Modern HVAC technology has made it so that human beings can live virtually anywhere on the planet—even Antarctica! As impressive as this technology is, it’s not always easy to know how to match the right heating, ventilation, or air conditioning equipment with the building into which it will be installed. 

In Colorado, having a furnace or heat pump is a necessity to get us through the cold winter months. But, how do you know what size furnace is appropriate for your residential or commercial building? 

With Furnaces, the BTU’s Matter

Generally speaking, the bigger a furnace is, the more heat it can produce. By ‘bigger’ we don’t necessarily mean physically bigger—we mean bigger in terms of the amount of British Thermal Units (BTUs) that the furnace is capable of emitting. 

Thanks to advances in engineering and manufacturing, a modern furnace that is only 2’ x 3’ in size could be capable of producing the same amount of BTUs as a 30 year-old furnace that occupies a whopping 5’ x 8’ footprint.

It used to be a rule-of-thumb that the larger a furnace was, the more BTUs it could produce. However, this is no longer the case, as the furnaces being manufactured today are much more efficient, better built, and better engineered. 

Factors to Consider When Choosing a Furnace

There are numerous factors to consider when sizing a new furnace. First among them is how cold it gets where you live.


Obviously, someone who lives in Phoenix, Arizona isn’t going to need the same horsepower out of their furnace as someone who lives in Durango. Here in Colorado, our climate is a pretty even mix of alpine and semi-arid, which means we need more ‘oomph’ out of our furnaces than those who live in many other parts of the country. 

what size furnace do I need 2If you live in an area west of Denver, you’re going to be using your furnace more often than someone who lives in, say, Aurora. Higher elevations necessarily mean colder average temperatures throughout the year. 

For those living in ‘alpine’ climates (virtually all of the Colorado locales above 7,500 feet in elevation), a furnace that can produce 50-60 BTUs per square foot of building space will be needed. For Denver residents and those who live east of the Rockies, that number drops to about 45-50 BTUs per square foot of building space. 

So, if you live in Aspen, Colorado, and you’re shopping for a new furnace to heat your 2,000 square-foot home, you’d be in the market for a furnace rated at 100,000-120,000 BTUs. 

Alternatively, if you live near the Denver International Airport, for example, your furnace for a similarly sized home would need to be rated at 90,000-100,000 BTUs.

Insulation and Building Construction

Other factors to consider when sizing a furnace are the amount of insulation within a home or commercial building, and what the building is composed of. 

Wood frame houses with thinly insulated walls are going to require much more heat to keep warm in comparison with brick houses that are heavily insulated. Similarly, log cabins with a thick, sealed exterior won’t need as ‘big’ of a furnace as a mountain home that only has a few planks of wood and some drywall separating the interior from the exterior. 

In many cases, it helps to look at the BTU rating of the furnace that is being replaced in order to arrive at a good estimate for how powerful the new furnace will need to be. 

The last factor involved in selecting the right furnace is its efficiency rating. 

Furnace Efficiency

A more efficient furnace is going to be able to produce more heat with less amount of natural gas or electricity. Generally speaking, electricity is more expensive than natural gas, however both types of furnaces can be highly efficient. It’s not uncommon for a furnace to have an efficiency rating of 80-95%+. 

A furnace that is rated at 80% efficiency and 80,000 BTUs simply isn’t going to perform as well as a furnace that is rated at 95% with an identical BTU rating. Remember that higher-efficiency furnaces are typically going to cost more to purchase; however, they often wind up paying for themselves fairly quickly thanks to the lower energy costs required to operate them. 

Need Professional Furnace Advice? Contact Front Range HVAC

If you’re on the fence about which furnace to choose, let us help you! Our team of HVAC experts is standing by, ready to work with you to identify the ideal furnace for your unique needs. We’ll even remove the old furnace and recycle it. 

To get started, contact us online today or call (303) 872-9060. We look forward to hearing from you!

Read More: What Should I Do If My Furnace Isn’t Producing Enough Heat?