What is AFUE & Furnace Staging?

Now that winter is coming let us help explain the difference between AFUE & Furnace Staging. What is a multi-speed motor? What is a variable speed motor? Does this all sound complicated? It really doesn't have to be. Let us break it down so it is easy to understand.

AFUE is Annual fuel utilization efficiency.
In a nutshell, the AFUE directly relates to how much money you pay the gas company vs how much you pay actually heats you home. For example, lets use a 80% AFUE furnace. Lets say your gas bill at the end of the month was $1.00. Although this will never happen, let's pretend :) A 80% AFUE furnace will put $0.80 of heat in your home and $0.20 will be wasted heat going out the chimney. A 96% AFUE furnace will put $0.96 cents of heat in your home and $0.04 out the chimney. Long story short, the higher the AFUE the less you pay the gas company at the end of the month.
Furnace Staging
Single Stage Furnace A single stage furnace will have one burner rate. This is how almost all furnaces before 1980 were made and most furnaces operate. For example, if you have a 100,000 BTU furnace when the thermostat tells the furnace to turn on it burns 100,000 BTU's of gas. This is very straight forward.
Two Stage Furnace A two stage furnace has two burner rates. For most furnaces on the market, it is a 70%/100% split. Let's use the 100,000 BTU furnace again for example. In first stage it will run at 70% or 70,000 BTU's. It will run at the rate for a period of 15 minutes. The idea behind this is when a furnace is sized for a home, it is sized that the outdoor temperature is at or near 0 degrees Fahrenheit. Most of the winter, you won't be at that temperature... or let's hope not! :) Realistically it usually is well above that temperature, so you are burning more gas than necessary to heat your home. The two stage option helps with that. 70% burner rate on an average winter will run in that first stage about 70-80 percent of the time. If after 15 minutes the house still isn't warm enough, it will then go into second stage or 100% capacity.
Modulating Furnace A Modulating furnace has a smart gas controller that can output 20-100% capacity based off what the house needs. There is no set burner rate and will require a smart thermostat that is made by the same manufacture as the furnace. This is the most efficient but also the most expensive option upfront. The thermostats from the furnace manufacture are also super expensive. This is the only drawback to a modulating unit.
Now Let's talk Motor options!
Multi-Speed Motor A Multi-Speed motor is just that. A motor that has more than one speed. This is the most common motor in most furnaces. It has a High, Medium, and Low setting that can be adjusted by your furnace installer to match your ductwork. These are a little expensive to run but work well.
Variable Speed Motor A Variable speed motor can run between 20-100 percent similar to the Modulating furnace. It does not require a smart thermostat though. The logic for the speed adjustments are built into the furnace and will speed up and down based off the houses needs. When you close vents, as the filter gets dirty, or as the temperature changes, the motor speed will change as well. This is the quietest and most efficient motor option.
I hope this makes things a little easier to understand and makes life a little easier when picking your home furnace needs this winter.
Posted August 19, 2019. Video Posted 11/22/2019.