SA-Series Technical Manual
41
Cambridge Air Solutions
DAMPER MOTOR
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
auxiliary switch which has been factory
set to operate when the damper is fully open. The
damper will spring closed when the heater cycles off.
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)
For SA250 heaters, the motor starter is
a motor rated contactor that switches
power to the blower motor.
For SA350 heaters, the motor starter
assembly consists of a motor rated
contactor, overload relay and auxiliary
contact. The overload relay protects
motor from excessive current or single
phasing. If overload relay trips, it must
be reset manually. The auxiliary contacts
are used in gas valve safety circuit as an indication
blower is operating, and as an optional exhaust fan
contact for interlocking other equipment with 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 blower in
approximately 3
1
/
2
minutes if either of the following
occurs: (a) Inlet temperature drops below LTC setpoint
(40, 45, 50, or 55˚F) in ventilation mode; or (b) the gas
valve fails to remain energized during a heating cycle.
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
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 (SS)
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)
SA250 heaters are furnished with a
24 Volt secondary transformer.
SA350 heaters are furnished
with a dual 115/24 Volt secondary
transformer.
A FLM-6.25 secondary fuse is provided in the Class I
transformer circuit to protect downstream components
from short circuit. Do not increase the fuse rating over
that which is specified.
TRANSFORMER (T2)
The Class II transformer provides 24
Volt supply for gas modulation and
remote control circuits. The
transformer output current is
inherently limited to protect
downstream components from short
circuit.
