The AFUE rating, or Annual Fuel Utilization Efficiency rating, measures the efficiency of your heater. This indicates how well your heater retains the heat it’s producing. It’s given as the percentage of energy that your boiler or furnace is actually converting into heat that you can use. For example, if your furnace is rated 90%, then 90% of the energy it takes in (via gas or oil) is used to heat your home, while 10% is heat you’re losing. Both furnaces and boilers are given AFUE ratings.
The AFUE rating is affected by many factors, including:
- The number of heat exchangers
- The number of burner settings
- Whether it runs on gas or oil (gas is typically more efficient)
- The materials
- The type of exhaust
- The type of ignition
The higher the AFUE rating is, the more efficient your furnace or boiler will be (and the lower your bills will be, too!).
Obsolete AFUE Rating: 56%-70%
If you have an older furnace or boiler, it may have a very low AFUE rating. An obsolete heater like this should be replaced, since it’s costing you money each month. Typically, a low-efficiency furnace or boiler has some or all of the following characteristics:
- Non-electronic ignition, so the pilot light is burning all the time.
- No exhaust fan to control combustion gases.
- Unsealed combustion, emitting more heated air into the environment rather than keeping it within the system.
- Heavier materials, like cast iron.
- One stage (on or off) and one blower speed.
Low AFUE Rating: 71%-79%
Prior to January of 2015, the minimum AFUE rating for a furnace or boiler was 78%. If your system was built prior to 2015, it might have a low AFUE rating. Heating systems with a low AFUE rating tend to have some of the following characteristics:
- No exhaust fan to optimally direct the combustion gases.
- Most likely an unsealed-combustion unit.
- One-speed blowers and single-stage on/off switch.
- Electronic ignition, so the pilot light isn’t always lit.
- Somewhat lighter heat exchanger materials, such as steel.
Mid-Range AFUE Rating: 80%-83%
According to the Department of Energy, a mid-efficiency central heating system would have an AFUE rating in this range. Such a system might have these characteristics:
- Electronic ignition.
- May have an exhaust fan to control the flow of combustion gases.
- Smaller size and weight, although the heat exchanger is likely steel.
- May have two stages: high and low, as well as off.
- May have a variable-speed blower.
High-Efficiency AFUE Rating: 85%-98.5%
These top-of-the-line heating sources use the latest technology to maximize efficiency. To achieve Energy Star certification, a boiler must have an AFUE of at least 87%. A gas furnace must have a rating of 90% in the Southern U.S. (including North Carolina) and 95% in the Northern U.S. An oil furnace can be Energy Star-certified with an efficiency of 85% or greater. These heat sources typically have:
- Electronic ignitions
- Multiple heat exchangers
- Sealed combustion chambers
- Modulating capabilities – rather than one or two settings, they can make fine adjustments in heat produced.
- Variable-speed blowers or an electronically commutated motor fan (ECM).
- For furnaces, condensing capabilities – the transfer of heat is so complete that water is formed as a byproduct.
What’s The Difference?
While a higher AFUE rating will save you more money in the long run, the savings decrease as you get closer to 100% efficiency. The biggest leap – a 9% savings – seems to occur when you switch from an 80% AFUE system to a 90% AFUE system. Beyond 90%, you’ll save about 1% with each additional percentage point. (So, a 92% AFUE will save you 2% more than a 90% AFUE).
Don’t get too hung up on AFUE ratings when you’re purchasing a new boiler or furnace. Any new appliance you purchase is likely to be significantly more efficient than a unit that’s ten or more years old. Currently, the highest AFUE rating for a furnace is about 98.7%.
Look on the cabinet or the user manual if your furnace or boiler to figure out its AFUE rating. If it’s less than the government-mandated 80%, you may want to consider investing in a new, high-efficiency heating source.