Chapter 5.1.6 - Basics Of Radiant Design

Maximizing radiant output is the goal of radiant panel design. Minimizing backloss
and edge loss is an important first step toward maximizing frontal heat output. Panel
planar location and airflow past the panel determine the split between radiant and
convection heat output. Evaluation of radiant panel efficiency is essential to determining
panel sizing and estimating system energy use. The analysis of the surface
heat balance applies the first law of thermodynamics to the surface of the radiant
panel. Figure 1.2 shows the thermal resistance factors, represented by R, the series of
resistors.

Although the ratio of heat delivered is an important design factor, it is essential
to factor in the thermal boundaries imposed by occupant thermal comfort to
develop a practical radiant system design.The chart in Fig. 1.3 shows the percentage

of dissatisfied people related only to discomfort due to cold or warm feet related to
floor temperature. The participants were in a condition of thermal neutrality. The
results were not gender-sensitive.

Another significant factor is the impact of air movement, whether natural or
mechanical, in reducing the surface temperature of the radiant heat panel, thereby
reducing the radiant output. In the built environment, air movement is primarily a
function of the heating system design. Coordination of air distribution design is
important for radiant panel sizing and design to assure desired performance, and
conformance to indoor air change code requirements. Studies of the effect of air
velocity over the whole body have found thermal acceptability unaffected in neutral
environments by air speeds of 50 feet per minute (fpm) or less. Subjective responses
indicate that there is no interaction between air speed and radiant temperature
asymmetry. Changes in percent dissatisfied due to draft and radiant asymmetry are
independent and additive.There was no significant difference between responses of
men and women to draft.

The chart in Fig. 1.4 shows the relationship of occupant thermal comfort to draft
in terms of the percentage of people dissatisfied (PPD) as a function of mean air
velocity.

Radiant panel planar location is the primary determinant of the split between
radiant and convection frontal heat output. In a laboratory study, zero-clearance
framed insulated heat panels placed upon the ceiling have 95 percent radiant output.

The same panel placed on the wall will have from 60 to 70 percent radiant frontal
outputs. Face-up floor placement further increases convection and reduces radiant
output to between 50 and 60 percent or less.These differences are important sizing
determinants. For example, an ASHRAE-sponsored University of Illinois research
project found the wattage requirements for identical radiant output to be 250 W for
ceiling location, 325 W for wall location, and 350 W for floor location. Radiant output
is a key factor in reducing the energy required to provide conditions of equivalent
human thermal comfort with a radiant system in comparison with a convection
heating system.

Maximizing radiant output is the goal of radiant panel design. Minimizing backloss
and edge loss is an important first step toward maximizing frontal heat output. Panel
planar location and airflow past the panel determine the split between radiant and
convection heat output. Evaluation of radiant panel efficiency is essential to determining
panel sizing and estimating system energy use. The analysis of the surface
heat balance applies the first law of thermodynamics to the surface of the radiant
panel. Figure 1.2 shows the thermal resistance factors, represented by R, the series of
resistors.

Although the ratio of heat delivered is an important design factor, it is essential
to factor in the thermal boundaries imposed by occupant thermal comfort to
develop a practical radiant system design.The chart in Fig. 1.3 shows the percentage

of dissatisfied people related only to discomfort due to cold or warm feet related to
floor temperature. The participants were in a condition of thermal neutrality. The
results were not gender-sensitive.

Another significant factor is the impact of air movement, whether natural...