ASHRAE Journal - February 2013 - 45

on those loops near the entrance when the system is in cooling mode.
The ventilation/dehumidification air system must also be
designed with condensation avoidance in mind. The apparatus dew point of the system should be well below the entering chilled water for the floor cooling system. For highly
dense occupancies, this differential could be as much as 10°F
(5.6°C). The air delivery system should be designed to provide
uniform distribution of air around the conditioned floor area.
Although uniform distribution may not be as critical as in a
space conditioned by an all air system, avoidance of stagnant
areas where local humidity levels can rise is important to condensation avoidance. For systems with locally intense latent
gains or with consistently open entrances, CFD simulations
modeling two species (dry air and water vapor), can assess
condensation risk.
For many of the spaces that could benefit from thermally
active floor systems, water features are an architecturally desirable element. Water features, especially splashing ones,
are effective evaporative humidifiers for the space, leading
to an increased differential between the supply air moisture
ratio and the room ambient condition. However, water features can be turned from a negative to a positive element
of the thermally active structure system by refrigerating the

water in the feature during the cooling season. Lowering the
water temperature feature to several degrees below the floor
entering chilled water dramatically decreases the evaporation rate from the water feature and provides a damper on
room moisture ratio increase. If the space ambient dew-point
temperature rises above that of the water in the water feature, the water feature itself provides safe dehumidification.
In the Hearst Corporation headquarters, the sensible cooling capacity of the thermally active floor is supplemented by
a large-area inclined water feature, called the “Waterfall,”
that runs parallel to the escalators to the lower level entrance
(Photo 2, Page 43).
Control sequences for avoiding condensation are relatively
simple. Ventilation systems that feature demand-controlled
ventilation will increase airflow in response to additional occupancy, thus raising dehumidification capacity in response
to increased interior latent load. Also, ventilation/dehumidification system supply air leaving temperature can be reduced
(reducing apparatus dew point) when space dew-point temperature rises. A safety measure is to incorporate a floor circulation pump shutdown control in response to space dew-point
temperature rising to within a safety margin of the space dewpoint temperature. The safety margin should be determined in
response to potential latent load asymmetry within the space.

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February 2013

ASHRAE Journal

ASHRAE Journal - February 2013

Table of Contents for the Digital Edition of ASHRAE Journal - February 2013