When we do energy assessments on houses, 95% of the time we find that the air conditioner is over-sized. An over-sized AC unit can cause all kinds of problems from high humidity, mold/allergy issues, high energy bills and increased wear and tear on the existing unit.
Bigger is not better.
When an air conditioner is over-sized, it has enough cooling capacity and will blow cold air into the space quickly, causing the thermostat to be satisfied with the temperature and shutting off the unit. The problem is the unit doesn’t run long enough to remove the humidity. In Arkansas, 30% of the energy consumed while in cooling mode is to remove humidity and if the unit isn’t running it is not removing humidity.
We don’t build homes the same way we use to.
The rule of thumb is one ton of air conditioning per 500 square feet of conditioned space. While that may have been true in the 1950s when people first started putting air conditioners in their homes, we don’t build houses the same way we did back then. Building codes now require builders to account for energy efficiency measures that allow smaller air conditioners we use.
Properly size your HVAC – do it, it matters.
To properly size an air conditioner a few things need to be done before the purchase is made. First, an energy assessment should be done on the home. One of the biggest problems we see with HVAC systems is that the ducts leak to the outside, often more then half of the capacity. Just sealing the ducts up can make an existing air conditioner over-sized. Another issue that can affect the size of the system is the amount of insulation in the ceiling above. Simply getting your home’s insulation to an R-38 can allow you to downsize even more. Finally, a load calculation should be done on the house using the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHREA) Manual J calculator to ensure that you are not over sizing the unit.
Downsizing will save you money.
Being able to downsize the unit will save you money on the replacement cost. Ideally, you would use the money saved on buying a smaller unit to upgrade to a more energy efficient variable speed air conditioner. With having a variable speed air conditioner, you not only set the desired temperature on the thermostat, but you also set your desired humidity level as well. When you are able to control the humidity, you can take control of your comfort in the home and can cut out many of the problems that cause health issues too. Having a properly sized HVAC system will save you up to 50% of your energy consumption during heating and cooling months, will lead to more comfort in the house, and can reduce dust mites and mold problems in the home.
Chris Kell – RESNET Certified HERS Rater | BPI Building Analyst | Energy Star 3.1 Certified