Carrier 39L - Manual (Page 48)

Airflow Switch — The airflow switch (AFS) is a snap-
acting SPDT switch that is factory installed in the PIC con-
trol box. It senses the air supplied by the 39L or 39NX unit
and provides the microprocessor module with a 24 vac dis-
crete signal for fan status. See Fig. 35.
A length of plenum tubing connects the switch to the probe
located on the fan side plate.
The airflow switch range is 0.05 to 2.0 in. wg with a dead-
band of 0.02 in. wg at minimum set point and 0.1 in. wg at
maximum set point.
Low-Temperature Thermostat (Fig. 36 ) — The
optional low-temperature thermostat (LTT) is factory wired
and installed. It is used to protect the chilled water coil from
freezing whenever abnormally cold air passes through the
coil.
The LTT consists of a 20-ft capillary tube that is serpen-
tined in the airstream on the entering side of the chilled wa-
ter coil. It has a range of 34 to 60 F and is factory set at
35 F.
The LTT is wired in series with the motor starter fan
relay. If any 1-ft section of the capillary tube senses cold air
at or below the thermostat setting, the fan shuts down. A manual
reset is provided to restart the fan after the abnormal prob-
lem is fixed. The temperature setting is field-adjustable.
To adjust the temperature set point, turn the adjustment
screw (located on the top of the case) until the position in-
dicator is at the desired temperature. (A clockwise rotation
increases the set point.)
DO NOT set low-temperature thermostat below 35 F.
Damage to freezestat may result.
If the temperature exceeds the set point by 5 F or more,
the reset button will restore the circuit.
Outdoor-Air Thermostat (Fig. 37) — Also called
the condensing unit status (CUST) switch, the outdoor air
thermostat is a temperature-actuated switch used in systems
with direct-expansion cooling. The thermostat is field-
installed in the outdoor condensing unit to prevent the sys-
tem from operating when the outdoor-air temperature is lower
than the condensing unit’s minimum temperature. When in-
stalled, the thermostat must be set to the minimum operating
temperature of the condensing unit. If the condensing unit
has an optional low-ambient control (Motormaster
௡ device),
an outdoor-air thermostat is not required.
The outdoor-air thermostat is an SPST switch; the con-
tacts open on temperature rise and the set point is adjustable
from 45 to 75 F (7.2 to 23.9 C). To increase the set point,
turn the indicator clockwise. See Fig. 37 for thermostat set
point adjustment and mounting hole locations. Thermostat
should be mounted inside condensing unit control box where
it can sense the outdoor air temperature but is protected from
rain and snow.
TURN SCREW CLOCKWISE
TO INCREASE AIRFLOW
TURN SCREW COUNTER-
CLOCKWISE TO DECREASE
AIRFLOW
NORMALLY
CLOSED CONTACT NORMALLY
OPEN CONTACT
CLOSED
CONTACT
SPDT SNAP
ACTING
SWITCH
MOUNTING BRACKET
HIGH-
PRESSURE
INLET
LOW-
PRESSURE
INLET
SPDT — Single-Pole, Double Throw
Fig. 35 — Airflow Switch (P/N HK06WC030)
C F
ADJUSTMENT
SCREW MANUAL RESET
BUTTON
Fig. 36 — Low-Temperature Thermostat
(P/N HH22CZ001)
Fig. 37 — Outdoor-Air Thermostat (Condensing Unit
Status Switch)
48