Service Manual

SERVICING
51
In all instances the limit control is wired in series with the
ignition control.
If the temperature within the furnace should exceed this
setting, the control will open, de-energizing the ignition
control which in turn will open the electrical circuit to the gas
valve.
The control will automatically reset when the temperature
within the combustion chamber is sufficiently lowered.
HIGH VOLTAGE!
Disconnect ALL power before servicing
or installing this unit. Multiple power
sources may be present. Failure to do so
may cause property damage, personal injury
or death.
1. Remove electrical power to unit. Some units may have
more than one source of power.
2. Remove the wires from the limit control terminals.
3. Using an ohmmeter, test for continuity across the two
terminals.
4. If limit test open allow unit to cool and retest.
5. If still open, replace the control.
S-301 TESTING AUXILIARY LIMIT
The auxiliary limit control is a preset nonadjustable control
mounted in the blower compartment area.
It is connected in series with the rollout switch wiring to the
gas valve. If its temperature should be exceeded, it will open,
interrupting the voltage to the gas valve causing it to open.
An additional limit (primary limit) control is required for safety
control of high temperature within the furnace or ductwork.
AUX. LIMIT
HIGH VOLTAGE!
Disconnect ALL power before servicing
or installing this unit. Multiple power
sources may be present. Failure to do so
may cause property damage, personal injury
or death.
1. Remove the wires from the auxiliary limit control termi-
nals.
2. Using an ohmmeter, test for continuity across the two
terminals. No reading indicates the control is open.
Push the red reset button, test again - if still open,
replace the control.
VOLT / OHM
METER
RED
RESET
BUTTON
COLO R
IDENTIFY ING
TAB
TESTING AUXILIARY LIMIT CONTROL
S-302 CHECKING FLAME ROLLOUT SWITCH
APD/GPD units are equipped with a temperature-activated
manual reset control. This control is mounted to the manifold
assembly and is wired in series with the auxiliary limit and
gas valve. The control is designed to open should a flame roll
out occur. An over firing condition or flame impingement on
the heat shield can also cause the control to open.
If the rollout control has opened, the circuit between the
ignition control and gas valve will be interrupted and the
ignition control module will go into lockout. The servicer
should reset the ignition control by opening and closing the
thermostat circuit. The servicer should look for the ignitor
glowing which indicates there is power to the ignition control.
The servicer should measure the voltage between each side
of the rollout control and ground while the ignition control is
try to power the gas valve.