When people think about investing in a new furnace for their home they should carefully consider the efficiency rating. Sadly, many people don’t understand what a furnace efficiency rating is and how it could affect their home. In this article, we will explain furnace efficiency in detail and show how it can change your home in many different ways.

4 Furnace Purchasing Variables

When you decide to invest in your new furnace, your choices will be guided by the following four variables.

  • The budget.
  • The monthly heating bill.
  • The size of the home.
  • Any occupants with respiratory illness.

All of these are important, but the exact order of importance will vary depending on your particular circumstances. Generally speaking, a furnace with a better efficiency rating will be more expensive. But, this will bring down the monthly heating bill if the heating system is the correct size for your home.

The Annual Fuel Utilization Efficiency Rating (AFUE) 

The AFUE rating will help you to determine how much energy will be required to heat your home and how much that energy will cost. The higher the AFUE rating, the less energy you will need for home heating. The AFUE rating is represented as a percentage; it’s a number that will show you how much heat that furnace will provide in comparison to how much energy will be needed to produce the required heat.

As an example: let’s say that your heating system has an AFUE rating of 90%, this would mean that 10% of the energy used will be lost and the 90% is the real amount of energy that’s used to heat your home.

So, this seems pretty simple, but it’s important to understand that the AFUE rating does not include any potential heat lost due to air leaks or inadequate insulation levels. Every home will have different ducts installed and varying levels of insulation. If the ductwork has been installed badly, there could be a lot of leaks where the heated air could escape, and this will lower the efficiency. If a home is poorly insulated the same applies, so if you want to get the best efficiency from your new furnace, you may have to get an HVAC company to check your home. Fixing air leaks and improving the insulation will help you to save energy and money on your heating and cooling.

Gas Furnace Types Explained

A gas furnace is an important appliance in your home, and you need it to be reliable, quiet and efficient. For our purposes here, we will only explore how your broad furnace choice could affect efficiency. Different furnace makes, and models will have different features and performance ratings, but that is beyond the scope of this article. Modern forced air furnaces broadly fall into two types; they are conventional and condensing furnaces.

The Conventional Furnace

This is a more traditional type of furnace; it uses an older method that exhausts the combustion gases out through a flue before the gas can cool. This will prevent the formation of moisture and condensation. This type of furnace exhaust prevents the heat exchanger from collecting the maximum possible amount of heat from the fuel used. This makes the conventional furnace less efficient, and it explains why most homeowners now purchase and install a condensing furnace instead.

The Condensing Furnace

A condensing furnace will capture heat even after the combustion gases have cooled. The furnace type uses a pair of heat exchangers, the first is the primary heat exchange and the second handles the condensed exhaust gases. These gases contain carbon dioxide and water vapor, which combined together create carbonic acid. This system will deplete all of the heat out of the exhaust gas until the water condensate drips out of the heat exchanger. The remaining flue gases are vented using a PVC pipe. The condensing furnace wastes less heat, and this makes it a more efficient heating system.

The Furnace Burner and Blower

Both conventional and condensing furnaces can be further defined according to how their burner and blower operates. This is referred to as a stage, and it explains how the burner and blower work together to heat the home. There are three main types of furnaces stages, and they all be found in conventional and condensing furnace designs.

Single stage furnace: This is a cheaper system, the burner and the blower only have a single “On” stage.

Two stage furnace: This is also known as a dual stage furnace. Electronic controls allow the user to have the burner flame and the burner to be on a high and low setting. The setting used will depend on the amount of heat required.

Modulating furnace: This furnace uses an electronic control system to control the burner and the blower motor. It allows very fine adjustments to the settings, and it can be modulated to keep the temperature in a room very close to the thermostat setting.

Fuel Efficiency Ratings Ranked

Any furnace can be categorized by its AFUE rating as shown below.

AFUE rating 55-72%: This would be regarded as a low efficiency furnace, it’s now obsolete for new furnaces, but many furnaces in homes fall into this category.

AFUE rating 78%: This is a low efficiency furnace, up until 1/1/2015 this was the minimum AFUE rating allowed for a new furnace.

AFUE rating 80-83%: This would be regarded as a standard or medium efficiency rating.

AFUE rating 90-98%: This would be an Energy Star certified high efficiency gas furnace.

As we showed earlier, the AFUE rating measures how much fuel is converted into heat in the space and in proportion to the amount of fuel that’s entered the furnace. Every home is required to have a minimum AFUE rating of 78%, but a furnace with a rating this low would typically be found in a manufactured home.

A furnace must have an AFUE rating of 90% or more to meet the Energy Star Program from the Department of Energy. There are over 800 furnaces that Energy Star approval and almost every furnace manufacturer makes at least one of these models.