Form I-X, P/N 150491 R7, Page 18
8.2 Limit and
Energy Cutoff
Controls
The heater is equipped with a non-adjustable high limit switch which shuts off the gas
in the event of motor failure, lack of air due to dirty lters, or restrictions at the inlet or
outlet of the unit. See Paragraph 9.3 for limit control check.
A unit manufactured prior to 10/2003 with a match-lit pilot also has an ECO con-
trol. The ECO control is calibrated to open at a much higher temperature than the limit
control, acting as a super high limit and providing redundant safety control.
WARNING: An ECO circuit interruption is a major failure caused by a malfunction of
the primary safety control or miswiring, and will require correction of the cause of
failure and the replacement of the limit control and wiring and the fan control before
the furnace can be returned to service.
An ECO interruption can be caused by the failure of the automatic reset limit in com-
bination with the following:
1) Automatic gas valve stuck in the open position.
2) Restricted airow over the heat exchanger due to motor failure, loose fan blade,
broken blower belt, or defective fan control.
3) Failed or ruptured gas pressure regulator.
4) Improper wiring.
8.3 Blocked Vent
Switch
The blocked vent switch is a heat-activated, manually reset, safety device that inter-
rupts the electric supply to the gas valve when the vent is 100% blocked. The sensor
is located near the relief opening of the drafthood. The reset button is located inside
the control compartment by the drafthood side.
If the sensor detects heated ue gases in the drafthood relief opening area, the blocked
vent safety device will activate to shut down the furnace. The cause of the switch shut-
ting down the furnace must be determined and corrected. The blocked vent switch is
designed to activate when the vent is blocked but may also be affected by a negative
building pressure or an inadequate vent system.
After the problem has been corrected, remove the furnace control compartment panel
and push the manual reset button on the blocked vent switch to restart the heater.
Replace the panel.
WARNING: In the event the blocked vent sensor causes the
heater to shutoff, determine the correct the cause. Failure to
do so could result in personal injury or death.
NOTE: All Model X units
manufactured after 10/2003
have a spark pilot and do
not have an energy cutoff
(ECO) device.
8.4 Gas Controls
8.4.1 Gas Valve
All furnaces are equipped with a 24-volt combination valve which includes the auto-
matic electric on-off valve controlled by the room thermostat, the pressure regulator,
and the manual shutoff valve. The standard gas valve allows for single-stage control
from a single-stage, 24-volt thermostat.
NOTE: Effective April 1991,
gravity-vented duct furnaces
are equipped with a blocked
vent shutoff system. Duct
furnaces manufactured prior
to April 1991 do not include
a blocked vent switch.
WARNING: If you
turn off the power
supply, turn off the
gas. See Hazard
Levels, Page 2.
8.1 Fan Control
1. A fan control provides for the following control of the eld-supplied blower.
(a) After the gas valve opens, there is a time delay of blower operation to prevent
the discharge of cold air.
(b) Blower operation continues after the thermostat is satised as determined by
the fan time delay.
2. To be sure that the blower can continue to operate, the power supply to the fur-
nace MUST NOT be interrupted except when servicing the unit.
3. If the customer wants the furnace off at night, the gas valve circuit SHOULD
BE OPENED by a single pole switch wired in series with the thermostat. Some
thermostats are provided with this feature. Multiple units controlled from a single
thermostat are shut off in the same manner. For proper operation, be sure the fan
control wiring is observed.
Service NOTES: To replace the fan control on units manufactured prior to 11/2004, a
replacement kit is required. Order P/N 209184. Prior to 10/2003, the fan control was
optional. Check the wiring diagram on the furnace.
8.0 Controls
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