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PLENUMS PRODUCT MODEL # DESCRIPTION
CMBRM12LLS STANDARD MIXING BOX, LOW LEAK, FITS 27″ W X 48″ D
PMB17Q-28LL STANDARD MIXING BOX, LOW LEAK, FITS 17 1/2″ W X 28″ D
PMB21-21ULL STANDARD MIXING BOX, LOW LEAK, FITS 21″ W X 21 3/4″ D
PMB21-28LL STANDARD MIXING BOX, LOW LEAK, FITS 21″ W X 28″ D
PMB23Q-21LL STANDARD MIXING BOX, LOW LEAK, FITS 23 1/2″ W X 21″ D
PMB24Q-28LL STANDARD MIXING BOX, LOW LEAK, FITS 24 1/2″ W X 28″ D
PMB24S-22LLS STANDARD MIXING BOX, LOW LEAK, FITS 24 5/8″ W X 22″ D
TMB120LLS1 STANDARD MIXING BOX, LOW LEAK, FITS 25″ W X 63 1/2″ D
TMB180LLS LOW LEAK: 12.5 TO 15 TON ODYSSEY
TMB240LLS1 LOW LEAK DAMPERS 20 TON ODYSSEY
TMB90LLS1 STANDARD MIXING BOX, LOW LEAK, FITS 25″ W X 47 1/2″ D
TMBMH23LLS STANDARD MIXING BOX, LOW LEAK, FITS 23 1/2″ W X 21 3/4″ D
YMBNC120S1LL STANDARD MIXING BOX, LOW LEAK, FITS 24″ W X 55″ D
CMBRM12S STANDARD MIXING BOX, INSULATED, FITS 27″ W X 48″ D
CMBRM34S STANDARD MIXING BOX, INSULATED, FITS 30″ W X 96″ D
DMB120S STANDARD MIXING BOX, INSULATED, FITS 24″ W X 48″ D
GMB20 STANDARD MIXING BOX, INSULATED, FITS 16 1/8″ W X 20 3/16″ D
PMB14Q-28Q FITS 14 1/2″ W X 28 1/2″ D
PMB15U-45Q FITS 15 3/4″ W X 45 1/2″ D
PMB17-21 STANDARD MIXING BOX, INSULATED, FITS 17″ W X 21″ D
PMB17Q-21 STANDARD MIXING BOX, INSULATED, FITS 17 1/2″ W X 21″ D
PMB17Q-21U FITS 17 1/2″ X 21 3/4″
PMB17Q-28Q FITS 17 1/2″ W X 28 1/2″ D
PMB17Q-28S STANDARD MIXING BOX, INSULATED, FITS 17 1/2″ W X 28″ D
PMB18Q-21S STANDARD MIXING BOX, INSULATED, FITS 18 1/2″ W X 21″ D
PMB21-21 STANDARD MIXING BOX, INSULATED, FITS 21″ W X 21″ D
PMB21-21US STANDARD MIXING BOX, INSULATED, FITS 21″ W X 21 3/4″ D
PMB21-22 STANDARD MIXING BOX, INSULATED, FITS 21″ W X 22″ D
PMB21-28S STANDARD MIXING BOX, INSULATED, FITS 21″ W X 28″ D
PMB23Q-21 STANDARD MIXING BOX, INSULATED, FITS 23 1/2″ W X 21″ D
PMB23Q-21U STANDARD MIXING BOX, INSULATED, FITS 23 1/2″ W X 21 3/4″ D
PMB23Q-23Q STANDARD MIXING BOX, INSULATED, FITS 23 1/2″ W X 23 1/2″ D
PMB24Q-21Q STANDARD MIXING BOX, INSULATED, FITS 24 1/2″ W X 21 1/2″ D
PMB24Q-28S STANDARD MIXING BOX, INSULATED, FITS 24 1/2″ W X 28″ D
PMB24S-22S STANDARD MIXING BOX, INSULATED, FITS 24 5/8″ W X 22″ D
PMB36Q-57N56 FITS 36 1/2″ W X 57″ D
PMB53-26 STANDARD MIXING BOX, INSULATED, FITS 53″ W X 26″ D
RMB10S1 STANDARD MIXING BOX, INSULATED, FITS 24 1/4″ W X 50 1/2″ D
RMB20S STANDARD MIXING BOX, INSULATED, FITS 34 1/2″ W X 59 1/2″ D
TMB120S1 STANDARD MIXING BOX, INSULATED, FITS 25″ W X 63 1/2″ D
TMB180S STANDARD MIXING BOX, INSULATED, FITS 27 5/8″ W X 79 1/2″ D
TMB240S1 STANDARD MIXING BOX, INSULATED, FITS 28 3/8″ W X 92 1/2″ D
TMB60S STANDARD MIXING BOX, INSULATED, FITS 20″ W X 36″ D
TMB90S1 STANDARD MIXING BOX, INSULATED, FITS 25″ W X 47 1/2″ D
TMBMH26S STANDARD MIXING BOX, INSULATED, FITS 26 1/2″ W X 21 3/4″ D
YMB363S STANDARD MIXING BOX, INSULATED, FITS 36 1/2″ W X 100″ D
YMB483S STANDARD MIXING BOX, INSULATED, FITS 44″ W X 103″ D
YMB603S STANDARD MIXING BOX, INSULATED, FITS 53″ W X 103″ D
YMBNC120S1 STANDARD MIXING BOX, INSULATED, FITS 24″ W X 55″ D
YMBNC120TD1212 STANDARD MIXING BOX, INSULATED, FITS 24″ W X 55″ D
YMBNC180S STANDARD MIXING BOX, INSULATED, FITS 30″ W X 73 1/2″ D
PMB28-44 TWINNED: FITS 21″ W X 28″ D
PMB28Q-51 TWINNED: FITS 28 1/2″ W X 51″ D
PMB21-28VLL VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 21″ W X 28″ D
PMB24Q-28VLL VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 24″ W X 28″ D
