TAB Journal Fall 2020 - 18

The percent of this HVAC unit's 521
MBH heating capacity represented by the
minimum OA airflow load component
(i.e. 1,600 CFM) is approximately 194
MBH, or 37.4 percent of the unit's total
heating capacity. If the HVAC unit is
changed from handling 20 percent OA to
50 percent OA, then the heating capacity
required to temper the OA to the leaving
air temperature of the HVAC unit is 401.4
MBH or 77.2 percent of the unit's total
heating capacity. It should also be pointed
out that under this scenario (i.e. 50 percent
OA airflow), the mixed air temperature of
return air and OA has dropped to 42.5ºF,
which is approaching the temperature that
could freeze the HVAC unit's cooling coil.
It is evident that if this unit's OA is
increased to 65 percent OA, then the
mixed air temperature would be in the
range of 32ºF and freeze the cooling coil.
If the HVAC unit is changed from
handling 50 percent OA to 75
percent OA, then the heating
capacity required to temper the
OA to the leaving air temperature of the
HVAC unit is 496.7 MBH or 95.5 percent
of the unit's total heating capacity.
Under this scenario (i.e. 75 percent OA
airflow) the mixed air temperature of
return air and OA would drop to 26.3ºF,
and definitely freeze the cooling coil.
Additionally, if most, or all the unit's
heating capacity is consumed warming the
OA airflow to an acceptable coil-leaving
air temperature, then the interior space
temperatures might not be suitable for
keeping occupants warm enough to be
able to be productive during wintertime
operation.

SUMMARY AND
CONCLUSIONS:
As can be readily seen in the above
scenario, just because utilizing 100 percent
OA on your existing HVAC equipment
can be helpful in inhibiting the spread of
COVID-19 in your facility, it does
not mean that you can
actually implement this
change on your existing
equipment without being
significantly penalized.

18

If you take this approach without doing
anything else, it is guaranteed that you will
be worse-off for implementing this change.
Not only will you have exceeded the
cooling capacity of your existing HVAC
equipment, and no longer be able to
cool the facility, but you will have laid
the groundwork for experiencing future
indoor air quality problems and organic
growth on interior facility and HVAC
unit surfaces, including the possibility
of culturing Legionella if a flooded or
clogged coil condensate drain pan is now
causing condensate to puddle on the floor
of the HVAC unit cooling coil, supply air
fan, or discharge air plenum sections.

" If you do not have
sufficient cooling
capacity to convert
your existing
HVAC equipment
to 100 percent
OA ventilation,
then don't even
attempt to do so,
because you will
only be creating
additional long-term
problems. "

These problems would be in addition
to COVID-19. Increasing the amount of
OA ventilation in your facility can help
minimize the spread of the COVID-19
virus by diluting the concentration of any
airborne virus in the space with clean and
filtered OA. However, do not attempt to
increase the OA airflow percentage on
your existing HVAC equipment unless
you are certain that the equipment has
sufficient cooling capacity to handle
the increased cooling load caused by
increased OA utilization.
If you do not have sufficient cooling
capacity to convert your existing HVAC

equipment to 100 percent OA ventilation,
then don't even attempt to do so, because
you will only be creating additional
long-term problems. The best and most
expedient way to increase your facility's
OA ventilation capacity is to add a
separate dedicated OA supply air handling
unit, properly sized to fully condition
100 percent OA airflow at summertime
conditions down to the required discharge
air temperature of your existing HVAC
equipment. Other suggestions for helping
contain the spread of any airborne
COVID-19 virus include the following:
1.	Have a professional engineer confirm
the actual cooling capacity available
on your existing HVAC equipment,
and ascertain whether or not any of
the HVAC air handling units can have
their specified design minimum OA
airflows increased by some nominal
percentage - say 10 to 15 percent
- without exceeding the units rated
cooling capacity.
2.	If your HVAC equipment controls
currently include a demand control
ventilation mode, disable it. The goal is
to dilute the air supplied to the facility's
interior spaces with as much outdoor
airflow as possible.
3.	Increase the amount of OA airflow
provided by the existing HVAC
equipment during facility unoccupied
operating hours, when it would be
acceptable to allow interior space
temperatures to be somewhat higher
than normal, as a means of purging
the facility with additional OA
ventilation during these times.
4.	If you have cooling coil discharge
ultraviolet lights, make sure that they
are operating properly, and utilize them
continuously because the Environmental
Protection Agency and others have
demonstrated that UV germicidal
irradiation cleaners can destroy
pollutants, bacteria, and viruses.
5.	Make every effort to ensure that cooling
coil condensate drain pans remain clean,
unflooded and unclogged, in order to
prevent the occurrence of standing
water which could stagnate, and lead to
the formation of Legionella.

