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Indoor Temperature Setpoints for Energy Efficiency

Submitted by Visitor on 5 January, 2008 - 10:59

The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard 55 - 2004 'Thermal Environmental Conditions for Human Occupancy' provides guidelines for acceptable ranges of temperature and humidity for indoor comfort (CSA Standard CAN/CSA Z412-00 - 'Office Ergonomics' contains similar recommendations). By Fanger's PMV model, the following recommended temperature ranges can be assumed to satisfy 80% of adult office workers. Consequently, some occupants will feel uncomfortable to some degree even if these values are met.

The following data show acceptable temperatures for people with typical winter and summer clothing doing sedentary work at 50% relative humidity and air speed not exceeding 0.15 meters/sec (30 feet/minute).

Predicted Optimum Temperature and Humidity of an Office Environment

Heating Season
Temperature Range 68-75°F (20-24°C)
Predicted Optimum 71°F (22°C) at 50%rH

Cooling Season
Temperature Range 73-79°F (23-26°C)
Predicted Optimum 76°F (24.5°C) at 50%rH

ASHRAE recommends a range of acceptable relative humidity levels of 30-60%. Very low humidity can lead to increased static electricity and health problems, such as skin irritation. An acceptable indoor temperature setpoint will vary with humidity, air velocity, clothing, and activity level. Radiation is also a factor. Someone sitting near a warm or cold surface such as a window, may have a different optimal temperature when all other factors are equal.

Temperature setpoints at different indoor humidity levels:

Summer (clothing insulation = 0.5 clo)
Relative humidity 30%: 76-82°F (24.5-28°C)
Relative humidity 60%: 74-78°F (23-25.5°C)

Winter (clothing insulation = 1.0 clo)
Relative humidity 30%: 69-78°F (20.5-25.5°C)
Relative humidity 60%: 68-75°F (20-24°C)

Temperature setpoints for various activities:

Temperature
°F °C
78 25 Optimal for bathing, showering. Sleep is disturbed
75 24 People feel warm, lethargic and sleepy. Optimal for unclothed people.
72 22 Most comfortable year-round indoor temperature for sedentary people.
70 21 Optimum for performance of mental work.
64 18 Physically inactive people begin to shiver. Active people are comfortable.

So what temperature setpoints should we use?

Capron Company standards require a minimum 2°F (1°C) deadband for acceptable control of zone air temperature. Increasing the deadband between heating and cooling setpoints will provide a corresponding decrease in energy consumption. Therefore, standard setpoints for office environments in the Baltimore/Washington corridor are preset for zone controllers as follows:

For LEED and most projects: 70°F (21°C) heating, 75°F (24°C) cooling
For best comfort (least complaints): 72°F (22°C) heating, 74°F (23°C) cooling

Unless directed otherwise, CCI will set zone controller setpoints installed on new projects to 70°F heating, 75°F cooling.

Note: Nominal values are shown for Celsius conversion of Fahrenheit.