TMB120VLLS1 VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 25″ W X 63 1/2″ D
TMB120VLLS1NI VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, NON INSULATED, FITS 25″ W X 63 1/2″ D
TMB180VLLS VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 27 5/8″ W X 79 1/2″ D
TMB180VLLSNI VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, NO INSULATION, FITS 27 5/8″ W X 79 1/2″ D
TMB240VLLS1 VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 30 3/8″ W X 92 1/2″ D
TMB240VLLS1NI VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, NO INSULATION, FITS 30 3/8″ W X 92 1/2″ D
TMB60VLLS VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 21 1/4″ W X 37 1/4″ D
TMB90VLLS VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, INSULATED, FITS 25″ W X 47 1/2″ D
TMB90VLLSNI VERTICAL MIXING BOX WITH LOW LEAK DAMPERS, NO INSULATION, FITS 25″ W X 47 1/2″ D
PMB17Q-21QV VERTICAL: FITS 17 1/2″ W X 21 1/2″ D
PMB17Q-28V VERTICAL: FITS 17 1/2″ W X 28″ D
PMB21-28VS VERTICAL: FITS 21″ W X 28 D
PMB23Q-21V VERTICAL: FITS 23 1/2″ W X 21″ D
PMB24Q-28V VERTICAL: FITS 24 1/2″ W X 28″ D
TMB120VS1 VERTICAL 8.5 TO 10 TON ODYSSEY
TMB180VS VERTICAL/UPFLOW 15 TON ODYSSEY
TMB240VS1 VERTICAL/UPFLOW 20 TON ODYSSEY
TMB90VS VERTICAL/UPFLOW 7.5 TON ODYSSEY
DMB120AR FITS 48 1/8″ W X 24″ D
PMB17Q-28AR BUILT IN RELIEF: FITS 17 1/2″ W X 28″ D, INSULATED
PMB21-21AR BUILT IN RELIEF: FITS 21″ W X 21″ D
PMB21-21UAR BUILT IN RELIEF: FITS 21″ W X 21 3/4″ D
PMB21-28AR BUILT IN RELIEF: FITS 21″ W X 28″ D
PMB24Q-28ARS BUILT IN RELIEF: FITS 24 1/2″ W X 28″ D
TMB120ARS RELIEF DAMPER 10 TON ODYSSEY
TMB180ARS RELIEF DAMPER 15 TON ODYSSEY
TMB90ARS RELIEF DAMPER 7.5 TON ODYSSEY
PMB21-28ARLL BUILT IN RELIEF, LOW LEAK: FITS 21″ X 28″
PMB24Q-28ARLLS BUILT IN RELIEF, LOW LEAK: FITS 24 1/2″ W X 28″ D
TMB120ARSLL RELIEF DAMPER, LOW LEAK DAMPERS 10 TON ODYSSEY
TMB90ARLL RELIEF DAMPER, LOW LEAK: 7.5 TON ODYSSEY
TMBMH23ARLL RELIEF DAMPER, LOW LEAK: FITS 23 1/2″ W X 21 3/4″ D
LMB090FRS FILTER RAILS: FITS 16″ W X 24″ D
PMB17Q-21UFR FILTER RAILS: FITS 17 1/2″ W X 21 3/4″ D
PMB17Q-28FRS FILTER RAILS: FITS 17 1/2″ W x 28″ D
PMB21-28FR FILTER RAILS: FITS 21″ W X 28″ D
PMB21H-24QFR FILTER RAILS: FITS 21 1/4″ W X 24 1/2″ D
PMB23Q-21UFR FILTER RAILS: FITS 23 1/2″ W x 21 3/4″ D
PMB24Q-28FR FILTER RAILS: FITS 24 1/2″ W X 28″ D
TMB120FRS FILTER RAILS: FITS 25″ W X 63 1/2″ D
TMB180FRS FILTER RAILS 15 TON ODYSSEY
TMB90FRS FILTER RAILS 7.5 TON ODYSSEY
PMB17Q-28FRVLL FILTER RAILS, VERTICAL, LOW LEAK: FITS 17 1/2″ W x 28″ D
PMB21-21ULLFR FITS 21″ W X 21 3/4″ LOW LEAK, WITH FILTER RACK
PMB21-28FRLL FILTER RAILS, LOW LEAK: FITS 21″ W x 28″ D

How Effective Are Mixing Boxes At Performing the Economizer Function?








ASHRAE JOURNAL ashrae.org DECEMBER 202022
How Effective
Are Mixing Boxes
At Performing the
Economizer Function?
BY STEPHEN WELTY, P.E., MEMBER ASHRAE; RICARDO ORTIZ
Taking outdoor air into the building to help cool a space and provide required venti-
lation is a well-known technique; to do it to maximum effect takes careful analysis
and proper selection of components and control strategies. A mixing box carries out
the economizer function of providing cooling assist and ventilation in buildings by
taking in outdoor air in a single, packaged device. This article examines the effective-
ness of using a mixing box for air mixing and explores strategies for improving its
effectiveness.
