When an HVAC contractor comes to your home for a new quote, they’ll probably ask you some questions about your comfort and budget. This is typical.
If it’s a quote for a new AC installation, though, an additional step should be taken: calculating the correct size for your equipment.
Unfortunately, this doesn’t always happen, and it hurts thousands of homeowners every year.
Air conditioners aren’t one-size-fits-all, and matching the correct power level to your home is crucial for several reasons. An improperly sized AC will cost you money, efficiency, your system is likely to break down sooner, and you’ll be less comfortable.
So let’s look at this important step, and why you should care about it when you replace your air conditioning unit.
What is Air Conditioner Size
Size can refer to the literal size of the air conditioner, but more traditionally it refers to the power level of a cooling unit.
This power level is indicated by a tonnage level, sometimes known as its capacity, which we define in more detail just below.
The easiest way to imagine it is to think about a small, single-level home, and then another that’s two stories and has 15-20 rooms. The cooling needs of these homes are vastly different!
While that’s an obvious example, correct sizing of an air conditioner involves a lot more than just the square footage of your home. Failure to account for these other factors is what often leads contractors to improperly size HVAC equipment.
AC Tonnage and Capacity
The tonnage of an HVAC system refers to the amount of BTUs (British Thermal Units) a system can move.
In layman’s terms, it’s how much airflow is produced relative to a home’s heating and cooling needs.
Central air conditioners generally range from 1.5 tons to 5 tons. Ductless mini-splits can go lower, but these are not designed to cool entire homes.
When you hear an HVAC representative talk about sizing, this is what they’re referring to.Your home may have a 3-ton system, for example. This is the equipment’s “size.”
Is this the right size for your home, though? This is where we have to dig into how sizing is calculated, and the many variables that affect sizing.
Manual J Load Calculation
A Manual J Calculation is an industry-standard method for calculating the correct size for HVAC equipment. It’s created and approved by the Air Conditioning Contractors of America.
A lot more goes into the calculation than you might be expecting.
Here’s an incomplete list of some of the factors that contribute to a size calculation:
- Square footage (total)
- Square footage of unconditioned spaces (crawl space, basement, patio, etc.)
- Total number of windows
- Size of windows
- How many windows face east/west
- Quality of windows (single-pane, triple-pane, etc.)
- Rooms without ductwork
- Height of the ceilings
- Is it single-story, two-story or other (loft, livable attic, etc.)
- Is it a slab home without a basement
- Quality of the insulation
- Quality of the siding
- Ductwork leaks
- Exposure of the home to the sun
Believe it or not, the quality, number and size of these many factors can take a “2-ton home” to a “3-ton home” or the reverse, or similar jumps in sizing. The amount of energy lost to various home construction factors is enormous, and this contributes to the overall cooling needs of your home.
The purpose of the calculation is to measure energy loss from a home – also sometimes referred to as the amount of heat lost – and this can happen in a variety of ways. If your windows are awful, for example, more cold air will escape and your air conditioner will have to work harder to compensate for this fact.
Or if a space isn’t conditioned, such as a garage, any rooms above it (often a master bedroom) will have higher cooling requirements since the warm air from the garage will seep into the bedroom.
Now take those examples and extrapolate them out to the 20-30 different variables that the Manual J procedure calculates. A lot of cooling energy can be lost and require a more powerful system!
Common AC Sizing Mistakes
The most common mistakes contractors make when sizing equipment are as follows:
- They only use the square footage of a home.
- They reference the previous system’s size, assume it’s correct, and install the same size.
On both, it’s entirely possible that either assumption will still produce the correct size for your air conditioner. For example, if your living space is between 700 and 1,000 square feet, usually you’ll end up with a 1.5 ton air conditioning system. This is why so many contractors use these methods!
But would you rather have a system that you’re 80% sure is the best size for your home? Or 100% certain? All it takes for that last 20% or so is the 15-20 minutes a full calculation takes.
Adjusting the Size of Your AC System
Let’s say your system is 20 years old and it’s well past time you upgraded it. It’s not a guarantee that your previous air conditioner was sized correctly. Or perhaps the home has changed enough that the sizing calculation has changed.
This can be due to a room addition, upgrades to insulation and windows, or simply an improper sizing calculation when the previous system was installed.
The easiest fix is simply to install an AC with higher tonnage, one that better matches the needs of your home.
However, this won’t always work. For example, if you’ve had a room added to the home, the cooling needs of the home are larger. But was ductwork installed that leads to the new room? And is the existing ductwork adequate for the increase in airflow that will come with a more powerful air conditioner?
These are problems that can make an upgrade more expensive than expected. Fortunately, you have other options to consider when upgrading your home cooling.
Ductless Options
A ductless mini-split is designed to heat and cool a single room or area. This can be ideal for home additions where it’s infeasible to add new ductwork to an existing central air system.
It’s often advisable to create an indoor door to section off this area of the home, to keep the cooling areas somewhat separate at times.
A window air conditioner or fan can assist as well, but these generally won’t provide the same even cooling as a central air unit or ductless unit.
Ductwork Modification
The other option is adding ductwork. Sometimes, though, adding ductwork isn’t the need but rather modifying or sealing existing ductwork.
If the airflow coming from your unit is too powerful for the ductwork to properly distribute, you’ll end up with an inefficient system and hot or cold spots in your home that aren’t being properly cooled.
This can be just as bad as having the wrong size of air conditioner to begin with.
If you are changing AC sizing with a new installation, make sure you discuss this with your HVAC partner, since it could contribute significantly to the work and cost of the installation.
The Problems With Oversized and Undersized Air Conditioners
The problems with an undersized cooling unit should be more obvious. The air conditioner will struggle to move enough air to keep your home cool.
This is especially true in two-story homes with sweltering master bedrooms or other corner rooms.
The dangers of an oversized AC unit are harder to imagine, but they can be just as bad.
Imagine that your HVAC system is a finely-tuned machine…which you don’t actually have to imagine, because this is exactly what it is. Now imagine that a lot more airflow power gets pumped through the same system. Several problems can occur:
- The air around the thermostat cools too quickly, causing the system to shut off before the upper levels or corner rooms are sufficiently cooled. This creates wide ranges of home temperatures.
- Inadequate humidity removal from the home as a result of this lack of holistic airflow, causing muggy dampness, problems with mold, mildew and pests.
- The ductwork can’t transport the air correctly, so backups in airflow occur, causing the system to work even harder and leading to long-term wear and tear on the system.
- You may have heard the term “short cycling” and it refers to this imbalance in airflow that causes your system to turn on and off repeatedly. Too much short cycling will cause you to lose money on energy costs and will cause your system to break down.
That’s why simply paying for “more” power isn’t always a good thing, and can in fact be worse for your home. Getting the right size for your home is the key to longevity, efficiency and comfort.
Finding the Perfect AC Size for Your Home
Proper installation is half the battle to having an efficient system and a comfortable home. This is why choosing the right HVAC contractor to work with can be so important.
Incorrectly size your system and you’ll pay for it for years. It doesn’t matter how efficient or powerful the unit should be. In practice, it won’t be good for you.
Finding a company unwilling to cut corners and willing to walk you through all the necessary steps to get you the best system is the best way to ensure that you have the right fit for your home.
