Types of Furnaces
Most older furnaces are one- or single-single stage, meaning that your furnace heats at one setting only. This kind of unit is the least expensive furnace in either the 80 or 90 percent categories, but it only has one speed of heating and its standard motor does not add to the efficiency of your air conditioner. HVAC companies size furnaces so that when it's 10°F outside, it should be 75°F degrees inside. Since it's rarely that cold, this means your furnace is usually running more than it needs to and will create hot and cold spots in your home. For example, it would be like having a car with only one heat setting. The car's heater would put out the heat you need, but it would be too much heat most of the time.
Single-stage units are the most commonly installed furnace and are easy to repair, but they are loud. Unfortunately, in order to close a sale, this kind of furnace is sometimes quoted by other companies as a builder's grade unit for less money. The builder's grade unit has all of the same disadvantages as a regular unit, but it uses less expensive parts which can cause it to fail more often.
Unlike a single-stage furnace, a two- or dual-stage furnace has two stages of heating. These separate stages reduce hot and cold spots. While this type of unit is about two percent more efficient than a single-stage furnace, it still uses a standard motor which does not add to the efficiency of your air conditioner.
A dual-stage furnace will run a little longer than a single-stage providing more even heat throughout your home. Two-stage units are quieter, but can require installation of wires to operate it properly and/or a two-stage thermostat to optimize its performance.
Two-Stage Variable Furnace
This type of unit has all of the benefits of a two-stage furnace, plus a high efficiency motor, which creates the optimum comfort level. Having a unit with this type of motor improves efficiency in heating mode while also increasing the efficiency of your air conditioner. The high efficiency motor is also self-adjusting, so if you have something restricting airflow (like a small problem with ductwork or a dirty filter), the dual-stage furnace adjusts to provide the right amount of air for the situation. Even though installation requires additional wires and a two-stage thermostat, the high efficiency motor can safely be left on all the time to improve air filtration and overall comfort in the home.
This furnace type is the most expensive, but that isn't necessarily a disadvantage. A two-stage variable speed furnace has the most features, delivering significant energy savings to you... and the comfort it provides is unparalled.
Unlike complicated efficiency ratings on air conditioners, furnace efficiencies are based on a percentage. Calculating this percentage, known as the annual fuel-utilization-efficiency (AFUE) rating, calculates both the gas it burns and the electricity it consumes to run its blowers and controls.
80% plus Efficiency
With this type of furnace, 80¢ to 85¢ of every dollar you spend on gas actually goes to heat your home. Compared to older furnaces that ran between 45% to 65% efficiency, this is a significant improvement.
90% plus Efficiency
With this type of furnace, 90¢ to 97¢ of every dollar spent goes to the gas going into heating your home, making this both the most efficient and safest furnace. Instead of using standard metal venting, this type of furnace requires a PVC pipe vent. It brings in air from outside, which it uses to burn the gas, thus lessening the likelihood of gas burning improperly because of lack of air. This also eliminates the need to bring cold air into your furnace room.