“Rules of thumb” are too often used to size comfort systems, which results in excessively oversized systems. . This leads to increased cost, wasted energy, and too-frequent on and off cycling, which can lead to comfort and efficiency issues. A trained contractor or qualified designer will determine a home’s specific heating and cooling loads to establish the right size equipment. This is achieved with sizing calculations based on recognized industry standards such as those developed by the Air Conditioning Contractors of America.