There are four main types of furnaces used in HVAC systems: single-stage, two-stage, variable-speed, and modulating. Single-stage furnaces are the most basic type and operate at one speed, either on or off. Two-stage furnaces have two settings, allowing them to run at a lower speed for more efficient operation. Variable-speed furnaces can adjust their speed to match the demand of the system, providing more consistent temperatures and improved energy efficiency. Modulating furnaces are the most advanced type and can adjust their output in very small increments, allowing for precise temperature control.