York YSCD, Manual, Page: 96 / 108

96
JOHNSON CONTROLS
FORM 145.05-NOM2 (807)
Coil Pressure Drop (FlexSys Units Only)
On FlexSys units a transducer is used to monitor the
pressure drop across the cooling coil. This pressure drop
is used to verify supply fan operation and also used to
control the by pass damper to prevent excessive by pass
air. The transducer is factory wired and tubing attached
and located on either side of the evaporator coil.
When verifying transducer operation, the technician
must insert a tee in each of the tubes connected to the
transducer to verify the differential pressure being
applied to the transducer. Once this pressure is known, a
comparison can be made of the duct pressure vs. output
milliamp values from the transducer. To check the
milliamp output from the transducer remove the wire
from the output (OUT) connection on the transducer and
connect your meter between the end of the wire and the
(OUT) terminal of the transducer. Table 20 shows the
relationship between the pressure applied to the duct
static pressure transducer and the output voltage. The
output is linear.
TABLE 20 – COIL PRESSURE DROP (FLEXSYS)
DIFF PRESS
(IN WC)
MA
OUTPUT
DIFF PRESS
(IN WC)
MA
OUTPUT
0.0 4.0 1.5 13.6
0.25 5.6 1.75 15.2
0.50 7.2 2.0 16.8
0.75 8.8 2.25 18.4
1.0 10.4 2.5 20.0
1.25 12.0
Humidity Sensor (FlexSys Units Only)
The humidity sensor outputs a 4 to 20 milliamp output
in response the relative humidity level of the supply
air. This value is used to determine the dew point of
the supply air entering the under fl oor space. The Unit
Control adjusts the supply air dew point to keep the
dew point temperature below the slab temperature of
the under fl oor area.
TABLE 21 – HUMIDITY SENSOR OUTPUT
RELATIVE
HUMIDITY
MA OUTPUT
RELATIVE
HUMIDITY
MA OUTPUT
0 4 55 12.8
5 4.8 60 13.6
10 5.6 65 14.4
15 6.4 70 15.2
20 7.2 75 16.0
25 8.0 80 16.8
30 8.8 85 17.6
35 9.6 90 18.4
40 10.4 95 19.2
45 11.2 100 20.0
50 12.0
FAULTS AND ALARMS
A “Fault” is declared when an abnormal operation is
detected. Usually this is related to a parameter operating
outside the accepted range, for example the discharge
pressure being above the discharge pressure high-
pressure limit. A
“Fault” may result in a temporary
shutdown of a system or the unit; however, when the
“
Fault ” condition is corrected the unit returns too
normal operation. The control will also close a contact
and turn on the LED at DO1 on the control board. This
set of dry contacts could be used to energize an external
device to alert the user that a
“Fault” or “Alarm” is
present.
An
“Alarm”
will require intervention to reset the unit.
Multiple “Faults” in a given time frame can result in
the initiation of an “Alarm” . An “Alarm” will result in
the lockout of the unit or one of its operating systems.
When an
“Alarm” is present, the “Alarm” light will
illuminate on the Keypad / Display. The control will
also close a contact and turn on the LED at DO1 on the
control board. This set of dry contacts could be used
to energize an external device to alert the user that an
“Alarm” is present. An “Alarm” can be reset in two
ways:
•
Removing the 24-volt input to the control. This
can be done by moving the slide switch in the
bottom right hand corner of the control to OFF,
and then back ON.
Service