Are you in the market for a new heating system for your home? If so, it probably won’t be long before you start hearing the term “AFUE.”

What does AFUE stand for? It’s Annual Fuel Utilization Efficiency. This is a rating that measures the efficiency of a heater. Basically, it’s a number that indicates just how good of a job your heater does at retaining the heat that it’s putting out.

As an example, imagine that the AFUE rating on your furnace is 90 percent. This means that 90 percent of the energy that the heater takes in is used to heat the house. At the same time, this number indicates that the heater is losing 10 percent of the energy.

What Factors Affect the AFUE?

Several factors may go into calculating the AFUE. These include:

  • How many heat exchangers are included in the system
  • How many burner settings there are
  • Whether the heater runs on oil or gas, with gas usually being the more efficient option
  • Materials used in the heater and the overall system
  • The kind of exhaust used by the system
  • The ignition type used by the system

Low AFUE Ratings

Homes that are older and accordingly equipped with an old boiler or furnace may have uncommonly low AFUE ratings. In fact, ratings in the range of 56 to 70 percent are considered obsolete.

When a heater is designated as obsolete, then it needs to be replaced as quickly as possible. This is because that outdated heater is actually costing you a great deal of extra money every month.

Here are some characteristics of a low-efficiency heater:

  • Uses a pilot light or other non-electronic ignition
  • Has no exhaust fan for combustion gases
  • Has unsealed combustion so that more heated air is allowed into the environment rather than in the system
  • Has only one stage: on or off
  • Has only one blower speed
  • Is made of heavier materials, such as cast iron

Similarly, AFUE ratings of between 71 and 79 percent are considered low. Any heating system that was installed before 2015 may have an AFUE rating at this level.

Systems with this AFUE rating may not have an exhaust fan, and they probably have an unsealed combustion unit. Moreover, these systems are likely to have one-speed blowers and single-stage on-off switches. Still, these units are an improvement over obsolete ones thanks to their electronic ignition and lighter heat exchanger materials, which may include steel.

AFUE Ratings in the Mid-Range

The Department of Energy has determined that a heating system with mid-level efficiency has an AFUE rating of between 80 and 83 percent. These systems definitely have an electronic ignition and are smaller in weight and size than earlier models. Typically, they have at least two stages, including settings for high and low. A variable speed blower makes these heaters more efficient, and there may even be an exhaust fan to help with controlling the flow of combustion gases.

Top of the Line AFUE Ratings

The latest, most efficient heaters have AFUE ratings in the range of 85 to 98.5 percent. A boiler cannot even achieve Energy Star certification unless its AFUE is at least 87 percent. In the Southern U.S., gas furnaces are required to have an AFUE of 90 percent to receive an Energy Star certification while Northern U.S. requires an AFUE of 95 percent. The standards for oil furnaces are slightly different, with Energy Star certification being available at 85 percent.

Heaters with the best AFUE ratings have several characteristics in common. These include electronic ignitions as well as numerous heat exchangers. If you examine these units, you will discover that they have sealed combustion chambers and either variable speed blowers or an ECM, or electronically commutated motor, fan.

Moreover, most of these high-efficiency models boast modulating capabilities. This means that instead of just having a couple of settings, it is possible for these machines to respond to fine adjustments in temperature.

How Furnaces and Boilers Work

In general, furnaces burn either natural gas or propane, although there are oil furnaces that utilize fuel oil instead.

Fuel is brought to the furnace or boiler from a holding tank or pipe via a fuel line. This fuel is under pressure, and when a valve is opened, the igniter heats up. This releases fuel into the burner where it ignites and begins producing heat.

Next, the heat rises, then travels through one or more heat exchangers. A blower fan moves cool air across the heat exchangers, thereby transferring heat into the air. This air is pushed through your home’s ductwork.

Not all of the heat that your furnace or boiler produces will actually be transferred through the heat exchangers. A portion of it will be lost through a vent. Included in this exhaust are combustion gases, like carbon monoxide, that would be dangerous to have in your home’s air.

About Heat Input vs. Output

An HVAC installer may refer to terms such as heat input and heat output when talking about the AFUE rating on a furnace or boiler.

Boilers and furnaces come in many sizes with varying sizes of gas valves and perhaps more than one set of burners. Heat input refers to the amount of heat that the unit is able to produce. This number is expressed in BTUs, or British Thermal Units.

A residential furnace may be sized anywhere from 40,000 BTU/hr up to 140,000 BTU/h. Conversely, boilers generally range from 50,000 BTU to 200,000 BTU, with most of them measuring between 60,000 and 150,000. Larger boilers also may be responsible for heating water for laundry, bathing and other household uses.

When you hear about heat output, then you’re talking about the amount of BTUs that actually get transferred into the air or water.

Accordingly, an HVAC expert may calculate the AFUE rating by using an equation such as:

90,000 BTUs of output divided by 110,000 BTUs input equals .82. Thus, the AFUE rating is 82 percent.

How Do I Know What Size Furnace I Need?

The proper size of a furnace may be determined by performing a load calculation. Such a calculation is able to tell you just how much heat your home will need when the weather reaches its coldest temperatures of the year. Typically, dozens of different factors may be used to perform the load calculation.

It makes sense to have an expert perform a load calculation for your home so that you can be certain that it is accurate. This helps to ensure that your furnace is properly sized for your home. Part of this calculation may involve undergoing a test to determine just how airtight your home actually is. This test will provide you with valuable information for how to make your home more energy efficient overall while also making certain that your new furnace isn’t larger or small than is required.

How Can Delaware Heating and Air Help You?

Most people don’t purchase a furnace, boiler or other components of an HVAC system more than two or three times in their lives. If it has been many years since your heating and cooling system was installed, then you may be shocked by how far the related technology has come.

Whether you are using oil, gas or electric as an energy source, you will discover that heating and cooling systems are far more efficient today than they have ever been before. It is possible that your current system has been wasting energy and money for years.

That means that it is time to explore the options that are available today through Delaware Heating and Air. We are a local company that takes pride in offering only the best systems, components and services to each of our customers.

If you have questions about AFUE ratings or anything else that is connected to your home or business’ HVAC system, call Delaware Heating and Air today.