The easiest way to see if your central HVAC is big enough is to look at the installation and manufacturing manuals for the HVAC system you have. Check the number of square feet that the manufacturer indicates will be controlled by the system. If you are using a system designed for much less square footage than you have, then the system probably will need upgrading. The average central air unit should handle between 3 to 6 tons, or between 36,000 and 72,000 british thermal units (BTUs) BTUs are the amount of energy needed to raise 1 pound of water one degree Fahrenheit, with 12,000 BTUs making a ton.
Ace Hardware offers the following basic formula to find out if your central air is properly sized based on square footage and BTUs: Multiply the square footage of the area to be cooled by 25. Then add 1,000 per window and 400 per occupant. Compare this number to the number of BTUs the central air system is designed to handle. If the number is higher than the BTUs your system is able to handle, then you'll probably need to get a larger system. A more complete calculator is available on the Association of Home Appliance Manufacturers' website at cooloff.org.
Sometimes an HVAC system is large enough to handle the number of square feet you have but is designed in such a way that it is inefficient. For example, if ducts and vents are extremely long, then your hot and cool air will need to travel very long distances before it can reach you, and much of the energy may be absorbed by the materials that make up the system. Look to see if there are unnecessary turns in the ductwork, whether there are just a few filters that clog more quickly than if many would be used, where the thermostats and vents are placed and what elements (e.g., antifreeze, heating coils) the system utilizes.
Temperature and Humidity
Consider whether your heating and cooling system is running continuously and what the relative humidity is like in your home. The larger your system, the more it will turn on and off instead of running all the time. This means it won't filter as much air and won't remove as much moisture, and relative humidity impacts how hot or cool the air will feel to you. If you have moisture problems or feel like you're constantly at the thermostat, try downgrading to a smaller system that will run more often. Do just the opposite if moisture is scarce and the climate is extremely constant.
Look at your energy bill. If you are paying an arm and a leg for a system that never seems to shut off, you're likely to be paying more because the system has to run continuously. This is an indication that the system is having trouble keeping up with the demands you are placing on it. If you live in a humid climate where running a central air system more often is preferable to control temperature and moisture, you don't necessarily need to upgrade. Just get a more efficient HVAC instead, such as those that meet Energy Star requirements.