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Normal
Operation - Heat Cycle - Upon a call for heat, the SNS 120 enters
the purge. At the end of the specific purge period, the unit enters the
trial for ignition (TFI). At the start of the TFI, the valve opens and
the unit begins sparking at a synchronous rate of 60 sparks/second. Sparking
continues until the desired trial time has elapsed. If the unit does sense
a flame, the valve is held open, to commence heating, until the thermostat
is satisfied.
Lockout if a flame is not established - If a flame is not established
during the trial for ignition, the valve is closed and the unit enters
lockout. The thermostat must be opened and then closed again to provide
another trial. The SNS 120 is equipped with quick reset circuitry so that
the power interruption to restart the unit can be nearly instantaneous.
The SNS 120 has an indicator lamp that is illuminated when the unit is
working correctly.
Re-ignition after loss of flame or flame failure - If there is
a loss of an established flame during a burn cycle, the valve remains
open and the spark is reestablished within 0.8 seconds. This immediate
ignition attempt after loss of flame is called 'spark restoration'. The
advantage of spark restoration is that an unstable flame will not cause
valve clatter. Once the flame is reestablished, usually shortly after
the spark restoration trial begins, the burn cycle will continue until
the thermostat is satisfied.
Power interruptions - The quick reset circuitry on the SNS 120 enables
it to react very well to short power interruptions. A power loss of about
30 milliseconds or less is not recognized by the unit meaning that the
purge, trial, or burn cycle will continue as if no interruptions had occurred.
A longer power loss causes the unit to recycle. Therefore, a power interruption
of any duration does not lead to lockout. Make sure that any loss of power
results in a repurging of any closed space prior to the re- initiation
of combustion. (ie: Power failure causes exhaust blower to stop, resulting
in a system purge prior to power application to the SNS unit.)
Mounting - The SNS-120 and SNS-122 are mounted to the wall of your
oven of system utilizing (2) offset brackets at the top and bottom of
the case. These provide a 1/2" air space behind the unit to allow
air flow and prevent dirt buildup. The flame probe (if separate) should
have its tip within and its insulator outside of the flame envelope. The
spark ignitor must be mounted in such a way that the spark gap of 0.1-0.15
inches (2.5-3.8 mm) is maintained. The tip of the ignitor must be placed
in the air- gas mixture for proper ignition.
Testing - The flame voltage can be measured at the flame probe (SNS-
122) or the high voltage electrode (SNS-120) with no adjustments in wiring.
See Figures 5 & 6. This voltage must be more than - 20 VDC, typically
in the -40 to -60 VDC range to prevent nuisance lockouts. A special test
lead set is available (FT- 003) for use with a standard digital multimeter
with an input impedance of 10 MegOhms, such as Radio Shack #22-166A. These
test leads divide the voltage by 1000 so that the voltmeter reading in
millivolts is actually the flame probe voltage in volts.
Approvals - The enclosed flame safety is approved by AGA, CSA, and
is UL Listed.
Specifications
Operating Voltage 117VAC ±15%
Current 25ma, excluding Valve
Gas Valve 117VAC nominal, 1A max.
Operating Temperature -40°C to +85°C
Humidity 100%
Purge time 5 Seconds
Trial for Ignition 15 ±3 sec
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