Section 1 - Ventilation
11 Honeywell Economizers 63-8594-02
Mixed Air Formulas
Formula for Measuring the Percentage of Outside Air in an Air Handler.
This formula is used to determine the
percentage of outside air (by volume) being
brought into a building from the outside. The
OA dampers can be adjusted by measuring
the MA, OA and RA to balance the correct
V
bz
. It is a test that should be conducted
during routine maintenance to ensure that the
correct percentage of ventilation is being
provided. Note the fan must be running with
the panels on the unit to take these
measurements. Drill a hole in the side of the
unit and insert temperature probe to measure
the MA temperature. The hole must be sealed
when measurements are completed.
Formula for Adjusting the Minimum Position Control.
This formula is used to make adjustments to
the mixed air controls. In ASHRAE 62.1 there
are two components of the percent of outdoor
air ventilation required, the human component
and the buildings effluent component. The
rates in the standard are based on the type of
human activity normally performed in the
building. For example: the base rate for office
buildings is 5 cfm per person and the building
effluent rate is 0.06 cfm per square foot of
space. Initially only two temperatures are
measured, return and outside air. The
minimum position control is then adjusted until
the mixed air temperature is equal to the result
of the formula. For design requirements for
CFM per person for all building types, refer to
ASHRAE standard 62.1 section 6 and/or local
or state building codes.
Return Air
Temperature
_
Mixed Air
Temperature
x 100% =
Volume (%) of
Outside Air
Return Air
Temperature
_
Outdoor Air
Temperature
M23917A
MIXED
AIR
SENSOR
ECONOMIZER
“OA”
“MA”
“RA”
C7400S
SENSOR
MS3103
ACTUATOR
COMMERCIAL
THERMOSTAT
Return Air
Temperature
X
% of
Return
Air
+
Outside Air
Temperature
X
% of
Outside
Air
=
Temperature
of Mixed Air
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