Cambridge Air Solutions
108
S-Series Technical Manual
3) The EAT (Entering Air Thermostat) circuit
functions automatically to turn off the burner when the
outdoor temperature reaches the EAT setpoint (45, 50,
55, 57.5, 60, 62, 64, 66, 68 or 70˚F).
4) The PT (Purge Timer) circuit function is preset at
the factory to provide four air changes within the heater
cabinet prior to an ignition attempt (normally set at 4 or
8 seconds). If inlet ducting is attached to the heater, the
delay time can be increased to 8, 16, or 32 seconds, as
applicable.
5) The patented LFS (Low Fire Start) circuit function
is provided to limit the initial heater firing for the
first 15 seconds of a heating cycle. The voltage to the
modulating valve is adjusted between 9 and 13 Volts DC
by an adjustable potentiometer on the PC board.
SERVICE SWITCH (SW1, SW2, SW3)
The service switches are mounted in the
electrical control enclosure. In the
”LOCAL” or ”OFF” position, the service
technician has local control of the heater.
These switches must be placed in the
”REMOTE” position for normal control
from the remote control station.
TRANSFORMER (T1, T2)
Heaters supplied with an output of
greater than 400,000 Btu/hr are
furnished with a dual voltage
transformer unless the supply voltage
is 115 volts. The transformer
furnished depends on the supply
voltage (208, 230, 460 or 575 volts). This transformer
provides the secondary control voltage of 24 and 115
Volts. Heaters rated below 400,000 Btu/hr or supplied
for 115 Volts are furnished with the 24 Volt secondary
transformer only. Secondary fusing is provided in all
Class I transformer circuits to protect the downstream
components from short circuit. Fuse sizing is as follows:
FNM-6.25 for 150 VA 24 Volt; FNM-1.6 for 150 VA
115 Volt; and FNM-2.25 for 200 VA 115 Volt. Consult
the heater wiring diagram to identify the proper fusing.
Do not increase the fuse rating over that which is
specified.
THERMISTOR SENSOR
The thermistor enclosure is packed inside the Remote
Control Station for shipment. It can be mounted on the
exterior sides or bottom of the Remote Control Station
(see Figure 1). It can also be mounted in a remote loca-
tion within 500 feet of the Remote Control Station using
18 gauge stranded, twisted-pair, shielded cable. The
thermistor is wired to the TEMP. SENSOR terminal
block on the thermostat.
THERMISTOR INSTALLATION
(See Figure 2)
1. Insert the star bushing through the opening in the
enclosure.
2. Run the thermistor leads through the star bushing.
3. Attach the cover to the enclosure over the thermistor
sensor.
4. Separate and strip the ends of the lead wires 3/8".
5. Open the terminal blocks for the sensor connections.
6. Insert the lead wires and close the terminal blocks to
the locked position.
NOTE: For remote sensors connected with shielded
cable, ground the shield at the thermostat end only.
Figure 2
Figure 3
Figure 1
Figure 2
Figure 3
Figure 1