Economizers and mixing boxes both consist of a
damper to let outdoor air into the system, a damper to
let return air mix with that outdoor air and an exhaust
damper to prevent over-pressurization of the building.
Mixing boxes can be implemented with all the damp-
ers in one product (built-in relief) or with the relief or
exhaust damper separate. An economizer diagram is
shown in Figure 1.
The main advantage of a mixing box is that it can
be installed as a single unit right before the air han-
dler return and can be implemented with filter racks
and ultraviolet lights to provide air filtration and
sterilization. Since the dampers are close to each other,
they can be mechanically linked and use a single actua-
tor, which reduces cost, simplifies the control strategy
and reduces potential failure points. Since all the econo-
mizer components are in a single location, maintenance
and periodic performance checks are easier.
Dampers installed in ductwork in an economizer
setup can allow for long sections of duct for full mixing
to occur before the cooling coil. Mixing boxes, how-
ever, usually have short sections before the cooling coil,
which can be a disadvantage for effective air mixing. In
worst-case conditions, poor mixing of airstreams could
Stephen Welty, P.E., is vice president, and Ricardo Ortiz is design engineer at Plenums of Florida near Tampa Bay, Fla.
TECHNICAL FEATURETECHNICAL FEATURE
This article was published in ASHRAE Journal, December 2020. Copyright 2020 ASHRAE. Posted at www.ashrae.org. This article may not
be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal,
visit www.ashrae.org.

DECEMBER 2020 ashrae.org ASHRAE JOURNAL 23
lead to coil freezing.
2
The other effect of poor mixing is
causing the economizer controls to operate to the detri-
ment of system efficiency since the sensors are sens-
ing nonrepresentative conditions.
3
ASHRAE Standard
90.1-2019
4
makes recommendations for the high limit
settings in various climate regions, but this assumes a
representative mixed air temperature is sensed.
Several CFD simulations were run to calibrate the
CFD model to test data published in ASHRAE Research
Report RP-1045, “Verifying Mixed Air Damper
Temperature Control and Air Mixing Characteristics.”
by Finaish, et al.
5
The report tested a wide range of mix-
ing box configurations and damper sizes, so a single
test configuration was chosen that was close in size and
configuration to a commercially available mixing box
for use with a 20 ton (70 kW) to 25 ton (87.5 kW) air han-
dler. The test setup is shown in Figure 2. Figure 3 shows the
commercial mixing box product. The test data available
is for a flow rate of 16,000 cfm (7551 L/s) at 15% outdoor
air (OA) and 30% OA. This paper focuses on the 30% OA
case.
The steps used to proceed from the validated CFD
simulation against test data of the setup in Figure 2 to the
commercial product in Figure 3 were as follows:
1. Adjust the CFD model of dimensions in Figure 2 to
match test data within 10% including a mesh sensitivity
study.
2. Make the return air (RA) damper the same size
as the outdoor air (OA) damper, changing it from
54.2 in. × 18.2 in. (1377 mm × 462 mm) to 67.9 in. ×
22.6 in. (1725 mm × 574 mm).
3. Change the box size to that shown in Figure 3
(90.5 in. × 28.375 in. [2299 mm × 721 mm]).
4. Change the RA and the OA damper size to that
shown in Figure 3 (24 in. × 88 in. [610 mm × 2235 mm]).
5. Drop the total flow to 10,000 cfm (4719 L/s) to match
the 25 ton (87.5 kW) rating.
Mixing effectiveness was described in the ASHRAE
Research Report
5
as both range effectiveness and ther-
mal statistical mixing effectiveness. Robinson
6
has
defined range effectiveness as:
E
Range
Range
Range
DS
US
=−
×1 100%
(1)
where
Range
DS
= (T
max
– T
min
)
Downstream
Range
US
= (T
max
– T
min
)
Upstream
And statistical effectiveness as:
E
SD
SD
Statistical
DS
US
=−
×1 100%
(2)
where
SD
DS
= Temperature standard deviation downstream
of the mixing device
SD
US
= Temperature standard deviation upstream of
the mixing device
FIGURE 1 Economizer diagram.
1
Outdoor Air
(OA)
Mixed Air
(MA)
Supply Air
Fan
(SAF)
Cooling
Coil
(CC)
Supply Air
(SA)
Heating Coil
(HC)
Filter
(F)
Recirculated Air (CA)
Dampers (D)
Exhaust Air
(EA)
Return Air Fan (RAF)
Return Air
(RA)
FIGURE 2 Test setup from ASHRAE Research Report RP-1045.
5
Atmospheric
Pressure
OA Damper
RA Damper
Airflow
54
3
16
in.
22
5
8
in.
12 in.
18
1
16
in.
67
7
8
in.
117 in.
38 in.
144 in.
FIGURE 3 Commercially available 20 ton to 25 ton mixing box.
28
3
in.
8
90
1
in.
2
33
3
in.
4
6
3
in.
8
88 in.
24 in.
8 in.
TECHNICAL FEATURE

ASHRAE JOURNAL ashrae.org DECEMBER 202024
As Robinson
6
points out, each effectiveness method
has its own strengths and weaknesses. Range effective-
ness is good for reporting extremes of temperature that
might lead to coil freezing or nuisance freeze stat trip-
ping. Statistical effectiveness is good at reporting the
uniformity of the flow for control response. However,
in practice, usually a single sensor is placed in each
airstream, which makes statistical effectiveness more a
tool for design and analysis and not relevant for control
strategies.
