The Importance of Properly Sizing Your AC Unit: An Expert's Perspective

As an HVAC expert, I have seen many homeowners make the mistake of purchasing an air conditioning unit that is too large for their space. This can lead to a variety of issues, including high energy bills and poor indoor air quality. In this article, I will discuss the importance of properly sizing your AC unit and how to determine the right size for your home.

The Rule of BTUs

When it comes to heating, the general rule is to have 30 to 35 BTU per square foot in Zone 1 and 35 to 40 BTU per square foot in Zone 2.However, when it comes to cooling, the rule is not as straightforward. Many people believe that a 5-ton AC unit will work perfectly for a 2000 square foot space, but this is not always the case. While a 5-ton unit may cool down the space quickly, it will not be able to effectively remove moisture from the air.

This can lead to a damp and uncomfortable indoor environment. To properly cool a space, you need to consider two-stage condensers and air controllers with variable speed drives. This is especially important when you are cooling a space that is too small for the equipment you have.

The Importance of Floor Area

When determining the right size for your AC unit, it is important to take into account the size of your house. One way to do this is by dividing the load or capacity by the floor area.

For example, a 3000 square foot house with a 3-ton AC unit would have 3 ÷ 3000 = 0.001 tons per square foot. While this may seem like a small number, it can give us a better understanding of the appropriate size for our AC unit. By investing in a nicer number, we can determine that 1000 square feet per ton is the ideal ratio. However, in reality, only a small percentage of homes have this ratio. In fact, out of 167 zones, only 53 had less than 1000 square feet per ton and only 20 zones had less than 700 square feet per ton.This means that only 12% of the areas in this group could have come close to having refrigeration equipment of the right size using the rule of 400 to 600 square feet per ton.

The average load of the 75 homes we studied was about 1200 square feet per ton. This is why it is crucial to properly size your AC unit.

The Importance of Proper Design

Designing an HVAC system starts with choosing the right size for your AC unit. It is important to look at the number of square feet per ton to determine if you are in the right ballpark. If the number is less than 1000 square feet per ton for newer homes, it could mean that either the number is wrong or the house is not as efficient as it should be. As an example, I have a 1920s vintage house with about 2400 square feet.

I have a 1-ton Midea modulating agitator (basically a mini-split in a can) mounted in my office loft window at the top of the stairs. With all interior doors open, it covers 100% of my 1% cooling load (1% local temperature tank = 83°F) at an average indoor temperature in the mid-70s F. This means that under design conditions, I have a ratio of one ton per 2400 square feet.Without the window unit turned on, my central air with 5-ton ducts is ridiculously oversized. The duty cycle never exceeds ~50%, even when outdoor temperatures reach 90°F.

This implies a load ratio of one ton per 1000 square feet at outdoor temperatures of 10°F or more above the 1% design temperature. As you can see, properly sizing your AC unit can make a significant difference in its efficiency and effectiveness.

How Many Square Feet Will a 3-Ton AC Unit Cool?

If you apply the general rule of 20 BTU per square foot, you can determine that a 3-ton AC unit is suitable for cooling spaces of approximately 1800 square feet. However, it is important to keep in mind that this is just a general rule and may not be accurate for every home. If you are in the market for a new AC unit, it is essential to do your research and find the best fit for your home. You can check out the best central air conditioners with prices for 2, 3, 4, 5, and 6-ton units to get an idea of what is available on the market.

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