Startup
42
OAU-SVX02F-EN
Safety Controls
Air Pressure Switch.
An air pressure switch is provided
as part of the control system to verify airflow through draft
inducer by monitoring the difference in pressure between
the draft inducer 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 though the ignition control
module. On units with two speed draft inducer operation,
a dual air pressure switch is used, monitoring high and low
speed pressures. The air pressure switches have fixed
settings and are not adjustable.
Rollout Switch (Manual Reset).
The furnace module
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 module should not be placed back
in operation until the cause of rollout condition is
identified and corrected. The rollout switch can be reset by
pressing the button on top of the switch.
High Limit Switch.
The furnace module is equipped
with a fixed temperature high limit switch mounted on the
vestibule panel that shuts off gas to the heater through the
ignition control module in the event of reduced airflow
over the heat exchanger tubes. Reduced airflow can be
caused by indoor fan failure, dirty or blocked filters, or
restriction of the air inlet or outlet to the unit. The high limit
switch will automatically reset when the air temperature
drops to approximately 30°F below the limit setpoint.
Determine the cause of the reduced air flow and correct.
Figure 34.
OAB/OAG indirect gas-fired furnace components
Inducer motor
Ignition
Controllers
On/Off gas valve
On/Off gas valve
Manifold gas
pressure tap
Manifold gas
pressure tap
Air-proving switch
Condensate drain
(TYP. 2)
Inducer blower
Modulating valve
inlet pressure tap
Flame
sensor
High limit
switch
Rollout
switch
Rollout
switch
Modulating
gas valve
Minimum/maximum inlet pressure =
7 IN. WC/14 IN. WC (1.7 KPA/3.5 KPA)
for natural gas and 10 in. WC/14 in. WC
(2.5 KPA/3.5 KPA) for propane
Minimum/maximum inlet pressure =
7 IN. WC/14 IN. WC (1.7 KPA/3.5 KPA)
for natural gas and 10 in. WC/14 in. WC
(2.5 KPA/3.5 KPA) for propane
