ASHRAE Journal - February 2021 - 46
ENGINEER'S NOTEBOOK
COLUMN
Kelley Cramm
Resolving High Humidity
In Spaces Using Chilled
Water Fan Coil Units
BY KELLEY CRAMM, P.E., MEMBER ASHRAE
Fan coil units are typically factory standard items with no opportunity for enhanced
features. Their ability to provide dehumidification is limited due to standard coil
depths and fin configuration. This column presents two case studies in which spaces
served by chilled water fan coil units experienced high humidity and mold growth.
Case Study One: Higher Education Building
The first case involved a classroom building at a higher
education campus located in the Midwest. The campus
maintenance staff indicated that they were routinely
measuring humidity in several rooms in the building
above 60% relative humidity (RH) during the cooling
season.
The building had been renovated approximately
eight years prior, and a new HVAC system was
installed. The system consisted of two-pipe fan coil
units that were located above ceilings. The units were
supplied with chilled water during the cooling season
and hot water during the heating season. Hot water
was not available during the cooling season. Fan coil
units serving occupied areas had outdoor air ducted
to them for ventilation. The outdoor air was mixed
with return air prior to connection to the fan coil
units. Because these units provided ventilation air,
the fans ran continuously during occupied hours. The
chilled water valves were modulated to maintain space
temperature.
46
ASHRAE JOURNAL
ashrae.org
FEBRUARY 2021
Shortly after the building was renovated, the campus
maintenance staff began getting high humidity complaints. Each summer, they would install temporary
dehumidifiers in rooms with the highest humidity to
help reduce it. We were asked to review the original
HVAC drawings and visit the site to identify the cause
and recommend options for solving the problem.
When we visited the site, the condition in the worst-case
room was 72.1°F (22.2°C) and 73.3% RH. The second room
we observed was at 70.2°F (21.2°C) and 63.9% RH (Photo 1).
We also observed visible mold on furnishings in the
rooms (Photo 2). When we reviewed data from the building management and control system, we noted room
temperature setpoints were 71°F (21.7°C) or 72°F (22.2°C)
and most rooms were very close to setpoint. We also noted
that the supply air temperature on many units was only a
few degrees below room setpoint (Figure 1).
The fan coil units, which were constant volume, were
sized for a fully occupied building. However, most of the
Kelley Cramm, P.E., is an associate and mechanical technical leader at Henderson
Engineers in Lenexa, Kan.
http://ashrae.org
ASHRAE Journal - February 2021
Table of Contents for the Digital Edition of ASHRAE Journal - February 2021
Contents
ASHRAE Journal - February 2021 - Intro
ASHRAE Journal - February 2021 - Cover1
ASHRAE Journal - February 2021 - Cover2
ASHRAE Journal - February 2021 - 1
ASHRAE Journal - February 2021 - Contents
ASHRAE Journal - February 2021 - 3
ASHRAE Journal - February 2021 - 4
ASHRAE Journal - February 2021 - 5
ASHRAE Journal - February 2021 - 6
ASHRAE Journal - February 2021 - 7
ASHRAE Journal - February 2021 - 8
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ASHRAE Journal - February 2021 - 11
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ASHRAE Journal - February 2021 - 46
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ASHRAE Journal - February 2021 - Cover3
ASHRAE Journal - February 2021 - Cover4
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