Getting the standard deviation of upstream tempera-
ture has some inconsistencies in the literature and some
practical difficulties in sample size for a standard devia-
tion. Finaish, et al.,
5
defines the upstream standard
deviation as:
SD
US
= 0.5(T
RA
−T
OA
) (3)
where T
RA
is the return air temperature and T
OA
is the
outdoor air temperature.
This definition differs from the definition in another
paper, where Sauer, et al.,
7
defines the upstream stan-
dard deviation as:
SD
VV
VV
TT
US
RA OA
RA OA
RA OA
=
+
−
()
(4)
where V
RA
and V
OA
are the velocity of the return and out-
door airstreams.
This definition of upstream standard deviation
accounts for different airstream velocities, but was
shown to vary widely when the RA damper area was
changed while holding flow rates and all other vari-
ables constant. The derivations of Equation 3 and
Equation 4 were not provided, and the authors of
this paper found no intuitive way to choose between
them.
A simple conservation of energy evaluation neglect-
ing kinetic and potential energy changes yields this
equation:
Q
R A
r
RA
c
p
T
RA
+ Q
OA
r
OA
c
p
T
OA
= Q
MA
r
MA
c
p
T
MA
(5)
Assuming the density, r
x
, and specific heat, c
p
, change
is negligible, the equation becomes:
Q
RA
T
RA
+Q
OA
T
OA
= Q
MA
T
MA
(6)
where Q
RA
, Q
OA
and Q
MA
are the return, outdoor and
mixed air volumetric flow rates, respectively. T
RA
, T
OA
and T
MA
are the return, outdoor and mixed air tempera-
tures, respectively.
Using the relationship between volumetric flows and
simplifying yields the following:
Q
RA
+ Q A
OA
= Q
MA
(7)
Q
RA
= (1 – %OA)Q
MA
(8)
Q
OA
= (%OA)Q
MA
(9)
T
MA
= (1 – %OA)T
RA
+ %OA × T
OA
(10)
where %OA is the percent of outdoor air.
This is the equation used in an economizer reference
manual
8
for finding the mixed air temperature.
Considering T
MA
as the average theoretical tempera-
ture, we can compute a more meaningful upstream
standard deviation for the statistical effectiveness.
SD
TT
n
US
iMA
i
n
=
−
()
−
=
∑
2
1
1
(11)
where the data set consists of T
RA
and T
OA
.
Equation 11 will be used in Equation 2 for reporting of
statistical effectiveness in this paper because it does not
skew the numbers based on damper sizes as Equation 4
does, and it provides a weighted average of tempera-
tures based on the percent outdoor air, unlike Equation 3.
Computer-Aided Design (CAD) Model
Preparation and Uncertainties
An air mixing box with dampers facing perpendicu-
lar to each other was selected for analysis since this
is the most common configuration. The analysis was
performed in steady state with adiabatic walls. Figure 2
shows the 3D model with dimensions created to match
Configuration 4 tested in Test 7, with the results pub-
lished in ASHRAE Research Report, RP-1045, “Verifying
Mixed Air Damper Temperature Control and Air Mixing
Characteristics.”
5
To prevent any unrealistic recircula-
tion in the CFD simulation, the inlets and outlet sur-
faces were extended by 100 in. (2540 mm) and 150 in.
(3810 mm), respectively. There is also some uncertainty
on how the real blades were positioned and their open-
ing angles when performing the test.
TECHNICAL FEATURE

DECEMBER 2020 ashrae.org ASHRAE JOURNAL 25
It was assumed that blades opened at 90 degrees
equals a 100% airflow through that damper. It was esti-
mated that a 30% airflow would occur with the blade
angle at 27 degrees. The exact location of the blades is
also unknown, which might be another factor increas-
ing error between the CFD and the real test. According
to ASHRAE Guideline 16-2018,
9
the return dampers are
usually low value and will behave more linearly for the
control response as parallel dampers. Also according
to Guideline 16-2018, the outdoor air damper does not
control airflow, so it could be parallel or opposed blade.
In this study both dampers are modeled as parallel blade
dampers.
CFD Preparation
A full-cloud computer-aided engineering (CAE)
software with various simulation types was used to
perform the simulations of the mixing box CAD mod-
els. In this case a convective heat transfer simula-
tion was selected with a turbulent flow algorithm. To
select the appropriate turbulent model, the Reynolds
number was calculated assuming the full flow rate of
16,000 cfm (7551 L/s) and is flowing in the direction
shown in Figure 2 with an average air temperature of
90°F (32.2°C).
The Reynolds number was calculated as 244,300,
which defines a turbulent flow. As recommended by
CFD literature, a K-Omega SST model was selected.
10
This model is a two-equation model, combining turbu-
lent kinetic energy and dissipation.
Boundary conditions were set as published in the
ASHRAE report
5
for Configuration 4 and Test 7 using the
normalized temperature rather than test temperatures:
• OA Inlet: 4,800 cfm (2265 L/s) and 30°F (-1.1°C);
• RA Inlet: 11,200 cfm (5286 L/s) and 70°F (21.1°C); and
• Mixed Air Outlet: 1 atm.
As previously mentioned, the range mixing effective-
ness and the statistical mixing effectiveness were used
to validate this CFD simulation to the published test
results. To achieve this, it was required to read the tem-
perature values in the same position as the test probes
in the test report. As shown in Figure 2, a probe grid was
used with 12 in. (305 mm) spacing between each probe,
4.5 in. (114 mm) from the side wall and 7 in. (178 mm)
from top and bottom walls. This produces three rows
of 10 probe points each. Note that these probe points,
which are physically visible in Figure 2 and initially
modeled as features, were replaced with virtual probe
points within the software to reduce mesh size and com-
putational time.
