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CONTROL
CONTROL POINT
CURVE
APPROX. deg. F (deg. C)
A
B
C
D
Demand Controlled Ventilation (DCV)
When using the EconoMi$er IV for demand controlled
ventilation, there are some equipment selection criteria which
should be considered. When selecting the heat capacity and cool
capacity of the equipment, the maximum ventilation rate must be
evaluated for design conditions. The maximum damper position
must be calculated to provide the desired fresh air.
Typically the maximum ventilation rate will be about 5 to 10%
more than the typical cfm required per person, using normal
outside air design criteria.
A proportional anticipatory strategy should be taken with the
following conditions: a zone with a large area, varied occupancy,
and equipment that cannot exceed the required ventilation rate at
design conditions. Exceeding the required ventilation rate means
the equipment can condition air at a maximum ventilation rate
that is greater than the required ventilation rate for maximum
occupancy. A proportional-anticipatory strategy will cause the
fresh air supplied to increase as the room CO
even though the CO
set point has not been reached. By the time
2
the CO
level reaches the set point, the damper will be at
2
maximum ventilation and should maintain the set point.
In order to have the CO
sensor control the economizer damper in
2
this manner, first determine the damper voltage output for
minimum or base ventilation. Base ventilation is the ventilation
required to remove contaminants during unoccupied periods. The
following equation may be used to determine the percent of
outside-air entering the building for a given damper position. For
best results there should be at least a 10 degree difference in
outside and return-air temperatures.
AT 50% RH
73 (23)
70 (21)
67 (19)
63 (17)
65
(18)
60
(16)
A
55
(13)
B
50
C
(10)
45
D
(7)
40
(4)
35
(2)
35
40
45
50
55
60
65
(2)
(4)
(7)
(10)
(13)
(16)
(18)
APPROXIMATE DRY BULB TEMPERATURE--degrees F (degrees C)
Fig. 43 --- Enthalpy Changeover Set Points
level increases
2
85
90
95
100
(29)
(32)
(35)
(38)
80
(27)
75
(24)
70
(21)
A
B
C
D
70
75
80
85
90
95
100
(21)
(24)
(27)
(29)
(32)
(35)
(38)
OA
) + (TR
(T
O x
100
T
= Outdoor-Air Temperature
O
OA = Percent of Outdoor Air
T
= Return-Air Temperature
R
RA = Percent of Return Air
T
= Mixed-Air Temperature
M
Once base ventilation has been determined, set the minimum
damper position potentiometer to the correct position.
The same equation can be used to determine the occupied or
maximum ventilation rate to the building. For example, an output
of 3.6 volts to the actuator provides a base ventilation rate of 5%
and an output of 6.7 volts provides the maximum ventilation rate
of 20% (or base plus 15 cfm per person). Use Fig. 42 to
determine the maximum setting of the CO
a 1100 ppm set point relates to a 15 cfm per person design. Use
the 1100 ppm curve on Fig. 45 to find the point when the CO
sensor output will be 6.7 volts. Line up the point on the graph
with the left side of the chart to determine that the range
configuration for the CO
EconoMi$er IV controller will output the 6.7 volts from the CO
sensor to the actuator when the CO
at 1100 ppm. The DCV set point may be left at 2 volts since the
CO
sensor voltage will be ignored by the EconoMi$er IV
2
controller until it rises above the 3.6 volt setting of the minimum
position potentiometer.
Once the fully occupied damper position has been determined, set
the maximum damper demand control ventilation potentiometer
to this position. Do not set to the maximum position as this can
result in over-ventilation to the space and potential high-humidity
levels.
40
105
110
(41)
(43)
HIGH LIMIT
CURVE
105
110
(41)
(43)
RA
x
) =T
M
100
sensor. For example,
2
sensor should be 1800 ppm. The
2
concentration in the space is
2
C06037
2
2
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