Despite common belief, the size of your AC isn’t determined by your home’s square footage.
The size of your AC is determined by something called a “Manual J Load Calculation.” This is a calculation done by an HVAC professional to assess how much heat should be removed from your home (per hour) to keep it cool.
Now that you know the basic foundation of how ACs are sized, you probably have even more questions, like:
- What exactly goes into a load calculation?
- How are AC systems sized?
- Why does AC size matter anyway?
Below, we’ll answer these three questions so that you’re prepared to buy the optimal size AC for your home.
Rather just have a professional determine what AC size is needed for your home? Contact Advantage Air Mechanical to learn more about our AC installation process. Our technicians will find the perfect AC size for your Tucson home and cooling needs.
What exactly goes into a load calculation?
Your AC system cools your home by removing heat, so a load calculation looks at how much heat needs to be removed from your home to keep it cool.
While the concept seems simple, a load calculation must take into account a variety of factors that impact how much heat needs to be removed from your home, including:
- Where you live (climate)
- Quality and location of the ductwork
- Quality and type of insulation
- Height of the ceilings
- Type of flooring (carpet vs wood)
- Size, layout, orientation of your home
- Number of and age of windows
- Number of people living in your home
- Number of doors in your home
- Appliances in your home
- Type of roofing
- Square footage of your home
Let’s look at a few variables in-depth:
- Where you live (climate): Arizonans living in a hot, dry climate will need a different AC size than, say, somebody who lives in New Jersey.
- Orientation of the home: If your home mostly faces the sun, it will quickly warm up.
- Number of people living in the home: The AC must be sized to factor in people and body heat.
- Appliances in the home: The oven/stove warm up the kitchen.
A quality HVAC technician will properly perform a load calculation and let you know what size AC you’ll need in British Thermal Units (BTUs). We’ll dive into the details of how AC systems are sized in BTUs, below.
How are AC systems sized?
Contrary to popular belief, AC systems are not measured by physical dimensions (i.e. cubic feet). They are sized by the amount of heat they can remove from your home per hour. The heat removal is measured by British Thermal Units or BTUs.
For reference, 1 BTU is the amount of heat produced by 1 lit match. So, a 13,000 BTU AC would remove the heat produced by 13,000 lit matches in an hour from your home.
One lit match is a tiny amount of heat to measure, hence why most residential AC systems fall between 12,000 to 60,000+ BTUs.
AC size is also expressed in tonnage, with one ton being the equivalent of 12,000 BTUs. It’s good to know the conversion rate when shopping for ACs because only the tonnage may be listed.
Here’s a quick reference guide to convert tonnage to BTUs:
Now that we’ve covered how ACs are sized, let’s discuss why size even matters at all.
Why does the AC size matter?
You may be thinking that a bigger AC system is “better” at cooling your home. However, bigger is not always better. Like we discussed above, many factors impact what AC size is needed for your home. Installing an AC system that is too big or small for your home might cost you money and cooling problems down the line.
If your AC is too big for your home … it will cool your home quickly and then shut off. This constant on-and-off is called short cycling and can cause issues like:
- High energy bills
- Uneven cooling throughout your home
- Shortened lifespan for your AC, resulting in expensive repairs or replacements
If your AC is too small for your home … it will struggle to properly cool your home and compensate by running all the time. This constant usage will lead to the same issues that an oversized AC has (e.g. high energy bills, uneven cooling and your AC quickly wearing out).
When you’re investing in something as expensive and long-term as an AC, it’s important to hire a high-quality technician who will take the time to do a proper load calculation so you avoid the issues we listed above.
If the technician says they can determine what AC size you need with only your home’s square footage, we highly recommend finding a different technician. When a technician doesn’t perform a load calculation and ignores all the factors listed above, you risk having the wrong size of AC for your home, which will most likely cost you money in the long run.