This probe grid is located using coordinates with the
origin in the centroid of the model. As the plenum size
is reduced in the study, the probes remain in the same
location with relation to the center of the model, making
probe points 1, 10, 11, 20, 21 and 30 no longer inside the
control volume being analyzed, as shown in the bottom
portion of Figure 4.
Mesh
The model was run with different mesh settings to test
the sensitivity of the results and how convergence was
affected by the mesh setting. The software allows for the
adjustment of mesh size using a “fineness” value from
one to 10, with 10 being the finest mesh. Simulations
were run with mesh sizes from six to 10. The simulation
failed with mesh fineness at six. With the fineness at
seven, the simulation was successful and convergence
reasonable, but with a high mixing effectiveness error
compared to the published test report. When refin-
ing the mesh to a fineness of eight, convergence was
improved, and the range and statistical mixing effec-
tiveness values were within 2% and 7% of the test data,
respectively.
Figure 5 shows a cut plot showing the temperature dis-
tribution using a mesh fineness of eight, which yields
range and statistical mixing effectiveness of 61.0% and
83.9% compared to 60.3% and 87.2% of the test data,
respectively. Pressure drop was calculated to be about
0.22 in. w.c. (55 Pa).
FIGURE 4 Probe grid layout for 117 in. × 38 in. and 90 in. × 28 in. plenum.
P1 P2 P3 P4 P5 P6 P7 P8 P9 P10
P11 P12 P13 P14 P15 P16 P17 P18 P19 P20
P21 P22 P23 P24 P25 P26 P27 P28 P29 P30
P2
P12
P22
P9
P19
P29
117 in.×38 in. Plenum
90 in.×28 in. Plenum
TECHNICAL FEATURE

ASHRAE JOURNAL ashrae.org DECEMBER 202026
Since the mesh fineness of eight yielded results within
the desired accuracy of 10% of the test results and
increased fineness increased computational time with-
out improving the results, it was decided to use the fine-
ness of eight for the rest of the simulations. The behavior
of the temperature distribution with all mesh settings
was similar between each other, but with isolated low
temperature points that were causing the increased
error using mesh fineness seven. In all cases, a hot air
layer was found at the bottom of the air mixing plenum
and colder/mixed air at the top. Low temperature spots
were at the top, left and right sections, which corre-
spond to Probes 2, 3, 8 and 9.
CFD Results
Once the CFD model was validated with the published
report as described above, the first change was to make
the RA damper the same size as the OA damper, while
keeping everything else constant. Figure 6 shows the tem-
perature distribution. This change decreased the mixing
effectiveness considerably by creating low temperatures
at Probe 1, 2, 9 and 10.
The second change was changing the plenum size to
90 in. × 28 in. (2286 mm × 711 mm) from 117 in. × 38 in.
(2972 mm × 965 mm), while keeping the damper sizes at
their original size. Results are like the previous change
with the temperature distribution following the same
behavior. This change decreased the mixing effective-
ness considerably by creating low temperatures at Probe
1, 2, 9 and 10.
The third case was making both the plenum size
90 in. × 28 in. (2286 mm × 711 mm) and making RA
dampers the same size as the OA damper. In this con-
figuration the temperature distribution changed
compared to all previous scenarios, as cold air coming
from the OA damper tends to go to the bottom and sides
of the plenum, while the hot air stayed in the middle of
the plenum (Figure 7).
Range and statistical mixing effectiveness were
reduced to 26.3% and 68.9%, respectively. It seems the
reduction of incoming return air speed with the increase
of RA damper area and the increase of mixed air speed
with the reduction of mixed air plenum area led to
increased stratification. The pressure drop increased
from the original of 0.22 in. w.c. to 0.27 in. w.c. (55 Pa to
67 Pa).
To finalize this study, the damper sizes were changed
to 24 in. × 88 in. (610 mm × 2235 mm) to match the
dimensions of the mixing box of Figure 3. This mixing
box was analyzed at 10,000 cfm (4719 L/s) total flow
rate, 30% of which is coming through the OA damper.
The mixing effectiveness was reduced even further as
shown in Figure 8, consequently being the configuration
with the most potential to cause freezing of the coils in
the air-handling unit due to its low temperature on the
bottom of the plenum. However, this configuration had
FIGURE 5 CFD simulation results with mesh fineness of eight.
70
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Temperature (°F)
FIGURE 6 Temperature distribution with same size dampers.
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Temperature (°F)
FIGURE 7 Plenum 90 in. × 28 in. with return air damper same size as outdoor air
damper.
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Temperature (°F)
TECHNICAL FEATURE

ASHRAE JOURNAL ashrae.org DECEMBER 202028
a very low pressure drop of 0.075 in. w.c. (18.7 Pa). The
bottom image of Figures 8 and 9 show the extended OA
inlet (blue) and the extended RA inlet (red) as well as the
extended mixed air section extending to the right. These
extended sections are required in the CFD modeling
to avoid unrealistic recirculation. The actual end of the
mixing box can be seen in Figure 9 where the tempera-
ture probes are located.
To improve mixing effectiveness, several baffle strate-
gies were evaluated to achieve the lowest pressure drop
possible and an acceptable level of mixing effectiveness.
