AC systems are rated by Tonnage or “Tons” The rule of thumb is for every 500 sq/ft you need one ton of AC/Heating capacity. There is a lot that goes into it and you should always consider having a ACCA (Air conditioning Contractors of America) approved Heat Load Calculation or “Manual J” done to determine the exact size your home needs. We screen all of our HomePros for this capability.
Higher tonnage is not always a better thing when it comes to AC systems. Choosing lower tonnage system have proven to be quite a disappointment to a homeowners. Systems that are too large “super cool” the area causing the thermostat to shut off quickly by not taking out enough humidity. The environment still has not had time to cool off thus it starts heating up the space again causing the entire system to turn off and on repeatedly throughout the day.
Systems that are too small will tend to run a lot longer and may never get the home to the desired comfort level. Properly sized systems will keep up with the demands of the home while not using too much electricity.
If you have too large of a system, it won’t run long enough to keep up with the humidity. In order to handle both temperature and humidity efficiently, an HVAC system should run for around 30 minutes at a time. If your system is staying on too long or shutting off after only around 10 minutes, you are not getting efficient performance.
A little simple math can help determine the size system you need.
Another rule of thumb is 20 BTUs per square foot. So, a 1000 square foot room would need 20,000 BTUs to cool or warm it efficiently. The region in which you leave as well as attic insulation and/or thermal barriers all play a large part in this as well as:
- Color of the roof
- Construction materials used when the house was built
- Exterior doors, Types and Quantities
- Foundation type
- Insulation type and R-values
- Landscaping that affects things like Shading, Sunshine or Constant Breeze)
- Size of the house in square footage
- Type of roof (Wood, Stone, Tile Etc)
- What temperature you want to maintain to be comfortable
- Where area or exact location where you live
- Which direction the house is facing
- Windows (type, ((wood, aluminum, vinyl)) location, and quantity)
The short answer is you really should use Licensed Contractor to do a Heat Load Calculation to determine the proper size of HVAC equipment needed. Don’t just let the contractor tell you they are doing the heat load calculation. If it is really done you will receive a multi page document with all of your calculated date on it. The heat load calculation is used to calculate the proper size of heating and cooling equipment for your home requires use of an procedure such as ACCA/ANSI Manual J. Using short-cuts or rules-of-thumb typically results in equipment that is incorrectly sized for your home. Visit ENERGY STAR Heating and Cooling for more information.