ASHRAE Journal - July 2020 - 30

TECHNICAL FEATURE

to certain climates and may not be
possible in all regions of the country
or world. Protecting them from the
weather can be as simple as providing a small tent.

Lesson Learned 3: Use a Pressure Monitor With
A Digital Display and Alarm for Each Negative
Pressure Room
During a pandemic, shortages of
equipment and supplies may exist.
If manometer alarms are not available, routine checks for negative
pressure are required. That means
assigning a qualified person to check
each of them every 15 minutes.

Lesson Learned 4: Where Fire-Rated Walls are
Penetrated, a Fire Watch is Required
Avoid penetrating fire-rated walls
with temporary, flexible ductwork
whenever possible. Anytime we
penetrate a fire-rated wall with a
duct or anytime we have an opening in a fire-rated wall, we need
to install a fire damper or a combination of a fire/smoke damper.
Obviously, during an emergency
installation of temporary, flexible
ductwork, there is not adequate
time for such installation. Most
code and authorities having jurisdiction allow such installations
without fire/smoke dampers as long
as the hospital assigns a full-time,
qualified professional as a "fire
watch."

Lesson Learned 5: Use Rooms With Full-Height
Walls or a Solid Ceiling, Whenever Possible, to
Prevent Impact To Adjacent Rooms
T-bar ceilings often do not allow
proper pressure control and can
experience excessive air leakage
or adversely affect the pressure
in adjacent rooms. Remember,
these are temporary setups and the
30

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room pressurization could easily
be affected by an accessible control knob that can be adjusted by
a patient or hospital staff. Manual
adjustment of the HEPA exhaust fan
speed may result in fluctuation of
the differential pressure to the corridor by over 90% (from 0.01 in. w.c.
to 0.10 in. w.c. [2.5 Pa to 25 Pa] or
above) depending upon the size of
the HEPA exhaust fan. We learned
this the hard way!

Lesson Learned 6: Do Not Use the Existing
Return Air Duct System For Exhaust
Using the existing return air duct
(Photo 2) may contaminate existing ductwork, spread infection and
create significant challenges for
infection control. In one case, the
facilities maintenance staff used
the direct digital control (DDC)
system and temporarily closed the
mixed air dampers of an existing
economizer to provide 100% outside
air (OA) to the space. They then
used the return air duct to exhaust
the air from a temporary negative
pressure room. This may be a costly
mistake due to the following three
inherent problems:
1. The system is not designed for
100% OA, and temperature control
of the entire hospital may be affected.
2. 100% OA on an economizer
is not really 100%. Economizer
dampers often have massive leakage rates and contaminated return
air will likely mix with supply air
and contaminate the entire hospital.
3. Let's assume everything works
out well for now. What happens
when we return to normal operation? We now have return air
ductwork that is contaminated. We

J U LY 2 0 2 0

PHOTO 2 Don't use the existing return air duct
system for exhaust (Lesson 6).

all have heard that when it comes
to HVAC, DDC is your best friend.
In this case, it could become your
worst enemy. "Do not do it," said
Joel Sanders, OSHPD Compliance
Officer.
Standard 170 allows recirculation
with HEPA filtration for specific
spaces including waiting rooms and
triage. For infection isolation rooms,
which is the intent of these negative
pressure rooms, Standard 170 does
not have the same allowance for
recirculation. All air from infection
isolation rooms must be exhausted
to the outdoors per Table 7.1, Design
Parameters-Hospital Spaces, of this
standard.8

Lesson Learned 7: Locate HEPA Exhaust Fan
Units to Ensure All Exhaust Ductwork Outside
The Room is Under Negative Pressure
Any temporary, flexible ductwork
installed in the corridor must be
under negative pressure in accordance with state and international
codes and the Uniform Mechanical
Code. By placing the HEPA exhaust
fan units outside to put the duct
under negative pressure, you can
prevent the contamination of the
corridor if there is a leak in the duct.
In addition, the ductwork should
be run to avoid fire sprinklers and
alarms in the corridor.
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ASHRAE Journal - July 2020

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