HVAC Sizing: What It Is and Why It Matters

HVAC sizing is a process that HVAC contractors use to determine the proper air conditioner, heat pump, air handler, or furnace combination you need to heat and cool your home. For the comfort and efficiency you deserve, you need an HVAC system with the capacity to get the job done, without being oversized or undersized. 

By Anne Fonda

If you’re thinking about replacing an old or inefficient HVAC system, don’t just assume that your current units are the right size for your home. Work with an HVAC professional to calculate your home’s heating and cooling load when you’re replacing a system, but also when you’re building a new home. Since HVAC components can last 15-20 years, you want to get both the air duct sizing and HVAC unit sizing right.

Every HVAC technician learns how to size an HVAC system accurately. But not every HVAC contractor goes to the trouble of doing it right. That’s why industry estimates are that up to 90% of HVAC systems are improperly sized or improperly installed.

Correct sizing involves the Manual J calculation and can also involve a separate calculation for duct sizing.  Proper sizing includes considering multiple factors, including:

Before an HVAC contractor arrives, you can also estimate the size of the heat pump or AC unit you need by using the square feet method. This tons per square foot calculation is not as accurate as the Manual J load calculation but can give you an idea of the system capacity you will need in a system replacement.

This calculation has evolved over the years due to construction material and insulation requirements used today vs historically.  Traditionally, this estimation would be 600 SF/ton for residential applications with different requirements for apartment buildings, commercial locations, and other applications.   

However, with the updated insulating materials used in new construction, it may be closer to 1000 SF/ton for residential. The SF/ton may also need to be adjusted for your home if you’ve made upgrades with energy-efficient windows, insulation upgrades, and more.

So if one HVAC contractor recommends one unit size and another recommends a larger capacity unit, ask them to show you their calculations to back up their recommendation.

Choosing the right HVAC system size requires some homework, doing some math, and working with a trained HVAC professional. To help you get an idea of what size AC or heat pump you need, read our blog What Size AC Unit Do I Need?

American Standard has AC units, heat pumps, air handlers, furnaces, and ductless systems suitable for everything from tiny homes to grand estates. Larger homes may need more than one HVAC system to achieve optimal comfort. 

When it comes to your comfort, you want an HVAC system that’s just right. Not too big and not too small. If your system is oversized, it will heat or cool your home before being able to remove humidity, which will leave you feeling sticky. An oversized system can also mean higher energy bills. 

An HVAC system that’s too small will cause problems as well. An undersized system will cause short cycling, where the system turns on, but turns off again before reaching the set temperature. The frequent starting and stopping can lead to unnecessarily high energy bills and increased wear and tear on your system.

To ensure accurate sizing of your HVAC system, work with the HVAC experts. Contact your local American Standard dealer to set up a quote for a new HVAC system that’s correctly sized for your home.

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need. 

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer