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.
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 31/2 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.
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.
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.