Furnace BTU Guide for Homeowners

When you’re shopping for a new furnace, you’ll run into numbers like 40,000 BTU, 80,000 BTU, or 100,000 BTU on product listings. But what do those numbers actually mean?

This furnace BTU guide breaks down the terminology, from BTU input vs. output, furnace efficiency, and AFUE. Having a better understanding of what the numbers tell you about each system makes it much easier to compare one furnace to another—so you can invest in the right one for you.

What Is a BTU and Why Does It Matter for a Furnace?

To start, let’s break down the main question when comparing furnace models: What is a BTU?

BTU stands for British Thermal Unit. It's a measurement of heat energy, where one BTU represents the amount of heat it takes to raise the temperature of one pound of water by one degree Fahrenheit. Fortunately, you don't really need to do anything with that definition when you're shopping for a furnace.

The important part is that BTUs are a way for manufacturers to describe a furnace's heating capacity. You'll typically see BTU information listed on a furnace specification sheet like this:

·      BTU Input

·      BTU Output

·      AFUE rating

These numbers work together to tell you how much energy the furnace uses, how much heat it produces, and how efficiently it runs overall.

What Does BTU Mean on a Furnace Listing?

When someone says they're shopping for a "60,000 BTU furnace," they might be talking about the furnace's input rating vs the amount of heat it delivers. That's an important distinction, because a furnace's BTU rating doesn't tell the whole story by itself.

Ultimately, you should be looking at the input and output ratings in conjunction with each other—along with the furnace's AFUE efficiency rating.

What's the Difference Between BTU Input and BTU Output?

It’s common for people to mix up BTU input vs. output. But here's the simple version:

·      BTU Input = the amount of fuel energy entering the furnace

·      BTU Output = the amount of usable heat the furnace delivers

For example, you might see a furnace listed at:

·      50,000 BTU input

·      40,000 BTU output

That doesn't mean something is wrong with the furnace. The difference is just showing the energy that isn't able to be converted into usable heat. The furnace takes in 50,000 BTUs of fuel energy and delivers 40,000 BTUs of heat. That’s the cost of running your furnace. It’s also where AFUE comes in.

Why Isn't a Furnace's BTU Output the Same as Its Input?

AFUE stands for Annual Fuel Utilization Efficiency. You’ll see it as a percentage, and it’s what tells you how efficiently a furnace converts its fuel into usable heat over the course of a typical heating season.

The reason why a furnace’s BTU output isn’t the same as its input is because it has to use up energy to heat your home. It’s not an equal exchange. Here are a few different examples of how the calculation works if you’re looking at different 50,000 BTU input units:

·      50,000 BTU input × 80% AFUE = 40,000 BTU output

·      50,000 BTU input × 90% AFUE = 45,000 BTU output

·      50,000 BTU input × 95% AFUE = 47,500 BTU output

The higher the AFUE, the better the heating performance. So if you're comparing two furnaces with the same input rating, the one with the higher AFUE will generally give you more usable heat. That's why it's important to look beyond the big BTU number on the product listing.

How Do BTU Ratings Relate to Furnace Size and Efficiency?

To recap, BTU ratings tell you about a furnace's heating capacity. AFUE is how you gauge its efficiency. Both matter when you're choosing equipment, but they answer different questions. You can think of it this way:

·      BTUs: How much heating capacity does the furnace provide?

·      AFUE: How efficiently does it turn fuel into usable heat?

A higher AFUE rating can mean less fuel is wasted, but that doesn't mean you should automatically choose the furnace with the highest efficiency rating—or even the one with the largest BTU input. The furnace still needs to be appropriately sized for the home.

When the furnace has too much capacity for the space, it can end up cycling on and off more often. It can heat the house up quickly, but then it’ll shut off and need to reboot again later. This can end up using more energy over time. On the other hand, a furnace that doesn’t have enough capacity may struggle to keep up with demand in the first place, meaning you won’t be comfy during a cold winter.

If you're trying to figure out how many BTUs in a furnace you actually need for your home, that's a separate sizing question. Your square footage, the climate where you live, insulation, windows, ceiling height, and other factors all come into play. Working with a local HVAC professional can help you run the numbers so you can find the right fit.

Where Can You Shop BTU-Rated Furnaces at the Best Price?

Shopping is pretty straightforward once you understand furnace BTU ratings. At Surplus City, you'll find brand-new HVAC equipment from major manufacturers with all of the product specifications clearly listed so you can compare your options. Our budget-friendly furnace inventory gives you access to:

·      A range of BTU input and output ratings

·      Major HVAC brands

·      Equipment that's in stock and ready to ship

·      Nationwide shipping

·      A 30-day warranty program on eligible new equipment

We've been in the HVAC equipment business since 1975 and we’re known for giving customers the best deals. Since we buy directly from manufacturers, we’re able to offer competitive prices without the traditional distributor markup.

If you already know the approximate heating capacity you need, you're ready to start comparing models. Browse the Surplus City furnace collection today and save!