The Ultimate Guide to Furnace Sizing

Confused about choosing the right furnace for your home? Call 720-779-7263 to book an appointment with our skilled installation technicians at JC Mechanical.

Furnace sizing presents a challenge to most homeowners. You might encounter unfamiliar terms like BTU and tonnage. Plus, the math equations involved can make anyone nervous about whether they’ll make correct calculations.

As a furnace installation company in Denver, CO, our team developed a guide to assist you in making the right choice. Below, you’ll discover how we assist homeowners in choosing the right-sized furnace for their homes.    

furnace sizing

Building Size

First and foremost, calculate your home’s square footage. You can find the square footage of a room by measuring the length and width of that area. After multiplying both numbers together, you’ll have the room’s square footage.

Repeat this process for each room, including common areas and small spaces like closets. List the numbers as you go. Once you’ve calculated the numbers for each room, add them together. Your total is the square footage of your home.

Although this process seems laborious, repeating it or having a second person check your math prevents mistakes that could affect your furnace needs.   

Regional Climate

The second factor you should consider is your regional climate. The United States contains five different climate zones, each with its own furnace needs. Check a map to figure out in which zone your home is located.

The state of Colorado contains two climate zones: zone four and zone five. Denver, CO, is in zone five. Thus, you’ll need a furnace that outputs more heat than a property in zone three or four.

BTU (British Thermal Units) refers to the heating requirements a furnace can fulfill. Climate zones require the following amount of BTUs to adequately heat a home:

  • Zone one: 35 BTU for each square foot
  • Zone two: 40 BTU for each square foot
  • Zone three: 45 BTU for each square foot
  • Zone four: 50 BTU for each square foot
  • Zone five: 60 BTU for each square foot

How to Calculate Furnace Size

Once you’ve collected your home’s total square feet and climate zone, you’ll multiply your zone’s BTU by your home’s total square footage. This equation should give you a close estimate to inform your furnace sizing needs. You’ll find examples listed below to help you make your calculations: 

  • A house with 1000 square feet located in zone five requires 60000 BTU. 
  • A house with 5000 square feet located in zone five requires 300000 BTU.
  • A house with 1000 square feet located in zone four requires 50000 BTU. 
  • A house with 5000 square feet located in zone four requires 250000 BTU.

Once you have your BTU estimation, you should still consider other factors. The efficiency rating, the type of furnace you choose, and various other elements will influence whether the furnace outputs consistent temperatures without overworking or negatively impacting your home.    

Types of Furnaces

You’ll encounter a variety of heating systems on the market. Make a decision based on:

  • Whether you have access to gas or electricity
  • Whether professional technicians recommend a specific type for your home.
  • Whether the furnace is compatible with your climate zone. 

Some furnaces work better for smaller buildings or warmer climates and won’t provide sufficient heating for your circumstances.

For example, a single-stage furnace may work well for a smaller, single-story home. It boasts lower installation costs and maintenance requirements for an affordable option. However, it doesn’t perform optimally in large houses and may not provide consistent temperatures.

Two-stage furnaces are more expensive and require more maintenance than their counterpart. However, they provide better heating capabilities for larger homes or houses with more than one story. 

Efficiency Rating

In addition to BTUs, you should also consider a furnace’s efficiency rating. In HVAC terms, the efficiency rating refers to how much power a system uses and loses during operation. A furnace with a higher rating will more effectively turn power into temperature control.

High-efficiency furnaces come with a more expensive price tag than low-efficiency ones. However, lower power or gas bills and optimal heating capabilities offset the initial costs. Choosing higher efficiency ratings also makes your home greener and more eco-friendly.  

Other Factors to Consider

Other factors in your home will also affect how it maintains and takes in heat. A house’s age and structure will influence the amount of BTUs needed to heat it. An older home often requires more BTUs, while newer homes have structural differences that better maintain internal temperatures.

Think about how the following appliances and structural differences apply when choosing a new furnace:

  • Different insulation types and amounts affect how your home retains heat. 
  • The number and size of your windows influence how quickly heat escapes a room. A large window allows more heat to escape. 
  • Lower ceilings allow a room to stay warmer than tall, vaulting ceilings. 
  • Roof color affects the amount of heat a home absorbs. Darker colors will draw more heat into your home than lighter colors. 

An HVAC technician can point out other pertinent factors that should influence your final decision. 

Choosing the Wrong Furnace Size

What happens if you choose the wrong size furnace for your home? How can it influence your comfort and wallet?

If your furnace is too big, it will operate in short, intense bursts that overheat your home. These short cycles cause excessive wear and tear on its components, leading to more repair and maintenance requirements. Plus, it’ll sharply spike your energy bills during Denver’s long cold season.

An undersized furnace might work well in milder winter temperatures. But once intense, below-zero temperatures hit, it will struggle to maintain a cozy temperature in your home. This leads to excessively long cycles that also affect your overall costs. 

Questions About Furnace Sizing? Let JC Mechanical Assist You!

Furnace sizing is essential to a comfortable, cost-effective home. The right-sized furnace will maintain optimal levels of comfort without sacrificing energy efficiency to do so. Our team at JC Mechanical can help you make the right choice with furnace repair and installation services. Call us at 720-779-7263 to schedule an appointment.