Figure 9 shows one of these strategies that incorporate
the use of an existing filter rail accessory as a baffle to
improve mixing. The mixing was greatly improved in
this study from range and statistical mixing effective-
ness of 15% and 52.2% to 77.8% and 90.4%, respectively;
pressure drop increased from 0.22 in. w.c. (55 Pa) from
the original test data to 0.45 in. w.c. (112 Pa) with the
baffle. This last simulation is the only one with all tem-
peratures well above potential freezing conditions.
Other simulations were run and several insights
gained on how to better design a mixing box for its par-
ticular purpose based on climatic region. For ventila-
tion only, a low pressure drop mixing box is preferable,
whereas for thermal mixing, adding baffles or filters can
dramatically improve mixing performance.
While it is understood that the extended outlet sec-
tion is not representative of a mixing box installed on an
AHU, this configuration was chosen because of the test
data available for CFD model validation. Future work
should include modeling filters using porous media
techniques to more closely represent actual conditions.
Recommendations
This analysis has shown too many variables in a mixing
box installation exist that could make thermal mixing
effectiveness low, especially in VAV systems with several
volumetric flow rate operating points. As such, a recom-
mendation for control temperature sensor placement
in the mixed airstream cannot be made due to all of the
variables and demonstrated poor mixing in many cases.
It is much better to focus on getting good thermal
mixing than be too concerned about where the sen-
sor goes. Unless extensive, system-specific analysis has
been done, mixing boxes without baffles or static mix-
ing devices can only be recommended for meeting the
ventilation requirement of ASHRAE Standard 62.1-2019
and not the energy-efficiency requirements of ASHRAE
Standard 90.1-2019.
Even for economizers with dampers in the duct sys-
tems, mixing effectiveness could be poor in certain
circumstances if not carefully specified, as can be seen
in Figure 8 where the airstreams remain stratified for a
long length of duct. The addition of mixing baffles or
filters will add some pressure drop to the system, but
according to Stephens, et al.,
11
the energy consequences
of higher pressure drop filters are likely small. This is
particularly true in light of the energy savings potential
of properly mixing the outdoor air and measuring a rep-
resentative mixed air temperature for a more effective
control response.
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Temperature (°F)
FIGURE 8 Plenum 90 in. × 28 in. with 24 in. × 88 in. dampers and 10,000 cfm
total flow rate.
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Temperature (°F)
Temperature Probes and
Outlet of Mixing Box
FIGURE 9 Mixing effectiveness improvement with baffles and filter rails.
TECHNICAL FEATURE

DECEMBER 2020 ashrae.org ASHRAE JOURNAL 29
The fan curve of a commercially available 25 ton
(87.5 kW) air handler
12
was reviewed to see the conse-
quences of a 0.25 in. w.c. (62 Pa) additional pressure drop.
It was found that it increased blower motor power by 0.5
bhp (370 W). A reasonable estimate for the power to run
the 25 ton (87.5 kW) AC air handler is about 33.5 bhp (25
kW), so that 0.25 in. w.c. (62 Pa) additional pressure drop
increased overall power consumption by only 1.5%.
It is possible to reduce cooling power by up to 13.4 bhp
(10 kW) in some cases by lowering incoming air tem-
perature, and in other cases it is possible to run only the
blower at about 5.4 bhp (4 kW) instead of the whole AC
system at 33.5 bhp (25 kW). So, a clear energy advantage
exists to increasing pressure drop to improve mixing
in the economizer or mixing box application to give the
control system a more representative mixed air tem-
perature signal and to avoid coil freezing.
References
1. 2019 ASHRAE Handbook—HVAC Applications.
2. Cramm, K. 2020. “Why don’t mixing boxes mix and what
should we do about it?” ASHRAE Journal (2).
3. Taylor, S. 2010. “Economizer high limit controls and why
enthalpy economizers don’t work.” ASHRAE Journal (11).
4. ANSI/ASHRAE/IESNA Standard 90.1-2019, Energy Standard for
Buildings Except Low-Rise Residential Buildings.
5. Finaish, F. H. Sauer, B. Van Becelaere. 2003. “Verifying Mixed
Air Damper and Air Mixing Characteristics.” ASHRAE Research
Project RP-1045, Final Report.
6. Robinson, K.D. 2000. “Rating air-mixing equipment.” ASHRAE
Journal (2).
7. Sauer, H.J., P. Hande, F. Finaish. 2008. “Mixing effectiveness
of various damper-plenum configurations.” Transaction of the
Missouri Academy of Science 42(2008):18 – 22.
8. Honeywell. 2013. “Design and Application Guide
for Honeywell Economizer Controls.” Honeywell.
https://tinyurl.com/yxcycn2m
9. ASHRAE Guideline 16-2018, Selecting Outdoor, Return, and Relief
Dampers for Air-Side Economizer Systems.
10. Solmaz, S. 2019. “Turbulence: Which Model Should I Select for
My CFD Analysis?” SimScale.com. https://tinyurl.com/y4qwygm3
11. Stephens, B., A. Novoselac, J.A. Siegel 2010.
“The Effects of Filtration on Pressure Drop
and Energy Consumption in Residential HVAC
Systems (RP-1299).” HVAC&R Research 16(3).
