ASHRAE JOURNAL ashrae.org DECEMBER 202022
Are Mixing Boxes
At Performing the
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
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,
DECEMBER 2020 ashrae.org ASHRAE JOURNAL 23
lead to coil freezing.
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.
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.
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
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
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
as both range effectiveness and ther-
mal statistical mixing effectiveness. Robinson
deﬁned range effectiveness as:
And statistical effectiveness as:
= Temperature standard deviation downstream
of the mixing device
= Temperature standard deviation upstream of
the mixing device
FIGURE 1 Economizer diagram.
Recirculated Air (CA)
Return Air Fan (RAF)
FIGURE 2 Test setup from ASHRAE Research Report RP-1045.
FIGURE 3 Commercially available 20 ton to 25 ton mixing box.
ASHRAE JOURNAL ashrae.org DECEMBER 202024
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
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.,
deﬁnes the upstream standard
is the return air temperature and T
outdoor air temperature.
This deﬁnition differs from the deﬁnition in another
paper, where Sauer, et al.,
deﬁnes the upstream stan-
dard deviation as:
are the velocity of the return and out-
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
A simple conservation of energy evaluation neglect-
ing kinetic and potential energy changes yields this
Assuming the density, r
, and speciﬁc heat, c
is negligible, the equation becomes:
are the return, outdoor and
mixed air volumetric ﬂow rates, respectively. T
are the return, outdoor and mixed air tempera-
Using the relationship between volumetric ﬂows and
simplifying yields the following:
+ Q A
= (1 – %OA)Q
= (1 – %OA)T
+ %OA × T
where %OA is the percent of outdoor air.
This is the equation used in an economizer reference
for ﬁnding the mixed air temperature.
as the average theoretical tempera-
ture, we can compute a more meaningful upstream
standard deviation for the statistical effectiveness.
where the data set consists of T
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
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.
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,
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
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
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.
This model is a two-equation model, combining turbu-
lent kinetic energy and dissipation.
Boundary conditions were set as published in the
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-
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.
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,
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
117 in.×38 in. Plenum
90 in.×28 in. Plenum
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.
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
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.
FIGURE 6 Temperature distribution with same size dampers.
FIGURE 7 Plenum 90 in. × 28 in. with return air damper same size as outdoor air
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.
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
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.,
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
FIGURE 8 Plenum 90 in. × 28 in. with 24 in. × 88 in. dampers and 10,000 cfm
total flow rate.
Temperature Probes and
Outlet of Mixing Box
FIGURE 9 Mixing effectiveness improvement with baffles and filter rails.
DECEMBER 2020 ashrae.org ASHRAE JOURNAL 29
The fan curve of a commercially available 25 ton
(87.5 kW) air handler
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.
1. 2019 ASHRAE Handbook—HVAC Applications.
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should we do about it?” ASHRAE Journal (2).
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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.
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for Honeywell Economizer Controls.” Honeywell.
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Dampers for Air-Side Economizer Systems.
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My CFD Analysis?” SimScale.com. https://tinyurl.com/y4qwygm3
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