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Step 4 —Check Safety Controls
The flame sensor, gas valve, and pressure switch were all checked
in the Start--up procedure section as part of normal operation.
1. Check Main Limit Switch(es)
This control shuts off combustion control system and
energizes air--circulating blower motor, if furnace
overheats. By using this method to check limit control, it
can be established that limit is functioning properly and
will operate if there is a restricted duct system or motor
failure. If limit control does not function during this test,
cause must be determined and corrected.
a. Run furnace for at least 5 minutes.
b. Gradually block off return air with a piece of
cardboard or sheet metal until the limit trips.
c. Unblock return air to permit normal circulation.
d. Burners will re--light when furnace cools down.
2. Check draft safeguard switch.
The purpose of this control is to cause the safe shutdown
of the furnace during certain blocked vent conditions.
a. Verify vent pipe is cool to the touch.
b. Disconnect power to furnace and remove vent
connector from furnace vent elbow.
c. Restore power to furnace and set room thermostat
above room temperature.
d. After normal start-- up, allow furnace to operate for 2
minutes, then block vent elbow in furnace 80 percent
of vent area with a piece of flat sheet metal.
e. Furnace should cycle off within 2 minutes. If gas does
not shut off within 2 minutes, determine reason draft
safeguard switch did not function properly and correct
condition.
f. Remove blockage from furnace vent elbow.
g. Switch will auto--reset when it cools.
h. Re--install vent connector.
NOTE: Should switch remain open longer than 3 minutes,
furnace control board will lockout the furnace for 3 hrs. To reset
furnace control board, turn thermostat below room temperature or
from HEAT to OFF and turn 115v power OFF, then back ON.
3. Check Pressure Switch This control proves operation of
the draft inducer blower.
a. Turn off 115--v power to furnace.
b. Disconnect inducer motor lead wires from wire
harness.
c. Turn on 115--v power to furnace.
d. Set thermostat to “call for heat” and wait 1 minute.
When pressure switch is functioning properly, hot
surface igniter should NOT glow and control
diagnostic light flashes a status code 32. If hot surface
igniter glows when inducer motor is disconnected,
shut down furnace immediately.
e. Determine reason pressure switch did not function
properly and correct condition.
f. Turn off 115--v power to furnace.
g. Reconnect inducer motor wires, replace outer door,
andturnon115--vpower.
h. Blower will run for 90 sec before beginning the call
for heat again.
i. Furnace should ignite normally.
Step 5 —Checklist
1. Put away tools and instruments. Clean up debris.
2. Verify that blower OFF--DELAY time is selected as
desired.
58ST
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