12. Trane. 2012. “Product Catalog: Split System
Air Conditioners Odyssey™ — TTA, TWE.” Trane.
https://tinyurl.com/yynnsvbj
https://bit.ly/3dTa21p
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TECHNICAL FEATURE
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Air Handlers and Gas Furnaces

PARTIAL LIST OF AIR HANDLERS AND GAS FURNACES WITH PLENUMS MIXING BOX ACCESSORIES

We are growing our list of OEM Air Handlers and Gas Furnaces and we need your help to do it.  While our list of Unitary Packaged Units for Curb, Curb Adapters and Air Distribution accessories is comprehensive, we have only recently begun to add Air Handler and Gas Furnace model numbers to our database.   We used to find accessories by using the width and depth of the air handler or gas furnace, but we decided to make it easier for our customers to find our products by adding unit model numbers.  If you would like to add Model numbers to our list, contact us or initiate a chat session with us.

OEM Manufacturer Company Name MODEL NO. Type
CARRIER 59MN7B060C1714 GAS FURNACE
CARRIER 59MN7B080C1714 GAS FURNACE
TRANE S9X1B040U3PSBA GAS FURNACE
TRANE S9B1B040U3PSAA GAS FURNACE
TRANE S9B1B060U4PSAA GAS FURNACE
TRANE S9X1B060U4PSBA GAS FURNACE
TRANE S9X1B080U4PSBA GAS FURNACE
TRANE S9B1B080U4PSAA GAS FURNACE
TRANE S9B1C080U5PSAA GAS FURNACE
TRANE S9X1C080U5PSBA GAS FURNACE
TRANE S9B1C100U5PSAA GAS FURNACE
TRANE S9X1C100U5PSBA GAS FURNACE
TRANE S9X1D120U5PSBA GAS FURNACE
TRANE S9B1D120U5PSAA GAS FURNACE
TRANE S9X1B040D3PSBA GAS FURNACE
TRANE S9B1B040D3PSAA GAS FURNACE
TRANE S9B1B060D3PSAA GAS FURNACE
TRANE S9X1B060D3PSBA GAS FURNACE
TRANE S9X1B080D4PSBA GAS FURNACE
TRANE S9B1B080D4PSAA GAS FURNACE
TRANE S9B1C100D5PSAA GAS FURNACE
TRANE S9X1C100D5PSBA GAS FURNACE
TRANE S9X1D120D5PSBA GAS FURNACE
TRANE S9B1D120D5PSAA GAS FURNACE
TRANE S9V2B040U3PSAA GAS FURNACE
TRANE S9V2B060U3PSAA GAS FURNACE
TRANE S9V2B060U4PSAA GAS FURNACE
TRANE S9V2B080U3PSAA GAS FURNACE
TRANE S9V2B080U4PSAA GAS FURNACE
TRANE S9V2C080U5PSAA GAS FURNACE
TRANE S9V2C100U4PSAA GAS FURNACE
TRANE S9V2C100U5PSAA GAS FURNACE
TRANE S9V2D120U5PSAA GAS FURNACE
TRANE S9V2B040D3PSAA GAS FURNACE
TRANE S9V2B060D3PSAA GAS FURNACE
TRANE S9V2B080D3PSAA GAS FURNACE
TRANE S9V2B080D4PSAA GAS FURNACE
TRANE S9V2C100D4PSAA GAS FURNACE
TRANE S9V2C100D5PSAA GAS FURNACE
TRANE S9V2D120D5PSAA GAS FURNACE
BOSCH BVA048 SPLIT SYSTEM AIR HANDLER
BOSCH BVA060 SPLIT SYSTEM AIR HANDLER
BOSCH BVA024 SPLIT SYSTEM AIR HANDLER
BOSCH BVA036 SPLIT SYSTEM AIR HANDLER
CARRIER 40RM007 SPLIT SYSTEM AIR HANDLER
CARRIER FV4CNF002 SPLIT SYSTEM AIR HANDLER
CARRIER FV4CNF003 SPLIT SYSTEM AIR HANDLER
CARRIER FV4CNF005 SPLIT SYSTEM AIR HANDLER
CARRIER 40RMS034 SPLIT SYSTEM AIR HANDLER
CARRIER 40RMS028 SPLIT SYSTEM AIR HANDLER
CARRIER FA4CN042 SPLIT SYSTEM AIR HANDLER
CARRIER FA4CN048 SPLIT SYSTEM AIR HANDLER
CARRIER FA4CN060 SPLIT SYSTEM AIR HANDLER
DAIKIN DAT090 SPLIT SYSTEM AIR HANDLER
DAIKIN DAT120 SPLIT SYSTEM AIR HANDLER
DAIKIN DAR090 SPLIT SYSTEM AIR HANDLER
DAIKIN DAR120 SPLIT SYSTEM AIR HANDLER
GOODMAN MBVC1200AA SPLIT SYSTEM AIR HANDLER
GOODMAN MBVC1600AA SPLIT SYSTEM AIR HANDLER
GOODMAN MBVC2000AA SPLIT SYSTEM AIR HANDLER
GOODMAN AVPTC24B14 SPLIT SYSTEM AIR HANDLER
GOODMAN AVPTC42D14 SPLIT SYSTEM AIR HANDLER
GOODMAN AVPTC48C14 SPLIT SYSTEM AIR HANDLER
GOODMAN AVPTC48D14 SPLIT SYSTEM AIR HANDLER
GOODMAN AVPTC60D14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT25B14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT29B14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT35B14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT37B14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT33C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT37C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT39C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT47C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT49C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT59C14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT47D14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT49D14 SPLIT SYSTEM AIR HANDLER
GOODMAN ASPT61D14 SPLIT SYSTEM AIR HANDLER
GOODMAN AMST24BU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST30BU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST36BU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST36CU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST42CU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST48CU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST48DU SPLIT SYSTEM AIR HANDLER
GOODMAN AMST60DU SPLIT SYSTEM AIR HANDLER
HEIL FVM4X2400A SPLIT SYSTEM AIR HANDLER
HEIL FVM4X2400AT SPLIT SYSTEM AIR HANDLER
HEIL FVM4X3600A SPLIT SYSTEM AIR HANDLER
HEIL FVM4X3600AT SPLIT SYSTEM AIR HANDLER
HEIL FVM4X4800A SPLIT SYSTEM AIR HANDLER
HEIL FVM4X4800AT SPLIT SYSTEM AIR HANDLER
HEIL FVM4X6000AT SPLIT SYSTEM AIR HANDLER
HEIL FVM4X6000A SPLIT SYSTEM AIR HANDLER
LENNOX ELA072 SPLIT SYSTEM AIR HANDLER
