Dry bulb temperature is the actual air temperature measured by a standard thermometer with an unshielded bulb exposed to air circulation. This is the temperature shown on thermostats and weather forecasts. Dry bulb temperature forms the basis for cooling and heating load calculations in HVAC system design.
Load Calculation Standards
ASHRAE standards establish design dry bulb temperatures for different geographic locations and design conditions. For cooling, engineers typically use temperatures representing 1 percent or 2.5 percent annual cumulative hours, generally between 90°F and 105°F depending on climate zone. For heating, design dry bulb temperatures range from negative 20°F in cold climates to positive 30°F in moderate zones.
System Sizing Impact
Cooling and heating capacity selection depends directly on outdoor design dry bulb temperature. A 5-degree increase in design temperature can increase cooling loads 15 to 20 percent, requiring larger equipment and higher operating costs. Indoor design dry bulb temperature for occupied spaces is typically 72°F for cooling applications. Accurate dry bulb temperature selection ensures equipment adequately handles peak conditions while optimizing efficiency during normal operating seasons.