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IM-PR 566442 1114 (Design Manual)

Figure 3-4 Percent persons dissatisfied (PPD) based on floor temperature. Note that a minimum of ~66°F is

advisable to achieve 10 PPD

50 59 68 77 86 95 104 °F

510152025303540

°C

FLOOR TEMPERATURE

LOCAL DISCOMFORT CAUSED BY

WARM AND COOL FLOORS

DISSATISFIED

Dewpoint temperature: When a surface temperature

is below the dewpoint temperature of the

surrounding air, condensation is likely to occur

on the surface. Generally speaking, as the relative

humidity of a space decreases, the dewpoint

temperature also decreases. A lower dewpoint

means a lower likelihood of condensation. If you

know the relative humidity and drybulb temperature

of a space, you can determine its dewpoint

temperature through referencing a psychrometric

calculator or a psychrometric chart.

Dewpoint temperature can be proactively controlled

through providing dehumidification to a space, as

explained in Section 3.6.3. To avoid condensation,

the dewpoint should be controlled to remain lower

than the supply temperature to a space.

Temperature drop: To support delivery of high

supply temperatures, which can provide higher

system efficiency, a temperature drop of 5 to 10°F

is recommended for commercial cooling.

Supply temperature: In radiant cooling, it is

advisable to maintain supply temperatures as

high as possible while still meeting design load

requirements for the space. Supplying higher

temperatures to the zone permits increases

in the efficiency of the cooling equipment and

reduces the risk of condensation when the supply

temperature is below the space dewpoint. The

trade-offs for supplying higher temperatures are

higher flow rates, higher head and potentially

tighter spacing requirements for the tubing (e.g.,

6" to 9"). Table 3-7 shows required mean cooling

water temperatures as a function of floor covering

R-value, thickness of concrete above the tubing,

tubing spacing and targeted cooling capacity.

Mean cooling water temperatures represent the

average water temperature in a panel. Supply

temperature can be calculated from the mean

cooling water temperature using the following

equation:

Ts =

∆T

MCWT

—

where:

Ts = Supply temperature (°F)

MCWT = Mean cooling water temperature (°F)

∆T = Temperature drop across the panel