Basic:
Single-stage furnaces and ACs are the most affordable but have the highest operating costs due to their low efficiency. They run at full capacity when on, so there are slight temperature fluctuations, and single-stage systems offer few performance features.
Furnace blower motors are either single-speed and run at full speed when on, or multi-speed motors that speed up and down in 3-5 steps during the heating cycle.
Better:
Two-stage furnaces and air conditioners offer a combination of moderate cost with improved performance that appeals to homeowners. They run on low up to 65% of the time to maximize efficiency and climate control. Two-stage systems run on high when a significant boost in heating or cooling is required due to rapidly changing outside temperatures or an adjustment to the thermostat.
Best:
Variable-capacity systems are the most expensive, but they produce the lowest heating and AC costs due to their higher efficiency. They also deliver the best climate control including maximum dehumidification in AC mode.