HOW ARE AIR CONDITIONING SYSTEMS DESIGNED?
At Colony Plumbing, Heating and Air Conditioning, we receive many calls asking why AC systems are not cooling to 69°F and below and why the AC unit is running constantly during hot spells. It is important for home and building owners to understand the criteria by which air conditioning systems are typically designed. These criteria which have been established by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) and are referenced by most energy codes. This memo presents the typical design criteria and explains their ramifications. It also explains why AC systems are not typically designed to cool rooms below 75°F and why they may not appear to work properly on very hot days.
WHAT ARE THE CRITERIA FOR AC SYSTEM DESIGN?
- Indoor design temperature is 75°F
- Outdoor design temperature is 85-90°F
- Outdoor relative humidity is 50%
WHAT DOES THIS MEAN?
What this means in practical terms is that a properly designed AC system will cool a building to 75°F on a day when it is 85-90°F outdoors with 50% relative humidity (the temperature varies depending on the geographic location of the building).
WHAT ARE THE CONSEQUENCES OF THESE DESIGN CRITERIA?
It is important to understand the following:
- The system is not designed to cool a building to less than 75°F on the hottest days.
- If the outdoor temperature and humidity exceed 85-90°F and 50%, the building or certain areas of the building may not be able to be cooled as low as 75°F.
- Temperatures and humidity may exceed these values for several days at a time during a “hot spell.”
Kohler and Lewis, Mechanical Engineers
When it is about 85°F and 50% humidity, the AC unit is not designed to hold in home temperatures below 75°F. If you have your thermostat set lower, the AC unit will have to work harder and longer to attempt to achieve your desired temperature. Understanding the anatomy of the unit will help you understand it’s capabilities.