ASHRAE Journal - March 2021 - 44

ENGINEER'S NOTEBOOK
COLUMN

Nathan Ho

Low System Pressure Case Study

Troubleshooting Campus
Chilled Water Systems
BY NATHAN HO, P.E., MEMBER ASHRAE

On one of the hottest days on record for Southern California, with temperatures
reaching 105°F (40.6°C), a medical institution in the region experienced a sudden loss
of cooling at one of its campus research facilities, and the author's team was called to
investigate. This column presents a case study in troubleshooting and solving a chilled
water (CHW) distribution issue related to low system pressure.
Background
The facility in question is a four-story mixed-use
medical research facility with one of the four stories
located underground. A year earlier, the facility underwent a major renovation with new roofing, HVAC equipment, controls, lighting systems and interior finishes.
In that project our team upgraded the cooling system,
replacing rooftop air-handling units (AHUs) that used
direct-expansion refrigerant coils with two new AHUs
using chilled water coils; these were served by a new
chilled water riser with connection to the campus'
chilled water utility distribution piping (recently added
under a separate project by another design-build team).
Start-up and commissioning were completed during the
summer, and for a year the building's CHW system proceeded to function as designed. Then came the mysterious cooling failure.

First 24 Hours-Investigation
Late morning on what was already a hot day, the
facility owner's project manager received a call from
users in a research suite on the underground level,
44

ASHRAE JOURNAL

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MARCH 2021

stating that room temperatures were substantially
warmer than setpoint. Sensing that the day would
become significantly hotter, and knowing that the
AHU serving the space is a 100% outdoor air unit, the
project manager proactively called in our designbuild team to investigate.
Upon arriving, we first checked the overheated areas
and confirmed that the supply air temperature at the
diffusers serving these spaces was indeed too warm to
cool the space. We quickly determined that the elevated
supply air temperature was not due to a failed duct
heating hot water control valve; the associated heating
coils serving these zones were cool to the touch, and
the measured supply air temperature was similar to
the supply air temperature found at the corresponding
rooftop AHU. (See the observations on the next page for
AHU-1's supply air temperature.)
Moving further upstream in the HVAC system, we
proceeded to the roof to check global system performance. Figure 1 shows the CHW system layout, with " CC "
Nathan Ho, P.E., is an associate principal and engineering group leader with P2S Inc., in Irvine,
Calif. He is the chair of ASHRAE SPC 110.



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ASHRAE Journal - March 2021

Table of Contents for the Digital Edition of ASHRAE Journal - March 2021

Contents
ASHRAE Journal - March 2021 - Intro
ASHRAE Journal - March 2021 - Cover1
ASHRAE Journal - March 2021 - Cover2
ASHRAE Journal - March 2021 - 1
ASHRAE Journal - March 2021 - Contents
ASHRAE Journal - March 2021 - 3
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ASHRAE Journal - March 2021 - Cover3
ASHRAE Journal - March 2021 - Cover4
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