Choosing the correct size HVAC — heating, ventilation and air conditioning system — for your home is important for both your finances and your personal comfort. An HVAC system that is too small must run more frequently, increasing your electricity bill and the need for repairs. On the other hand, an HVAC system that’s too large doesn’t run often enough to successfully perform secondary tasks such as regulating indoor humidity. The industry standard for determining the correct HVAC system size is complex and generally best performed by an experienced professional, but you can use some simpler methods for closely approximating your needs.
Step 1 – Obtain a copy of Manual J, published by the Air Conditioning Contractors of America and widely considered the authority on HVAC sizing. The manual covers a complex calculation that takes into account everything from the type and location of each window in the home to the presence of appliances such as a dishwasher or second freezer. Appliances like these emit small amounts of heat that can affect a building’s overall heating and cooling needs.
Step 2 – Download the Speed Sheet Excel worksheet from the Air Conditioning Contractors of America to assist in your calculations.
Step 3 – Enter information about your geographic area, such as latitude and elevation, into the spreadsheet. Measure the size of all windows and doors, making note of the direction each faces. Use each sheet to determine the relevant values for your flooring, windows and doors. Follow all other Manual J directions as specified to determine the specific needs of your home.
Step 4 – Measure the square footage of your home. Divide the total square footage by 600 to determine the size of the air conditioning unit you require in tons. According to The Weather Channel, a home requires one ton of air conditioning for every 600 square feet. One ton of air conditioning is the amount of air conditioning power required to freeze one ton or 2,000 pounds of water for 24 hours.
Step 5 – Use the size of your existing HVAC unit if you’re confident that it’s the right size. Use this method cautiously, because there may be an opportunity for savings and increased efficiency with a more accurately sized unit.