get the proper size equipment for your home.
For a gas furnace, which most of the US uses, your first choice will be the heat output of the furnace, measure in British Thermal Units, or BTUs. For example, furnaces might range from 40,000 BTUS to 100,000 BTUs. For a 1500 square foot home, a 60,000 BTU furnace is probably adequate. Large homes often have multiple "zones" and multiple furnaces to heat them. During the initial visit we will calculate the required size of furnace and in many cases replace the existing unit with one of the same size.
Sizing the equipment properly is important. If your furnace is too big for your house, it will run at full power and turn off long before it gets to an efficient heat and level. The next choice will be in efficiency ratings.
Second consider the different efficiency ratings available.
When it comes to cost, you will find about a 10% differential between a 90% furnace and a 94-95% furnace. If your furnace was designed and installed back in the early 90s or late 80s it typically had an efficiency rating of about 60-80%, meaning that 80% of the gas or oil burned went to heat your house, and the rest was lost as waste heat up your chimney or vent. Today you will find furnaces with 90, 92, 94 and 95% efficiency ratings.
Third, decide whether you want a single stage or 2 stage furnace.
Older furnaces are single-stage furnaces. A single stage furnace runs and burns at almost full power, all the time it is on. This is an inefficient method, since you would save fuel by starting to burn slowly, then as the unit warmed up, burning more, and doing the opposite at the end of the cycle. The same is true regarding the ambient temperature outside. If it is not too cold, you can heat your house with less power by burning less fuel, instead of burning at full power.
Now there are two stage furnaces on the market which can run at about 65% of capacity or 95%, depending on the circumstances. They still make single stage furnaces, but when it comes time to choose, you may want to consider the 2 stage option.
The final option is the blower.
The blower is a fan that pushes all the hot air from the air handler, through the ductwork, and out through the vents in your house. Old blowers ran at a single speed. The newest blowers are available variable speed, meaning they can run low, high, or anywhere in between, depending on what stage the furnace is in and how heat can most efficiently be sent into the home. These motors are very efficient and use less energy in most cases than a 100W light bulb, meaning you save money in gas and in electricity costs. The variable speed blowers run more often, and at lower speeds, than old-fashion blower, which keeps the warm air circulating better in your home instead of piling up at your ceiling, away from where you are sitting. It also means your air moves through your filter more often, resulting in cleaner air.