LENNOX ELA090 SPLIT SYSTEM AIR HANDLER
LENNOX CBA27UHE-060 SPLIT SYSTEM AIR HANDLER
LENNOX A2ECON31M-1-120 SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHGE-075 SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHGE-100 SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHGF-100 SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHSA-HM4221 SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHGM090ZL SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHGM120ZL SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHCLA2090CBR SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHCLA2120CBR SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHCLA2150CBR SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHCLA2180CBR SPLIT SYSTEM AIR HANDLER
RHEEM/RUDD RHCLA2240CBR SPLIT SYSTEM AIR HANDLER
TRANE BCVE024 SPLIT SYSTEM AIR HANDLER
TRANE TWE126 SPLIT SYSTEM AIR HANDLER
TRANE BCVE048 SPLIT SYSTEM AIR HANDLER
TRANE BCVE060 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0B18 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0B24 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0B30 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0B36 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0C037 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0C042 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0C048 SPLIT SYSTEM AIR HANDLER
TRANE TEM4A0C060 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0B24 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0B30 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0C36 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0C42 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0C48 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0D48 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0C60 SPLIT SYSTEM AIR HANDLER
TRANE TEM6A0D60 SPLIT SYSTEM AIR HANDLER
TRANE TAM4A0B42 SPLIT SYSTEM AIR HANDLER
TRANE TWE101 SPLIT SYSTEM AIR HANDLER
TRANE TWE120 SPLIT SYSTEM AIR HANDLER
TRANE TWE090 SPLIT SYSTEM AIR HANDLER
TRANE TWE076 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0A18 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0A24 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0B30 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0B36 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0C42 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0C48 SPLIT SYSTEM AIR HANDLER
TRANE GAM5B0C60 SPLIT SYSTEM AIR HANDLER
TRANE BCHD36 SPLIT SYSTEM AIR HANDLER
TRANE TWE201 SPLIT SYSTEM AIR HANDLER
TRANE TWE240 SPLIT SYSTEM AIR HANDLER
TRANE TWE251 SPLIT SYSTEM AIR HANDLER
TRANE TWE300 SPLIT SYSTEM AIR HANDLER
TRANE TWE150 SPLIT SYSTEM AIR HANDLER
TRANE TWE156 SPLIT SYSTEM AIR HANDLER
TRANE TWE180 SPLIT SYSTEM AIR HANDLER
TRANE TWE072 SPLIT SYSTEM AIR HANDLER
TRANE TWE051 SPLIT SYSTEM AIR HANDLER
TRANE TWE060 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B040U2 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B040U3 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B060U3 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B060U4 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B080U3 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B080U4 SPLIT SYSTEM AIR HANDLER
TRANE S9X2C080U4 SPLIT SYSTEM AIR HANDLER
TRANE S9X2C080U5 SPLIT SYSTEM AIR HANDLER
TRANE S9X2C100U4 SPLIT SYSTEM AIR HANDLER
TRANE S9X2C100U5 SPLIT SYSTEM AIR HANDLER
TRANE S9X2D120U5 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B040D2 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B060D3 SPLIT SYSTEM AIR HANDLER
TRANE S9X2B080D4 SPLIT SYSTEM AIR HANDLER
TRANE S9X2C100D5 SPLIT SYSTEM AIR HANDLER
TRANE S9X2D120D5 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0A24 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0B30 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0C36 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0C42 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0C48 SPLIT SYSTEM AIR HANDLER
TRANE TAM9A0C60 SPLIT SYSTEM AIR HANDLER
YORK NC120 SPLIT SYSTEM AIR HANDLER
YORK ND120 SPLIT SYSTEM AIR HANDLER
YORK NC090 SPLIT SYSTEM AIR HANDLER
YORK NC180 SPLIT SYSTEM AIR HANDLER
YORK ND180 SPLIT SYSTEM AIR HANDLER
YORK NL180 SPLIT SYSTEM AIR HANDLER
YORK NM180 SPLIT SYSTEM AIR HANDLER
YORK TM9E026A08MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E040A10MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E060A10MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E060B12MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E080B12MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E080C16MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E100C16MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E080C20MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E100C20MP12 SPLIT SYSTEM AIR HANDLER
YORK TM9E120D20MP12 SPLIT SYSTEM AIR HANDLER
YORK NL120 SPLIT SYSTEM AIR HANDLER
YORK NL090 SPLIT SYSTEM AIR HANDLER
YORK ND90 SPLIT SYSTEM AIR HANDLER