What's the Difference between an 80% and 90% Efficiency Furnace?

When buying a new furnace, you can often choose between a standard efficiency and a high-efficiency furnace. Based on only the name, it can be tempting to go for a high-efficiency furnace – but is it really the best option? In this post, we’ll go over the key differences between an 80% and 90% efficiency furnace so you can confidently make that choice.

What is an efficiency rating?

Space-heating furnaces are rated according to their Annual Fuel Utilization Efficiency (AFUE). It represents the amount of energy that goes into heating the home. For instance, if the furnace offers a rating of 80%, then 80% of the energy it uses goes into heating, while the rest is wasted. In other words, the furnace generates 80 BTUs of heat from the 100 BTUs of gas that it burns. The 20 BTUs are lost as waste heat – it’s important to note that this is bound to happen with fuel-fired furnaces, boilers, etc.

What’s the difference between 80% and 90% efficiency furnaces?

Although it generates more heat, it doesn’t guarantee that the 90%-efficiency furnace is the right choice. But why would someone want to buy a low-efficiency furnace? The answer lies within the way the furnaces are installed.

Typically, a low-efficiency furnace (80%) draws air from inside the home. Through open combustion, it burns gas by getting the air around it and then spews the waste gas to the outside. On the other hand, a high-efficiency furnace (90%) draws air from the outside of the home via a PVC pipe. Through sealed combustion, the 90%-efficiency furnace recovers the waste gas which is then diverted into a condensing plate. This guarantees that more heat is retained.

Here are a few more differences between an 80% efficiency and a 90% efficiency furnace:

#1 Cost

To install a high-efficiency furnace (90%), you’ll have to install a valve to allow the outside air to enter the unit. This can be very costly, so it’s unlikely that your savings will be enough to make up the difference.

#2 Consistency

A high-efficiency furnace (90%) can provide an even temperature in the home. Some of these furnaces come with multi-stage burners that can change the output according to the conditions. For instance, on days when it’s not too cold, the furnace can change its speed and power to decrease your energy costs.

#3 Humidity

High-efficiency furnaces draw air from outside of the home, so the air will always be clean. As a result, this improves the humidity levels of the home. A low-efficiency furnace, on the other hand, uses and reuses the indoor air to the point where it starts to feel stuffy.

#4 Safety

A standard-efficiency furnace (80%) does not pose a safety risk given that it’s in a vented attic. However, if you’re planning on installing a furnace in an unvented area, a high-efficiency furnace is a safer choice.

Which furnace should you install?

The ideal type of furnace depends on the layout of your house, as well as how much you’re willing to spend to install it. Contact an HVAC professional for more information on the pros and cons of each furnace so you can purchase the right unit for your household’s needs.