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The CO
2
sensors (33ZCSENCO2) factory set for a range of
0 to 2000 ppm and a linear voltage output of 0 to 10 vdc.
Figure 19 shows ventilation rates for various CO
2
set points
when outside air with a typical CO
2
level of 350 ppm is used.
Refer to the instructions supplied with the CO
2
sensor for elec-
trical requirements and terminal locations. The zone controller
requires 24 vac 25 va transformer to provide power to the
sensor.
To convert the CO
2
sensor into a duct-mounted CO
2
sensor,
the duct-mounted aspirator (33ZCASPCO2) will need to be
purchased.
To accurately monitor the quality of the air in the condi-
tioned air space, locate the sensor near the return air grille so it
senses the concentration of CO
2
leaving the space. The sensor
should be mounted in a location to avoid direct breath contact.
Do not mount the space sensor in drafty areas such as near
supply ducts, open windows, fans, or over heat sources. Allow
at least 3 ft between the sensor and any corner. Avoid mounting
the sensor where it is influenced by the supply air; the sensor
gives inaccurate readings if the supply air is blown directly
onto the sensor or if the supply air does not have a chance to
mix with the room air before it is drawn into the return air
stream.
To accurately monitor the quality of the air in the return air
duct, locate the sensor at least 6 in. upstream or 15 in. down-
stream of a 90 degree turn in the duct. The downstream loca-
tion is preferred. Mount the sensor in the center of the duct.
IMPORTANT: If the sensor is mounted in the return air
duct, readjust the mixed-air dampers to allow a small
amount of air to flow past the return air damper when-
ever the mixing box is fully open to the outside air. If the
damper is not properly adjusted to provide this mini-
mum airflow, the sensor may not detect the indoor-air
quality during the economizer cycle.
ZC
AIR
TERMINAL
UNIT
OCTOPUS
HEAT
SAT
2 FT. MIN.
PRIMARY
AIR INLET
ZC
AIR
TERMINAL
UNIT
HEAT
SAT
2 FT. MIN.
PRIMARY
AIR INLET
→
→ →
→
Fig. 17 — Supply Air Temperature Probe (Part No. 33ZCSENSAT) Locations
UNIT
WITH
ELECTRIC
REHEAT
ZC —
Zone Controller
UNIT WITH OCTOPUS
3.25
(8.3)
5.625
(14.3)
1.125
(2.9)
0.25
(0.8)
5
(12.7)
Fig. 18 — Indoor Air Quality (CO
2
) Sensor
(33ZCSENCO2)
303