Remote Control Station Components

THERMISTOR (RTS)
The thermistor is wired to the temperature sensor terminal block. If remote location of the sensor is desired, the connecting cable must be wired between the thermistor and this point. The thermistor enclosure (SH-1) is packed in the remote control station for shipment. It can be mounted on the exterior sides or bottom of the remote control station or a remote location within 500 feet using 18 gauge stranded, twisted-pair, shielded cable.

Gas Control System

The gas control system utilizes Maxitrol's Series 44 modulation controls. The discharge temperature sensor, which is mounted in the discharge duct, transmits a resistance signal back to the amplifier that corresponds to the discharge temperature. The amplifier creates a voltage output to drive the modulating valve to maintain the preselected temperature.

The system can be operated either intermittently as a space heater at maximum discharge temperature
(EDL - Space Heating Mode) or continuously as a makeup air heater with the discharge temperature modulating to maintain the space temperature (EDSM - Continuous Make-Up Air Mode).

EDL - Space Heating Mode - The discharge temperature is controlled by the MAX setting on the amplifier (100-160˚F). The heater cycles on and off based on the temperature settings of the space thermostat.

EDSM - Continuous Make-Up Air Mode (requires CE-Smart) - The discharge temperature modulates to maintain the space temperature selector. The adjustable temperature selector is typically mounted inside the remote control station and is set to the desired space temperature and it transmits a resistance output to the amplifier to control the discharge temperature. The non-adjustable space sensor is mounted in the space and transmits a resistance signal back to the amplifier that corresponds to the space temperature. The discharge temperature sensor is mounted in the discharge and transmits a resistance signal back to the amplifier that corresponds to the discharge temperature. The amplifier creates a voltage output to drive the modulating valve to maintain the selected space temperature by controlling the heater output between the MIN and MAX setting on the amplifier. The MIN dial setting determines the minimum discharge temperature the heater will deliver (40 to 80˚F). The MAX dial setting determines the maximum discharge temperature the heater will deliver (100 to 160˚F). When the space temperature drifts from the temperature selector setpoint by more than 3˚F, the modulating valve is driven to the MIN or MAX discharge temperature, as applicable.

Gas Control Components

AMPLIFIER (AMP)
The amplifier creates a voltage output to drive the modulating valve to maintain the selected discharge temperature.
For EDL Mode, the discharge temperature is set at the amplifier.
For EDSM Mode, the range of the discharge temperature is set at the amplifier, however, the adjustable temperature selector controls when more or less heat is required.

COMBINATION VALVE
The combination valve serves as a manifold pressure regulator and redundant gas shut-off valve. The valve operates on 24 Volts AC power. The combination valve is rated for a maximum gas supply pressure of 14" WC.

DISCHARGE AIR TEMPERATURE SAMPLING BOX
The discharge air temperature sampling box is located on the side of the discharge, and is accessible from the electrical control enclosure side of the unit. The sampling box houses the high limit and the DTS. This box should not be tampered with unless performing the Calibration Procedures.

DISCHARGE TEMPERATURE SENSOR (DTS)
The DTS is mounted in the discharge of the heater. It senses the average discharge temperature and transmits a resistance signal back to the amplifier that corresponds to the discharge temperature.

FLAME ROD (FR)
The flame rod senses the presence of flame and signals the flame safeguard relay. The presence of flame is detected by the flame rectification of the AC signal that is supplied to the flame rod thus creating the DC response. The resulting current flow produced can be measured with a DC microammeter. The reading should be steady and between 2.0 and 6.0 microamps (mA).

FLAME SAFEGUARD RELAY (FSR)
The flame safeguard relay supplies 24 Volts AC to the igniter for 4 seconds before the gas valve is energized. If the burner does not light (flame is not established within 7 seconds), this control will lock out gas flow until the control is reset. If flame signal is lost during burner operation, the control will allow one retry for ignition.

HIGH PRESSURE REGULATOR (HPR)
The HPR option is required when the gas supply pressure exceeds the nameplate rating for the heater. Unless otherwise specified, the HPR assembly is also furnished with a high gas pressure manual shut-off valve and a tap for measuring the upstream gas pressure. The HPR must be a positive lock-up type regulator which must be vented to outdoors. It is sized according to the gas supply pressure and the capacity requirements of the heater.

IGNITER (IGN)
The hot surface igniter is the ignition source for lighting the gas in the burner. It is made of silicon carbide which is very fragile. Care should be used in handling. It operates on 24 Volts and the current ranges from 1.3 to 1.7 amps. It will reach temperatures in excess of 2400˚F during the ignition trial.

MODULATING VALVE (MV)
The MV responds to a 4 to 24 Volt DC signal from the amplifier to modulate the flow of gas to the burner. The low fire adjusting screw is located on the side of the valve under the dust cover.

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
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.

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.