What is AFUE? Annual Fuel Utilization Efficiency is the term used to rate the efficiency levels of a gas furnace. The higher the AFUE rating the more energy efficient the furnace is and the lower your gas bill will be. It measures the amount of heat actually delivered to your home compared to the amount of fuel that you supply to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat - the other 20% is lost out the chimney or flue. Compare this to an older furnace that may only deliver 60% efficiency. With today's technology, we have furnaces available that are capable of reaching up to 98% AFUE ratings!
Modulating vs. Single-Stage vs. Two-Stage To understand Modulating Technology, just think of your car. A modulating furnace operates much like your cruise control, constantly making small adjustments to keep the car at its desired speed. It's a fuel-efficient way to drive, and also more comfortable than the lurching that comes with rapid acceleration and deceleration.
A Single-Stage system shuts on and off at full output only, similar to driving with either a lead foot or completely off the accelerator.
A Two-Stage system operates at two heat inputs, similar to driving at two available constant speeds. Two-Stage heat is more comfortable than Single-Stage heat, but a Fully Modulating system provides the best comfort solution.
Fixed Speed vs. Variable Speed Standard systems operating using fixed speed motors use more energy than variable speed motors. Variable speed also balances temperatures and minimize hot and cold spots. The continuous air flow provided through variable speed motors also improves filtration and humidity control.