ASHRAE Journal - February 2020 - 56

COLUMN ENGINEER'S NOTEBOOK
Kelley Cramm

Why Don't Mixing Boxes
Mix and What Should
We Do About It?
BY KELLEY CRAMM, P.E., MEMBER ASHRAE

Many air handlers today include a mixing box to combine the outside air (OA) and
return air (RA) streams upstream of heating and cooling coils. If the climate is
cold, there will be times when the outside air temperature is below freezing. If the
airstreams are not well-mixed, there is a potential to cause a low temperature detector to trip and shut the unit down. Even worse, the cold portion of the airstream could
hit a water coil located downstream and cause it to freeze and burst.
This column describes the scope of the problem and
supply outside air needed for ventilation, the mixing
presents some case studies of projects with mixing
box provides a convenient way to connect the airstreams
boxes that didn't work well, including how the problem
to the unit, and for the two airstreams to mix. Photo 1
was resolved. It also presents strategies to increase the
shows a typical mixing box.
likelihood of creating a well-mixed
Mixing boxes come in various conPHOTO 1 Typical mixing box (courtesy of AAON).
airstream before it reaches a vulnerfigurations with multiple options
able water coil in an air handler.
for OA and RA connections. Most
This column does not address OA/RA
manufacturers offer top/back, top/
damper sizing or control. See ASHRAE
side, side/side, and back/side opening
Guideline 16-2018, Selecting Outdoor,
combinations. Some also offer bottom
Return, and Relief Dampers for Air-Side
connections. Figure 1 shows examples
Economizer Systems, to learn about
of mixing box arrangements.
those topics.
Air stratification is the tendency
of two or more airstreams to remain
Mixing Boxes and Performance
separated in layers. This is a comA mixing box, or mixing plenum, is a section typically
mon occurrence in mixing boxes.1,2,3 Mixing box
located in an air handler. It has connections for OA and
performance can be difficult to predict. There are
RA. It may include separate OA connections for minicurrently no standard test methods to test and rate
mum ventilation and economizer. It usually includes
mixing box performance.4,5 In addition, conditions
control dampers for the OA and RA and may include
Kelley Cramm, P.E., is an associate and mechanical technical leader at Henderson Engineers
filters downstream of the air inlets. For air handlers that in Lenexa, Kan.
56

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ASHRAE Journal - February 2020

Table of Contents for the Digital Edition of ASHRAE Journal - February 2020

Contents
ASHRAE Journal - February 2020 - Intro
ASHRAE Journal - February 2020 - Cover1
ASHRAE Journal - February 2020 - Cover2
ASHRAE Journal - February 2020 - Cover2a
ASHRAE Journal - February 2020 - Contents
ASHRAE Journal - February 2020 - 3
ASHRAE Journal - February 2020 - 4
ASHRAE Journal - February 2020 - 5
ASHRAE Journal - February 2020 - 6
ASHRAE Journal - February 2020 - 7
ASHRAE Journal - February 2020 - 8
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ASHRAE Journal - February 2020 - 10
ASHRAE Journal - February 2020 - 11
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ASHRAE Journal - February 2020 - 37
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ASHRAE Journal - February 2020 - Cover3
ASHRAE Journal - February 2020 - Cover4
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