Determining what size of central air conditioner your home needs is quite complicated and never something you should do by yourself. If you’re replacing an existing AC, you may think you just need to install the same size as your current system. While this is sometimes the case, the truth is that a surprisingly large number of homes have an AC that is either undersized or oversized.
When sizing an AC system, certified HVAC technicians typically use a formula known as a Manual J load calculation. This calculation takes into account the following six factors to allow the technician to accurately determine exactly how many BTUs of cooling the AC needs to produce for it to work as effectively and efficiently as possible.
1. Square Footage
This first factor is one of the most important since a larger home will obviously have much higher cooling requirements than a small home and thus need a bigger AC. When looking at the square footage, only the actual living space is considered, not things like the garage or attic. If you have an addition to your home or a converted garage or attic, these areas would be considered if you want these areas connected to the central system.
2. Climate Zone
Square footage is important because there are general recommendations regarding the number of BTUs of cooling you need for every square foot. These recommendations are different for each part of the country since cooling requirements are obviously higher in hotter and/or more humid areas than in places with cooler, drier summers.
The International Energy Conservation Code and the US Department of Energy separate the country into seven climate zones based on average temperature and humidity. Most of coastal North Carolina, including the Wilmington area, is in zone 3A, a warm, moist climate. The general recommendation for this area is that an AC system should produce around 40-45 BTUs per square foot of the home. With this information, a technician can start getting at least a rough estimate of how large an AC system needs to be based on the size of the house.
3. Age of Home and Insulation Level
We said that square footage and climate zone only give the technician a rough estimate because the basic BTU recommendations don’t apply equally to every home. This is especially the case with older homes or homes that aren’t as well constructed. In either case, the BTU requirements will almost always be at least a bit higher.
One reason is that many older homes aren’t adequately insulated. This is partly because insulation recommendations have increased in the past decades, and the quality of insulating products has also improved. Another reason is that insulation in walls and attics deteriorates over time, so it isn’t quite as effective at blocking heat. This is especially true for the types of fiberglass insulation used in many attics.
Another reason older homes usually have higher cooling requirements is that they tend to be more drafty. Newer homes are built with more advanced materials, making them much more airtight. In an older home, you usually have quite a few gaps in the structure that allow heat and humidity to seep in from outside. This means the house will get hotter more quickly and thus need a larger AC to cool it effectively.
4. Windows and Doors
Exterior doors and windows are among the biggest sources of heat gain in most homes, especially if they’re not well insulated or properly sealed. South- and west-facing windows and skylights are the biggest sources due to the direct sunlight that shines through them during the hottest times of the day. Whether the windows are shaded by large trees or other buildings, along with the U value of the windows, also makes a difference.
5. Attic Ventilation and Roof Structure
A poorly ventilated attic will also increase the overall cooling requirements of the home. A properly ventilated attic should always stay near the same temperature as whatever it is outside. If the attic doesn’t have sufficient ventilation, most of the heat gain it receives through the roof will get trapped inside. This can easily lead to the temperature increasing as high as 140 degrees by the afternoon on hotter days, which causes the temperature inside the home to increase as well.
The type of roofing materials the home has and the color of the roof also play a role in how hot the attic gets. Some roofing materials absorb much more heat, while others are better at reflecting sunlight and don’t get as hot. A darker-colored roof will also absorb more heat than a lighter-colored roof, which generally leads to the cooling requirements at least slightly increasing.
6. Number of Occupants
The last factor is the number of people who live in the house. As with windows and doors, the formula calls for the technician to factor in additional BTUs for every occupant.
AC Tonnage
Once the technician has performed all of their calculations, they can use the BTU requirements to determine how many tons the AC unit needs to be. Every AC ton is the equivalent of 12,000 BTUs of cooling. Central ACs for residences are available in half-ton increments from 1.5 to 5 tons. When determining tonnage, the technician will round up whenever necessary. This is because it’s always best to opt for a slightly larger AC than you might need rather than running the risk of the unit being too small.
Why Getting the Right Size AC Is Essential
An undersized AC will generally never work well enough to keep your home fully comfortable on hotter, more humid days. It will also use much more energy than necessary since it will have to run so much and work so hard to try and keep your home cool. When the system has to work too hard, it will almost always lead to a shorter lifespan.
While you may think that bigger is always better for an AC, this isn’t the case. A slightly oversized system is generally not an issue, but an AC that is much too big absolutely is. Not only will an oversized system cost more to install, but it will also use lots of extra energy. Oversized ACs also tend to have a much shorter lifespan due to an issue known as short cycling. This is when the system typically only runs for 10 minutes or less since it provides so much cooling in a short time. This is an issue because it typically leads to the system cycling on and off much more often, which puts lots of extra strain on it and increases wear and tear. Short cycling also contributes to much greater energy usage since an AC draws a huge amount of power each time it has to turn on.
If you plan on installing a new AC, the technicians at Green Dot Heating & Air are ready to help ensure you get the right system for your home. We specialize in AC installation and offer financing for new systems on approved credit. Our technicians are also experts in HVAC maintenance and repairs, so contact us if you need any home comfort service in the Wilmington area.
