INDOOR ENVIRONMENTAL QUALITY

Temperatures and Occupancy Settings

Temperature and relative humidity measurements are often collected as part of an indoor environmental quality investigation because these parameters affect the perception of comfort in an indoor environment. The perception of thermal comfort is related to one's metabolic heat production, the transfer of heat to the environment, physiological adjustments, and body temperature. Heat transfer from the body to the environment is influenced by factors such as temperature, humidity, air movement, personal activities, and clothing. The ANSI/ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy specifies the combinations of indoor environmental and personal factors that produce acceptable thermal conditions to a majority of occupants within a space [ANSI/ASHRAE 2013b]. Assuming slow air movement (less than 40 feet per minute) and 50% indoor relative humidity, the operative temperatures recommended by ASHRAE range from 68.5^oF to 75^oF in the winter, and from 75^oF to 80.5^oF in the summer. The difference in temperature ranges between the seasons is largely due to clothing selection. ASHRAE also recommends that indoor relative humidity be maintained at or below 65% [ANSI/ASHRAE 2013b]. The EPA recommends maintaining indoor relative humidity between 30 and 60% to reduce mold growth [EPA 2012]. 

Occupied and Non-Occupied Settings

Buildings with simple HVAC systems often operate the ventilation system during occupied hours and then turn them off completely at night or other periods when the building is unoccupied.  While turning the system off may save energy, depending on outdoor conditions, it often increases demand on the HVAC system when it is turned back on. Essentially, the equipment has to operate longer and harder to reach desired indoor temperature and humidity set-points.  It can also create issues with condensation if temperatures indoors become warm during the “off” periods. This is particularly true in areas with warmer climates. More sophisticated HVAC systems with programmable thermostats or building automation systems allow for the ventilation equipment to be “set back” during unoccupied periods. This method still allows the indoor temperature and humidity to drift further from the occupied set-points, but eventually the HVAC system will come on to prevent fluctuations as extreme as they might otherwise be with the equipment powered off.  This “set back” method still provides significant energy savings, but it does not require the system to work as long or as hard to bring the indoor conditions back to set-points in preparation for building occupancy.  Whether the system runs continuously, is powered off during unoccupied periods, or is “set back” when empty, the indoor temperature and humidity conditions should always meet recommendations found in ANSI/ASHRAE 55-2013 and ANSI/ASHRAE 62.1-2013 any time the building is occupied.