Operative temperature is calculated based on air temperature (dry bulb), mean radiant temperature (MRT), and air velocity. At air velocities below 40 fpm, it is the average of MRT and air temperature. MRT basically is a weighted average of the surface temperatures surrounding an occupant. That is the basic definition; see ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, and 2013 ASHRAE Handbook—Fundamentals for more information.

Why should we care about operative temperature? Because the acceptable thermal-comfort ranges shown in ANSI/ASHRAE Standard 55 are based on operative temperature, not dry-bulb air temperature. The thermal-comfort chart for mechanically conditioned spaces shown in ANSI/ASHRAE Standard 55 (Figure 1) may look like a psychrometric chart, but there is one important difference: The x-axis is operative temperature, not air (dry-bulb) temperature. Use that chart with caution, and be sure to read the fine print.

Besides dry-bulb air temperature, MRT, and air velocity, ANSI/ASHRAE Standard 55 addresses three other factors affecting thermal comfort: metabolic rate, clothing insulation, and humidity. When evaluating the level of thermal comfort in a space, consider all six factors—especially if you have occupants who report feeling cold or warm.

Yet Another Reason Not to Like Mondays

Most non-residential buildings are unoccupied at night and on weekends, their HVAC systems off unless setback and setup temperature setpoints are reached. The operators of many of these buildings start the HVAC systems earlier on Monday than other days of the week. They do this because it takes longer to bring building temperature up (or down) after a weekend of being shut down. But even if space-temperature setpoints are met by the beginning of the occupied period, there still might be comfort complaints. At PECI’s offices in Portland, Ore., the operations team receives more comfort complaints on Monday than any other day of the week, even though the occupied space-temperature setpoints are met. Operative temperature may be the reason for this.

After a HVAC system has been shut down for a weekend during winter or when a HVAC system is operated only to maintain setback temperature, a building’s interior surfaces typically will be cold. These include the interior side of exterior walls, floors, ceilings, and furniture. Because surface temperature takes longer to raise than does air temperature (because of the thermal mass related to surfaces), surface temperatures may be low at the start of an occupied period, even with an earlier start of the HVAC system. This could cause uncomfortable conditions for occupants, especially those sitting near exterior walls. The same holds true for high interior surface temperatures during summer: If surface temperatures are much higher than the air temperature, occupants may feel warm, even if the air temperature is on setpoint.