How to Size a Furnace & Why it Matters

How to Size a Furnace & Why it MattersDid you know that when it comes to furnaces, size matters? A furnace that is either too small or too large can cause a lot of problems and those problems can cost money. This is why it is important to invest in a correctly-sized furnace with the proper BTU rating.

First Things First: What is a BTU?

A “BTU” refers to what is known as a “British thermal unit.” In a nutshell, a BTU describes how much energy is needed to produce a certain amount of heat. The higher the BTU rating, the better, and the bigger the home, the more BTUs are required for energy-efficient heating. Your furnace’s BTU rating is typically correlated to its size, so it’s important to take both into account.

What Happens if Your Furnace is Incorrectly Sized?

Much like Goldilocks and her three bears, you can wind up with three different furnace sizes: too big, too small, and just right. Here is what happens with each.

1. A Too-Large Furnace

A furnace that is too large is naturally inconsistent. Since it finishes heating the home quickly, it turns off quickly. However, it soon turns back on to keep the temperature accurate. The constant turning on and off reduces the furnace’s lifespan. It also increases your monthly utility bill.

2. A Too-Small Furnace

On the other hand, a furnace that is too small cannot keep up with the workload. It either dies early or does not fully heat the home. Additionally, since it has to put extra work into producing heat, your heating costs might climb.

3. A Just-Right Furnace

A just-right, energy-efficient furnace keeps your utility bills low and consistent while maintaining a normal lifespan.

What Size Furnace Does Your Home Need?

Now comes the hard part. How do you size your furnace? Calculating your furnace size primarily depends on your location and the square footage of your home.


If you live in a naturally warm climate, your furnace probably won’t require as many BTUs as it would in a cold climate. So the first thing is to determine your “zone.” Each “zone” has a different BTU standard ranging from 30-60 BTUs. You can check your zone to BTU conversion online.

Square Footage

Next, determine the square footage of your home by multiplying the length and width of each room. Then, calculate the number of BTUs required per square foot as applies to your zone.

If you need help determining what size to buy, a trustworthy HVAC company can help you figure it out. Just take your time, do your research, and ask plenty of questions.

How Can ABE Heating & Cooling Help?

As a licensed Rheem dealer, we can introduce you to some of the most energy-efficient and cost-effective systems currently available on the market. ABE Heating and Cooling offers great products with iron-clad warranties and superior installation services, all at manageable prices suitable for any budget. We are located in Brighton, Colorado, and serve the entire Denver Metro Area, and if you would like to learn more about high-efficiency furnaces please contact us today to discuss the possibilities.