Mean radiant temperature (MRT) is the uniform surface temperature of a radiantly
black enclosure in which an occupant would exchange the same amount of radiant
heat as in the actual nonuniform space.Along with air speed, humidity, and the more
common dry-bulb temperature, thermal radiation control is essential to human thermal
comfort. That condition of mind that expresses satisfaction with the thermal
environment is defined as thermal comfort.

Because air speed and relative humidity in an occupied space are normally within
recommended limits, the main comfort variables are air and MRT.Yet, because drybulb
air temperature is the only input factor to the thermostat, the impact of MRT is
unrecognized.

Everyone has experienced some MRT impacts. Perhaps the number one MRT
impact is that caused from ever-changing solar radiation that is impacted by clouds,
the rotations of the earth, and building design. A change in the presence of solar
radiation can easily produce a double-digit in-space MRT swing.A totally different
MRT influence comes from floors that are more than a few degrees above or below
room dry-bulb air temperature. This may be due to factors such as the impact of
location above an unheated garage, a hot boiler room, or a vacated space. Mean radiant
temperature swings occur in low–thermal mass materials such as windows, corrugated
steel, and uninsulated partition walls. Mean radiant temperature influences
occupant thermal comfort throughout the year in the built environment.

Another significant, though generally overlooked, MRT impact appears when
space occupancy is materially changed. An auditorium fills or empties. The traditional
thermostat is reactionary—it will not recognize the MRT occupancy change
but will only respond when the dry-bulb air temperature in the vicinity of the control
rises or falls in response to the impact of the occupancy change on air temperature.
In the case in which the heating system is on when the space fills, the system
continues to heat to the thermostat set point.This could result in significant temperature
overshoot, because each occupant contributes approximately 100 W of heat to
a space being heated. Alternatively, the additional 100-W occupancy load will not be
seen by the cooling system until the air temperature reaches the set point. In either
case, if MRT were sensed, the control would respond immediately to the increased
or decreased load requirements, avoiding the conditions caused by the inability of a
conventional thermostat to maintain the dry-bulb set point.

Morris L. Markel, a man with a lifetime of experience designing electric heating
equipment, developed a device, the Comfort-Check Meter (Fig. 4.1), which analyzes
in-space occupant thermal comfort.The device is quick, handheld, and can be set up
to record 24 h of readings for subsequent download through the RS-232 port directly
into a computer spreadsheet for analysis.The Comfort-Check Meter is discussed in
more detail in Sec. 4.2 of this chapter.

The building and design community has recognized the MRT impact on occupant
thermal comfort. They have sought to reduce or manage MRT impacts by building
tighter buildings; requiring higher building material R values; installing thermally
broken doors and windows to reduce thermal bridging; and using spectrally selective
glazing, favorable building orientation design, and other factors. The objective is to
reduce MRT swings so that radiant asymmetry is reduced to acceptable occupant
thermal comfort levels.

These changes, as positive as they are, do not eliminate the impact of MRT on
daily occupant thermal comfort because systems are controlled solely by dry-bulb
air temperature. For example, along with the improvement in building construction
and materials, the practice of temperature setback is becoming a building code standard.
But the MRT impact of internal building surface temperatures is not taken
into account in the economic analysis, productivity, or equipment sizing. People are
cold (and possibly less productive) on Monday morning, as the warm-up recovery is
in progress.

ASHRAE Research Project 1114, Develop Simplified Methodology to Incorporate
Thermal Comfort Factors for Temperature Setback/Setup into In-Space Heating
and Cooling Design Calculations, should provide important information that will
include MRT and define temperature setback-setup in terms of occupant thermal
comfort. It will do this with a mathematical model that will predict the timedependent
thermal comfort characteristics of vacant or occupied space.

In addition to those MRT influences that occur naturally or in the course of
building use, there are situations in which hybrid systems, a subject covered in Sec. 7,

involve using both radiant and convective systems.ASHRAE Research Project 907,
Design Factor Development to Obtain Thermal Comfort with Combined Radiant and
Convective In-Space Heating and Cooling Systems, developed information that is
covered in detail in Sec. 7. Although the focus is on system analysis, this information
assists in understanding the in-space thermal comfort impacts of common-spot comfort
influences, such as wall cooling units, fireplaces, hot-air registers, radiators, baseboards,
space heaters, and radiant ceiling, wall, or portable heaters.

Mean radiant temperature is an obvious and important determinant of thermal
comfort and energy efficiency. It must be incorporated into design and engineering
programs to demonstrate the benefit. Obvious comparative benefit will drive build-
ing, system, control, and energy management system MRT implementation.When
that occurs, the building and HVAC industry use of the word comfort will accurately
project occupant experience in the built environment.

Mean radiant temperature (MRT) is the uniform surface temperature of a radiantly
black enclosure in which an occupant would exchange the same amount of radiant
heat as in the actual nonuniform space.Along with air speed, humidity, and the more
common dry-bulb temperature, thermal radiation control is essential to human thermal
comfort. That condition of mind that expresses satisfaction with the thermal
environment is defined as thermal comfort.

Because air speed and relative humidity in an occupied space are normally within
recommended limits, the main comfort variables are air and MRT.Yet, because drybulb
air temperature is the only input factor to the thermostat, the impact of MRT is
unrecognized.

Everyone has experienced some MRT impacts. Perhaps the number one MRT
impact is that caused from ever-changing solar radiation that is impacted by clouds,
the rotations of the earth, and building design. A change in the presence of solar
radiation can easily produce a double-digit in-space MRT swing.A totally different
MRT influence comes from floors that are more than a few degrees above or...