S-Series Technical Manual
107
Cambridge Air Solutions
SPACE TEMPERATURE SELECTOR (STS)
The STS is part of the EDSM control
system. The STS senses the space
temperature and provides a resistance
signal back to the amplifier that
corresponds to the temperature
variation from setpoint. A 3˚F drift
from the set temperature will cause the
heater to modulate to the extreme end of the preset
range on the amplifier.
HEATER COMPONENTS
AIRFLOW SWITCH (AF)
The airflow switch senses the pressure
drop across the burner. It is factory set
and not adjustable. It is designed to
prevent burner operation if the airflow
drops significantly below the minimum
design airflow of the heater.
DAMPER MOTOR (D, D1)
The damper motor operates on 24
Volts AC and switches power to the
motor starter when the damper
blades are fully open by the closure
of the damper end switch. The
damper end switch is an adjustable
internal auxiliary switch which has been factory set to
operate when the damper is fully open. The damper
motor will power closed when the heater cycles off.
The damper motor will not close unless the blower
service switch is in the ”REMOTE” position. For
detailed instructions refer to Damper Motor
Replacement & Adjustment (page 89).
DISCONNECT SWITCH (DISC)
The disconnect switch is provided on all
heaters and includes the disconnect rod and
lockable operating knob. The disconnect
switch must be in the ”OFF” position to
gain access to the control enclosure. Once the control
enclosure is open, experienced service technicians may
activate the electrical circuit by twisting the disconnect
rod clockwise to assist in troubleshooting. The
disconnect rod must be turned back to the ”OFF”
position before attempting to secure the enclosure
door.
EXHAUST FAN CONTACT (EFC)
The exhaust fan contact is an auxiliary dry contact
that is mechanically interlocked to the heater’s motor
starter and is provided with terminals located in the
electrical enclosure. The EFC is typically wired into an
exhaust fan control circuit to activate the exhaust fan
as a slave to the heater.
FUSE BLOCK (FU1, FU2)
The fuse block provides line fusing for
branch circuit protection. It is wired in
conjunction with the non-fused
disconnect switch.
HIGH TEMPERATURE LIMIT (HL)
The high limit opens when discharge
temperature exceeds 180˚F. This limit
must be manually reset.
MOTOR STARTER (MS, OL, AUX)
The motor starter assembly consists of
a motor starter, overload relay and
auxiliary contact. The overload relay
protects the motor from excessive
current or single phasing. If the
overload relay trips, it must be reset
manually. The auxiliary contacts are
used in the gas valve safety circuit as
an indication the blower is operating, and as an
optional exhaust fan contact for interlocking other
equipment with the operation of the heater.
MULTI-FUNCTIONAL PC BOARD
The multi-functional PC board provides
five (5) separate functions as follows:
1) Blower Relay (CR1) which is energized
on ”call for blower” from the remote
control station.
2) The LTC (Low Temperature Cutout) circuit
functions to shut down the blower in approximately
3
1
/
2
minutes if either of the following occurs: (a) The
inlet temperature drops below the LTC setpoint (40,
45, 50, or 55˚F) in the ventilation mode; or (b) the gas
valve fails to remain energized during a heating cycle.
