Furnaces
Air Conditioning Sizing: Why It Matters
By HVAC.com•August 1, 2023
Choosing the right size air conditioner for your home requires more than knowing the square footage. A thorough evaluation of your house will help you understand how BTUs, climate, and, yes, square footage all factor into your air conditioning tonnage needs.
When it comes to optimal AC performance, size does matter. A unit that’s too small for your home will cause the system to run constantly and leave you with an ever-rising energy bill. If the system is too big, the air will be cooled quicker than the system can dehumidify the space. You’ll be left with a humid, sticky environment. Plus, there’s an increased risk of mold growth inside the system and ductwork.
To keep your family comfortable and avoid bank-breaking energy bills, we’re introducing the best way to answer, “What size AC unit do I need?”.
Types of air conditioning systems
Before sizing your air conditioner to your home’s needs, determine what type of system you want to install. There are a variety of cooling systems available, but we’ll cover the three primary styles below.
Ductless air conditioners
Ductless air conditioners rely on a series of pipes to connect an outdoor unit to one or more indoor units to cool your home. The pipes are filled with refrigerant that absorbs and releases heat to maintain the desired temperature. Ductwork isn’t required with this system. Single-zone and multi-zone systems are available and best serve homes where a different temperature is desired in specific spaces.
Packaged HVAC systems
Packaged AC systems are the all-in-one solutions for homeowners who need to conserve space or need flexible options for where the HVAC equipment can be installed. The primary components, like the evaporator, compressor, and condenser, are stored in one cabinet. The outdoor cabinet is typically placed on a concrete slab outside the home. Packaged systems deliver cool air through the home’s ductwork.
Split-system air conditioners
Split-system air conditioners are likely what you envision when you think of an HVAC unit. These systems include an outdoor unit and an indoor unit that work together to create the desired temperature and indoor humidity level for your home. Refrigerant runs through both units during the cooling or heating cycle and the conditioned air is delivered through your home’s ductwork.
How many BTU per square foot?
A BTU, or British Thermal Unit, is a rating that measures the energy your AC uses in one hour to remove heat from your indoor air. The BTU of your HVAC system should be based on the square footage of your home, insulation quality, climate zone, and ductwork.
Generally, you need 20 BTU for every square foot of space in your home. The chart below can guide your BTU choice:
| Home Square Footage | BTU (British Thermal Unit) | Tonnage |
|---|---|---|
| 600 – 1,000 | 18,000 | 1.5 |
| 1,000 – 1,300 | 24,000 | 2 |
| 1,300 – 1,600 | 30,000 | 2.5 |
| 1,600 – 1,900 | 36,000 | 3 |
| 1,900 – 2,200 | 42,000 | 3.5 |
| 2,200 – 2,600 | 48,000 | 4 |
| 2,600 – 3,200 | 60,000 | 5 |
The larger your home, the higher BTU you’ll need to support your comfort. One of the most accurate ways to measure the right size AC for your home is with a Manual J calculation.
Manual J calculation
A Manual J calculation is like an energy audit of your home. Who performs the Manual J calculation can vary depending on your region. In some instances, the HVAC company will complete the audit. It’s also possible for a general contractor or energy consultant to complete the work.
The cost of a Manual J depends on the size of your home. For most homeowners, you can expect to pay between $200 and $300. If your local HVAC expert performs the audit, ask if the service is included with your system replacement or installation.
We suggest having the inspection complete rather than simply replacing your old HVAC unit with the same size system. A Manual J inspection will consider:
| 🗜️Ductwork quality |
| 🌬️Home insulation quality |
| ☀️Amount of direct sun exposure |
| 👩👩👦👦Number of people living in home |
| ♨️Heat-generating appliances |
| 🌡️Geographical climate and average temperatures |
| 🚪Number and quality of exterior doors and windows |
| 🧱Home construction materials (i.e. brick, wood, etc.) |
| 🔥Home features that modify internal temperatures (i.e. fireplace, sunroom, etc.) |
Each of these elements has the power to affect the temperature and comfort of your home. By choosing an HVAC size based on square footage alone, you may purchase a unit that’s too small or large for your needs, and that can come with a hefty cost.
AC climate zones
While the square footage and makeup of your home will give you great direction on sizing your HVAC unit, we can’t forget climate.
Take two versions of the exact same home. Place one house in San Antonio, Texas, and the other in Gaylord, Michigan. Despite the same dimensions, quality of insulation, ductwork, windows, and doors, the Michigan home needs a smaller tonnage unit than the house in Texas.
The U.S. Department of Energy offers a guide to help homeowners easily determine their climate zone. Your local HVAC expert will also be able to guide you on the proper tonnage needed for your climate zone demands.
Best HVAC size for your home
Choosing the right size air conditioning system for your home doesn’t have to be complicated. Once you determine your climate zone and schedule your Manual J inspection, it’s time to choose the best HVAC brand. Once you know the size unit you need you can start comparing different leading products that are the same size – keeping your price estimate right on the money.