In the news these days you will find energy companies asking homeowner to keep their thermostats 78° and up. If you put your thermostat at 78° and don’t feel comfortable, why is that? Humidity! Most people feel hotter when the humidity is high (above ~55%) Most air conditioners run a drag race, meaning the turn on and “floor it” running at 100% until they cycle off again. (newer, hi-tech, hi-efficiency, higher cost equipment solves this issue) These systems concerned themselves with lowering temperature with lowering humidity as a secondary consideration. In Florida we have to take the “It’s the humidity, stupid!” standpoint and consider dehumidification as the primary function of an ac system. The worst part is (if you are married you know this) one person can feel comfortable while the other is freezing (or burning up). More control of your indoor environment requires a greater investment. In the end you have to balance your equipment purchase, your energy bill, and your comfort.
68° – The Danger Zone
The refrigerant in an air conditioner has to be much cooler than the desired indoor temperature. When a homeowner sets their thermostat to 68° the internal system temps have to come perilously close to the freeze point and bad things start to happen. The low temperature can cause icing up making everything even colder. The contracted volume of gas in the system can cause the compressor to start pumping its oil into the system plugging the coil and starving the compressor of lubrication. Modern air conditioners are designed to maintain a 50° coil while outputting air cooled 20° below the incoming air. A 68° room temperature upsets that balance and forces the indoor coil to dip below the factory designed temperature. (some of the newer hi-tech systems have logic to attempt cope with these situations).
The IECC regulates the indoor design temperature for use in performing load calculations. The IECC specifies residential_heating_and_cooling_load_calculation_requirements.pdf that the minimum cooling temperature shall be 75°F. Table IA of ACCA Manual J requires that the outdoor winter and summer design temperatures be based on the 1 percent value for summer. In areas where humidity is an issue (Florida!), an oversized system will degrade the humidity control. A properly sized system will run almost continuously at design conditions, and provide the proper level of dehumidification during the cooling season. (if you want your ac system to cool under 75 degrees on a hot summer day, your system needs to be over-sized. In Florida, over-sized systems have massive problems with humidity control (you WILL need a supplemental dehumidifier) especially on the days the outdoor temperature is several degrees below the summer highs. Also expect descreased system lifespan due to short cycling.)