TAB Journal Fall 2020



TAB Journal Fall 2020

Table of Contents for the Digital Edition of TAB Journal Fall 2020

TAB Journal Fall 2020 - Cover1
TAB Journal Fall 2020 - Cover2
TAB Journal Fall 2020 - 1
TAB Journal Fall 2020 - 2
TAB Journal Fall 2020 - 3
TAB Journal Fall 2020 - 4
TAB Journal Fall 2020 - 5
TAB Journal Fall 2020 - 6
TAB Journal Fall 2020 - 7
TAB Journal Fall 2020 - 8
TAB Journal Fall 2020 - 9
TAB Journal Fall 2020 - 10
TAB Journal Fall 2020 - 11
TAB Journal Fall 2020 - 12
TAB Journal Fall 2020 - 13
TAB Journal Fall 2020 - 14
TAB Journal Fall 2020 - 15
TAB Journal Fall 2020 - 16
TAB Journal Fall 2020 - 17
TAB Journal Fall 2020 - 18
TAB Journal Fall 2020 - 19
TAB Journal Fall 2020 - 20
TAB Journal Fall 2020 - Cover3
TAB Journal Fall 2020 - Cover4
https://www.nxtbook.com/ygsreprints/AABC/tab-journal-winter-2021
https://www.nxtbook.com/ygsreprints/AABC/G122399AABC_Fall2020
https://www.nxtbook.com/ygsreprints/AABC/G120138AABC_Summer2020
https://www.nxtbook.com/ygsreprints/AABC/G118248AABC_Spring2020
https://www.nxtbook.com/ygsreprints/AABC/G116302AABC_Winter2020
https://www.nxtbook.com/ygsreprints/AABC/G114573_AABC_Fall2019
https://www.nxtbook.com/ygsreprints/AABC/G111085_tab_summer2019
https://www.nxtbook.com/ygsreprints/AABC/g108931_tab_spring2019
https://www.nxtbook.com/ygsreprints/AABC/g104992_tab_winter2019
https://www.nxtbook.com/ygsreprints/AABC/g100221_tab_fall2018
https://www.nxtbook.com/ygsreprints/AABC/g97743_tab_summer2018
https://www.nxtbook.com/ygsreprints/AABC/g94804_tab_spring2018
https://www.nxtbook.com/ygsreprints/AABC/g91043_tab_winter2018
https://www.nxtbook.com/ygsreprints/AABC/g86727_tab_fall2017
https://www.nxtbook.com/ygsreprints/AABC/g81312_tab_summer17
https://www.nxtbook.com/ygsreprints/AABC/g78003_tab_spring17
https://www.nxtbook.com/ygsreprints/AABC/g73432_tab_winter17
https://www.nxtbook.com/ygsreprints/AABC/g7002_tba_fall2016
https://www.nxtbook.com/ygsreprints/AABC/g65897_tab_summer2016
https://www.nxtbook.com/ygsreprints/AABC/g62758_tab_spring16
https://www.nxtbook.com/ygsreprints/AABC/tab_winter2016
https://www.nxtbook.com/ygsreprints/AABC/tab_fall2015
https://www.nxtbook.com/ygsreprints/AABC/tab_summer2015
https://www.nxtbook.com/ygsreprints/AABC/g50853_aabc_spring2015
https://www.nxtbook.com/ygsreprints/AABC/g48663_aabc_winter2015
https://www.nxtbook.com/ygsreprints/AABC/g46741_aabc_fall2014
https://www.nxtbook.com/ygsreprints/AABC/g43965_aabc_summer2014
https://www.nxtbook.com/ygsreprints/AABC/g42323_aabc_spring2014
https://www.nxtbook.com/ygsreprints/AABC/g40447_aabc_winter2014
https://www.nxtbook.com/ygsreprints/AABC/g38793_aabc_fall2013
https://www.nxtbook.com/ygsreprints/AABC/g36202_aabc_summer2013
https://www.nxtbook.com/ygsreprints/AABC/g34728_aabc_tabspring2013
https://www.nxtbook.com/ygsreprints/AABC/g32884_aabc_tab-winter2013
https://www.nxtbook.com/ygsreprints/AABC/g30873_aabc_tabjournal_fall2012
https://www.nxtbook.com/ygsreprints/AABC/g28877_aabc_tabjournal_summer2012_2
https://www.nxtbook.com/ygsreprints/AABC/g25452aabc_tabspring12
https://www.nxtbook.com/ygsreprints/AABC/g23327aabc_fall_2011
https://www.nxtbook.com/ygsreprints/AABC/g21694aabc_summer11_final
https://www.nxtbook.com/ygsreprints/AABC/g19912nxtbk
https://www.nxtbook.com/ygsreprints/AABC/g19105_aabc_tabjournalwinter11
https://www.nxtbook.com/ygsreprints/AABC/AABC/p17871_aabc_fall10
https://www.nxtbook.com/ygsreprints/AABC/g16041aabc_tabjournal
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