Form I-XE/CRGB/RPB, Mfg No. 131782 Rev 4, Page 25
controls or miswiring, and will require correction of
the cause of failure and the replacement of the fan
and limit control and wiring before the unit can be
returned to service. See Hazard Levels, page 2.
16. Fan Control
1. A fan control provides for the following control of the blower.
(a) After the gas valve opens, there is a time delay of blower opera-
tion to prevent the discharge of cold air.
(b) Blower operation continues after the thermostat is satisfied as
determined by the fan time delay.
2. To be sure that the blower can continue to operate, the power sup-
ply to the furnace
MUST NOT
be interrupted
except
when servic-
ing 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. For location, see
FIGURE 19
, Item 12.
Service NOTE:
To replace the fan control on units manufactured
prior to 11/04, a replacement kit is required. Order P/N 209184.
17. Blocked Vent Switch (Indoor
Models XE/HXE)
The blocked vent switch is a heat-activated, manually reset, safety
device that interrupts 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 inside the control compartment by the
drafthood side.
If the sensor detects heated flue gases in the drafthood relief opening
area, the blocked vent safety device will activate to shut down the
furnace. The cause for the switch shutting down the furnace must be
determined and corrected. The blocked vent switch is designed to acti-
vate 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 side
panel and push the manual reset button on the blocked vent switch to
restart the furnace. Replace control panel.
WARNING: In the event the blocked vent sensor
causes the furnace to shut off, determine and correct
the cause. Failure to do so could result in personal
injury or death.
NOTE
: Effective April 1991, this model of indoor, gravity-vent fur-
naces is equipped with a blocked vent shut-off system. These units
manufactured prior to April 1991 do not include the blocked vent switch.
18. Combustion Air Proving Switch
(Power-Vented Model RPB)
The combustion air proving switch ensures that proper combustion
airflow is available. The switch is a single-pole, double-throw switch,
which senses pressure caused by the flow of combustion air from the
venter. The switch is designed to close when a decreasing pressure is
sensed in the outlet duct of the gas collection box. For location, see
FIGURE 19
, Item 2.)
On start-up when the furnace is cold, the sensing pressure is at the most
negative level, and as the furnace and the flue system warm-up, the
sensing pressure becomes less negative. After the system has reached
equilibrium (approximately 20 minutes), the sensing pressure levels off.
If a restriction or excessive flue length or turns cause the sensing pres-
sure to become less than the switch setpoint, the pressure switch will
function to shut off the main burners. The main burners will remain off
until the system has cooled and/or the flue system resistance is reduced.
The table below gives approximate water column negative pressure
readings and switch setpoints for sea level operating conditions.
Startup Cold
Equilibrium
Factory Setpoint
-1.05" w.c.
-.73" w.c.
-.58
±
.05" w.c.
DANGER: Safe operation requires proper venting
flow. Never bypass the combustion air proving
switch or attempt to operate the unit without the
venter running and proper flow in the vent system.
Hazardous condition could result. See Hazard
Levels, page 2.
19. Reverse Flow, Limit Control
(Outdoor Models CRGB & RPB)
These outdoor furnaces are factory equipped with an automatic reset
reverse flow limit control. This control is located in the blower compart-
ment, mounted in the blower junction box adjacent to the blower inlet
opening, and is wired in series with the main limit control mounted on
the heat exchanger duct side. For location, see
FIGURE 19
, Item 32.
In case of belt breakage or motor failure, the limit control will be opened
by the high temperatures caused by reverse flow from the heat ex-
changer to the blower compartment, thus breaking the circuit to the
automatic electric gas valve and preventing burner operation.
20. Optional High Ambient Limit
Control (Outdoor Models)
The optional high ambient limit control functions to shutoff the burner
when the entering outside air reaches a set temperature. The tempera-
ture setting is field adjustable from 0-100°F. For location, see
FIGURE
19
, Item 21.
21. Operating Valve
All furnaces are equipped with a 24-volt combination valve which in-
cludes the automatic electric on-off valve controlled by the room ther-
mostat, the pressure regulator, the safety pilot valve, and the manual
shutoff valve. The standard gas valve allows for single-stage control
from a single-stage, 24-volt thermostat.
WARNING: The operating valve is the prime safety
shutoff. All gas supply lines must be free of dirt or
scale before connecting the unit to ensure positive
closure. See Hazard Levels, page 2.
22. Optional 2-Stage Operation -
Heating Only Application
The standard combination control valve is replaced with a two-stage
combination gas control valve providing for low fire or high fire opera-
tion controlled by a two-stage thermostat. First stage (low fire) is fac-
tory set (not field adjustable). Both high and low stages are controlled
by a Servo regulator, maintaining constant gas input under wide varia-
tions in gas supply pressure. See instructions packed with the unit for
specific gas valve specifications, wiring, and operating instructions.
23. Optional 2-Stage Operation -
Makeup Air Application
Two-stage makeup air units are equipped with a two-stage gas valve,
but instead of control from a two-stage room thermostat, the outlet air
temperature is monitored and controlled by a two-stage ductstat. When
the discharge air temperature drops to the setpoint, low fire is ener-
gized. If low fire cannot satisfy the ductstat setting, high fire is ener-
gized.
Makeup air applications are usually adjusted to discharge an outlet air
temperature between 65°F and 75°F. In all applications, the allowable
