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11.3 Operating and Safety Controls
Safety systems are required for proper performance of
the gas furnace. The gas furnace shall not be permitted to
operate with any safety system disabled. If a fault is found
in any of the safety systems, then the system shall be
repaired only by a contractor qualified in the installation
and service of gas-fired heating equipment, using only
components that are sold and supplied by Addison.
• Combustion Air Pressure Switch: An air pressure
switch is provided as part of the control system to
verify airflow through induced draft fan by monitoring
the difference in pressure between the fan and the
atmosphere. If sufficient negative pressure is not
present, indicating lack of proper air movement through
heat exchanger, the switch opens shutting off gas
supply through the ignition control module. The air
pressure switch has fixed settings and is not adjustable.
• Rollout Switch (Manual Reset): The furnace is
equipped with manual reset rollout switch(es) in the
event of burner flame rollout. The switch will open on
temperature rise and shut-off gas supply through the
ignition control module. Flame rollout can be caused
by insufficient airflow for the burner firing rate (high
gas pressure), blockage of the vent system or in the
heat exchanger. The furnace should not be placed
back in operation until the cause of rollout condition is
identified. The rollout switch can be reset by pressing
the button on the top of the switch.
• Primary High Limit Switch: To prevent operation
of the furnace under low airflow conditions, the
unit is equipped with a fixed temperature high
limit switch mounted on the vestibule panel. This
switch will shut off gas to the furnace through the
ignition control module before the air temperature
reaches 250 °F (121.1 °C). Reduced airflow may be
caused by restrictions upstream or downstream of
the circulating air blower, such as dirty or blocked
filters or restriction of the air inlet or outlet to the
unit. The high limit switch will shut- off the gas when
the temperature reaches its set point and then reset
when the temperature drops 30 °F (16.7 °C) below
the set point, initiating a furnace ignition. The furnace
will continue to cycle on limit until the cause of the
reduced air flow is corrected.
Ignition Control Module: Ignition control modules
are available having a number of different operating
functions. Refer to Sequence of Operation and
Control Diagnostic data sheets provided in the
instruction package (located in the control section
when the unit ships) for a detailed description of the
control features, operation and troubleshooting for the
model control installed.
11.4 Wiring
All electric wiring and connections, including electrical
grounding, must comply with;
United States: Refer to National Electric Code®, NFPA 70
- latest revision.
Canada: Refer to Canadian Electric Code, CSA C22.1 Part
1 - latest revision.
Check rating plate on unit for supply voltage and current
requirements.
If any of the original control wire supplied with the gas
furnace must be replaced, replace it with type THHN 221°
F (105°C), 600 V, 16 gauge wire or equivalent.
11.5 Sequence of Operation
11.5.1 Sequence of Operation for Two-Stage
Furnace with 75-200 MBH (21.9-58.6kW) Input
When system is powered up, 24VAC will be applied to
the ignition control terminals 24VAC / R and to the timer
relay control.
The ignition control will reset, perform a self check
routine, flash the diagnostic LED for up to four seconds
and enter the thermostat scan standby state. The amber
light on the timer relay control will be lit indicating it is in
the ready position.
Call for Heat:
1. Controller provides contact closure (1st and 2nd stage)
on call for heat.
2. 24 VAC is supplied to ignition control terminal TH,
provided limit switch is in closed position.
3. The control will check that pressure switch con-
tacts are open (ignition control terminal PSW is not
powered).
4. Combustion blower is then energized at high speed.
5. When the airflow switch closes, a 15 second pre-purge
period begins.
6. At end of pre-purge period, the spark commences
and the 1st stage gas valve is energized for the trial for
ignition period. (If the burners fail to light or carryover
during a trial for ignition, the control will attempt two
additional ignition trials. If no flame is present at the
flame sensor within 10 seconds, the spark and gas
valve will be de-energized. A 15 second inter- purge
period begins and the combustion blower continues to
run. After the inter-purge period another ignition trial
will take place. If burner fails to light or prove the flame
sensor following the two additional trials the control
will go into lockout. The valve relay in the ignition
control will be de-energized shutting off the gas valve
immediately and the combustion blower following
PR Series Installation, Operation, and Maintenance Manual
