Equipment sizing is one of the most important things to know before AC installation. A more technical process than many people realize, sizing an air conditioner requires a thorough evaluation of your home. A simple calculation of a building’s square footage isn’t enough (although it’s an important step). In this guide, we’ll explain why an air conditioner must be properly sized and discuss various aspects an AC contractor considers.
Why Does AC Size Matter?
An air conditioner’s size is determined by how large a space it’s designed to cool. If the unit has just the right capacity for your home, it will work at peak efficiency. This can keep your energy bill down, reduce strain on the system, keep humidity levels manageable, and maintain optimal indoor air quality.
Too small an AC will run too long yet your home may never get cool enough. If too large, an AC unit can cool the space too quickly and turn off too soon to properly dehumidify it. The unit will also turn on and off too frequently. Either way, the system can overwork itself, which increases wear and tear. Combined with running inefficiently, more frequent repairs and a shorter time until replacement means a higher cost of ownership.
Central AC Size vs. Home Size
Air conditioners come in many different sizes. However, an AC unit’s size doesn’t necessarily reflect how big an air conditioner is physically. It refers to its cooling capacity, which is calculated in British Thermal Units (BTUs). The more BTUs an AC is rated for, the greater its cooling capacity. Tons is another term and is equal to 12,000 BTUs, but it refers to how much heat it can remove in an hour.
Here’s a look at one comparison of home size vs. AC BTUs and tons:
| Home Size | AC Size In BTUs | AC Size In Tons |
| 1,000 to 1,300 square feet | 24,000 | 2 |
| 1,300 to 1,600 square feet | 30,000 | 2.5 |
| 1,600 to 1,900 square feet | 36,000 | 3 |
| 2,200 to 2,600 square feet | 48,000 | 4 |
| 2,600 to 3,200 square feet | 60,000 | 5 |
Generally, HVAC professionals consider 20 BTUs per square foot, but the actual AC capacity you need for your home is more complex than that. First, here’s a look at calculating room size, which is relatively easy. The square footage of a room is determined by multiplying the length of the room by its width. Then you can add up all the rooms inside your home to get the BTU rating you need. To estimate the required AC size in tons, divide the BTU number by 12,000.
As we’ve mentioned, the proper AC size isn’t only determined by square footage. To prepare for air conditioner installation, a few other factors must be considered. When accounting for different variables, the total BTU count may be increased or reduced by a certain amount. The examples below highlight how this works:
- Sunlit Room: Increase the required BTU count by 10%.
- A Room with Few Windows: Reduce the calculation by 10%.
- A Room Shared by Two or More People: Add 600 BTUs per person.
- The AC Is In a Kitchen: Increase the capacity by 4,000 BTUs.
- Ceiling Height: Add 1,000 BTUs for each foot past a standard ceiling height of 9 feet.
What Is a Manual J Calculation
A Manual J load calculation accurately measures the required cooling capacity for your home. It can cost up to $300 but is like a complete energy audit. Depending on where you live, an HVAC company, general contractor, or energy consultant can perform a Manual J calculation. It helps an installer provide a tailored solution that meets your individual needs.
In addition to some factors already mentioned, a load calculation by an air conditioner installer in Portland can account for the following:
- Insulation: Good insulation helps keep your home airtight and blocks heat transfer. A Manual J calculation considers the type of insulating material and its R-value; the higher this number, the more effective it is at preventing heat flow.
- Climate Zone: The U.S. is divided into several climate regions. The warmer your climate, the more frequently your AC must run and the higher its efficiency requirements. Less cooling power is required if you live in a cooler or milder climate. Your climate zone also determines how many BTUs per square foot are needed.
- Windows: The more windows in your home and the larger they are, the more impact they have on energy efficiency. A Manual J calculation considers these variables and whether you have more energy-efficient windows such as double- or triple-pane or glazed models.
- Doors: An AC contractor will also consider the number of exterior doors. Usually, they’ll add 1,000 BTUs for every exterior door in your home. Older, drafty doors increase the cooling power needed; weather stripping and newer, more tightly sealed doors can decrease the required cooling load.
- Sun Exposure: If your home faces south, it will receive more direct sunlight than if it faces north. Therefore, increased heat absorption will make it harder to cool your home, and you’ll need a larger AC.
- Your Home’s Facade: Darker paint colors and denser materials like brick absorb heat more readily. Therefore, your home’s color and siding factor into a load calculation. The calculation also takes into account whether there are trees, awnings, and other sources of shade.
- Heat-Producing Appliances: A load calculation accounts for heat-generating appliances in your kitchen, laundry room, and other areas, which increase the number of BTUs needed to cool your home.
- Condition of Ductwork: If you have leaky ductwork, conditioned air can escape and outdoor air can enter, making your AC system less efficient. Assuming you opt not to replace old ductwork, you’ll need a stronger air conditioner to compensate. Duct size is an issue too, as improperly sized ducts can lead to friction loss and airflow velocity issues, putting pressure on the new unit.
- Layout/Interior Features: The layout of your home can determine AC size. A load calculation can factor in whether there are walls and partitions, your house has an open layout, or it has a sunroom, fireplace, or other features that can influence indoor temperatures. A two-story home will also need more cooling power than a single-story home.
Equipment Sizing vs. Type of Air Conditioner
Equipment sizing works the same way regardless of the air conditioner type. An AC installation professional will use the same load calculation procedure. However, depending on your home, different cooling systems have different potential when it comes to energy efficiency.
Therefore, a technician’s recommendations may vary whether you choose one of the following:
- Split System: It consists of an outdoor and indoor unit linked by refrigerant lines, and uses ductwork to deliver conditioned air.
- Packaged AC: Combines all the components in one unit, including the condenser, compressor, and evaporator, and uses ductwork to deliver cool air.
- Ductless AC: Consists of one outdoor unit connected to one or more indoor units linked by a series of pipes. It eliminates ductwork, improves efficiency, and allows for zoning so the temperature of different rooms or spaces can be controlled independently.
No matter what type of AC system you install, proper equipment sizing can ensure it runs efficiently, provides consistent cooling, and lasts longer.
Schedule Air Conditioner Installation In Portland
At Sky Heating & Air Conditioning, we provide custom AC design services that include proper sizing and high-quality AC installation. Our licensed technicians are fully trained, prompt, honest, reliable, and attentive to detail. They are also well-versed in heating and cooling system repair and maintenance, while our plumbing and electrical technicians can tend to your home service needs as well. When it’s time for AC repair or replacement in your Portland-area home, call (888) 900-2109 or contact us online for immediate help.