ASHRAE Journal - February 2021 - 47

COLUMN ENGINEER'S NOTEBOOK

time the building was less than fully occupied, especially
PHOTO 1 Humidity and temperature levels for the room with the worst conditions (left) and the second-worst conditions (right).
in the summer. The fan coil units were controlled to meet
space temperature setpoint. In the cooling season during
times of low occupancy, the chilled water valves would
modulate toward the closed position to prevent overcooling the spaces. This caused the discharge air temperature
from the fan coil units to rise. Since the air entering the
unit was a mixture of humid outdoor air and room air, the
humidity of this air was high.
When the chilled water valve was partially closed, the
unit didn't adequately dehumidify the incoming air. The
supply air temperatures we observed ranged from 58.7°F
(14.8°C) to 70.8°F (21.5°C). The leaving air conditions were
slightly cooler than room temperaPHOTO 2 Mold on furnishings in the rooms shown in Photo 1.
ture, but with very high humidity,
driving the humidity in the space
up. Without intervention, the
humidity in the room would continue to move closer to 100%.
We offered several options to help
remedy the problem. The ﬁrst and
most comprehensive option was to
install a dedicated outdoor air unit
to condition and dehumidify the
outdoor air. The unit would contain
FIGURE 1 BMS screen shot. The supply air temperature on many units was only a few degrees below room
setpoint.
a total heat recovery wheel, chilled
water coil and hot water heating/
reheat coil. The conditioned outdoor
air would be ducted directly to each
space. All outdoor air to existing fan
coil units would be removed, and the
fan coil unit fans would cycle to maintain space temperature. The chilled
water control valves would modulate
to maintain a 55°F (12.8°C) discharge
air temperature. This option would
be the most expensive ﬁrst cost, but
would provide the best control of
space temperature and humidity levels at the lowest
space temperature, but would use more energy due to
energy cost.
the electric reheat. The building electrical service would
The second option was to install an electric reheat coil
also need to be upgraded to provide enough power to
at each fan coil unit. The fan coil unit chilled water conserve these coils.
trol valve would be modulated to maintain a discharge
The third option was to install CO2 sensors in each
occupied room that was served by a fan coil unit. The
air temperature of 55°F (12.8°C) or lower to maintain
associated fan would be turned off if the space temspace humidity, and the electric reheat would be staged
to maintain the space temperature setpoint. This option perature and the CO2 setpoints were met. If either the
space temperature or CO2 was above the setpoint, the
would be effective at controlling humidity levels and
FEBRUARY 2021

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ASHRAE Journal - February 2021

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