561276-UIM-C-0712
Johnson Controls Unitary Products
37
SECTION IX: SAFETY CONTROLS
CONTROL CIRCUIT FUSE
A 3-amp fuse is provided on the control circuit board to protect the 24-
volt transformer from overload caused by control circuit wiring errors.
This is an ATO 3, automotive type fuse and is located on the control
board.
BLOWER DOOR SAFETY SWITCH
This unit is equipped with an electrical interlock switch mounted in the
burner compartment. This switch interrupts all power at the unit when
the panel covering the blower compartment is removed.
Electrical supply to this unit is dependent upon the panel that covers the
blower compartment being in place and properly positioned.
ROLLOUT SWITCH CONTROLS
These controls are mounted on the burner assembly. If the temperature
in the area surrounding burner exceeds its set point, the gas valve is
de-energized. The operation of this control indicates a malfunction in
the combustion air blower, heat exchanger or a blocked vent pipe con-
nection. Corrective action is required. These are manual reset controls
that must be reset before operation can continue.
PRESSURE CONTROLS
Pressure Sensor -
This furnace is equipped with a pressure sensor in
the burner compartment near the combustion blower. This sensor mon-
itors combustion airflow through furnace and piping systems. If any of
the conditions listed below are detected by the pressure sensor, the
control board will prevent a hazardous condition from occurring by
speeding up the combustion blower motor in order to maintain ade-
quate combustion airflow. If the combustion blower is already turning at
full speed, the furnace control will then start reducing the input to the
furnace in order to maintain proper combustion with the amount of com-
bustion airflow available. If there is not enough combustion air available
to give proper combustion even at the minimum input rate (35%), the
control will close the gas valve and shut off the burners. The sensor will
detect the following conditions.
1.
Blockage of vent piping or vent terminal
2.
Failure of combustion air blower motor or blower wheel.
3.
Blockage of combustion air piping.
Pressure Switch -
This furnace is equipped with a pressure switch
mounted on the draft inducer. This switch monitors the flow through the
vent system. The switch will close at the beginning of each cycle when
adequate combustion airflow is established. However, this switch may
be open under certain conditions when the burners are lit. The pressure
sensor is the primary flow sensor.
Condensate Pressure Switch -
This furnace is equipped with a pres-
sure switch that will shut the furnace burners off if the condensate drain
line is blocked so that the water does not drain properly from the fur-
nace.
LIMIT CONTROLS
Limit Switch -
This furnace is equipped with a high temperature limit
control mounted to the left side of the furnace vestibule panel. This limit
switch will open and shut off gas to the burners if it detects excessive air
temperature in the furnace, which can be caused by any of the following
conditions:
1.
Dirty filter
2.
Failure of the circulating blower motor or wheel
3.
Too many supply or return registers closed or blocked.
Temperature Sensor -
This furnace is also equipped with a tempera-
ture sensor mounted to the vestibule panel, near the limit switch. This
sensor monitors the temperature of the air being supplied to the home.
If the sensor detects air temperature higher than normal, the furnace
control will speed up the circulating blower motor in order to try to
increase the amount of airflow being delivered, thereby reducing the air
temperature.
SECTION X: NORMAL OPERATION AND
DIAGNOSTICS
NORMAL OPERATION SEQUENCE
The furnace control calculates the optimum firing rate each time the
wall thermostat R and W contacts close or open (at the beginning and
at the end of each call for heat) based on information from the thermo-
stat and past demand. UNLIKE CONVENTIONAL SYSTEMS, THE
WALL THERMOSTAT DOES NOT SIMPLY TURN THE FURNACE ON
AND OFF. THE FURNACE CONTROL CALCULATES THE DEMAND
AND MAY CONTINUE TO FIRE THE FURNACE DURING PORTIONS
OF THE THERMOSTAT "OFF" CYCLE.
When the wall thermostat R and W contacts close, indicating a call for
heat, the following sequence occurs:
1.
The inducer is energized and ramps up its speed until airflow is
proven by the pressure switch and by the pressure sensor on the
control board.
2.
The hot surface ignitor is energized.
3.
After a 17-20 second igniter heat-up, the gas valve opens and the
burners light.
4.
When the control senses that flame is present, the circulating
blower starts at low speed.
5.
The furnace fires at 70% of full rate for 30-45 seconds, then drops
to the minimum (35%) firing rate.
6.
The firing rate is automatically adjusted to meet demand, increas-
ing gradually to maximum (100%) firing rate if the thermostat is not
satisfied within a defined time.
7.
When the thermostat R and W contacts open (thermostat is satis-
fied) the furnace control recalculates the demand and a new firing
rate.
a.
If demand exceeds the minimum firing rate, the burners will
continue to fire at a recalculated reduced firing rate, decreas-
ing if the thermostat remains off for a defined time.
b.
If demand does not exceed the minimum firing rate, the burn-
ers will shut off immediately.
8.
After the burners shut off, the circulating blower will continue to run
until the temperature sensor detects that the supply air tempera-
ture has dropped to the desired level, which should take from 30 to
90 seconds.
Main power to the unit must still be interrupted at the main power
disconnect switch before any service or repair work is to be done to
the unit. Do not rely upon the interlock switch as a main power dis-
connect.
Blower and burner must never be operated without the blower
panel in place.
