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530.18-N11Y
on the 72 KW, 240V heater, heater contactors (8M & 9M)
will be energized.
c) The thermostat will cycle the electric heat to satisfy the
heating requirements of the conditioned space.
HEAT ANTICIPATOR SETPOINTS
It is important that the anticipator setpoint be correct. Too high
of a setting will result in longer heat cycles and a greater
temperature swing in the conditioned space. Reducing the
value below the correct setpoint will give shorter "ON" cycles
and may result in the lowering of the temperture within the
conditioned space. Refer to Table 11 for the required heat
anticipator setting.
TABLE 11 - HEAT ANTICIPATOR SETTING
HEATER
VOLTAGE
KW
18
36
208/230-3-60
54
72
18
36
460-3-60
54
72
18
36
575-3-60
54
72
GAS HEATING SEQUENCE OF OPERATION
The following sequence describes the operation of the gas heat
section.
CONTINUOUS BLOWER
With the room thermostat switch set to "ON", the supply air
blower will operate continuously. The normally closed contact
"K5-1" provides 24 volt power to the "3M" contactor. The "3M-1,
2 & 3" power contacts close and the blower motor operates.
INTERMITTENT BLOWER
With the room thermostat system switch set to the "AUTO" or
"HEAT" position and the fan switch set to "AUTO", the supply
air blower will operate after the room thermostat calls for heat
and the time delay relay closes.
The "TH1" closes, the heat relay "RW1" is energized. The
"RW1-1" power contact closes energizing the line voltage draft
motor. The "RW1-2" contact is also closed. As the speed of the
draft motor reaches approximately 2500 RPM, the centrifugal
switch contact located on the end of the draft motor shaft closes
to power the first stage ignition module "IC1".
After a brief pre-purge time, ignition module "IC1" will start the
first stage ignitor sparking and will open the redundant valve
located inside the first stage main gas valve "GV1" to allow a
flow of gas to only the first stage carryover tube. See Figure
13. Only after the pilot flame has been ignited and the presence
of pilot flame detected at the "IC1" by a signal sent back through
the flame sensor is sparking terminated and the first stage main
gas valve opened.
Gas flows into each of the main burners and is ignited from the
carryover tube flame.
If "IC1" fails to detect a pilot flame, it will continue to try for a
maximum of 85 seconds to ignite the pilot tube. If the pilot flame
is not detected, then "IC1" will lock out furnace operation for 5
minutes, then retry ignition sequence.
At the same time power was supplied to the "RW1", a parallel
circuit activates "TDR" which closes the "TDR" contact after
approximately 16 seconds and energizes "K5" which closes
"K5-2" and starts the blower by energizing "3M".
When "TH2" closes, heat relay "RW2" is energized. The
"RW2-1" contact is closed energizing the second stage ignition
module "IC2". "IC2" will immediately start the second stage
ignitor sparking and will open the redundant valve located
16
SETTING, AMPS
TH1
TH2
0.29
-
0.29
0.29
0.29
0.58
0.58
0.58
0.29
-
0.29
0.29
0.29
0.29
0.29
0.29
0.29
-
0.29
0.29
0.29
0.29
0.29
0.29
inside the second stage main gas valve "GV2" to allow a flow
of gas to the second stage carryover tube. See Figure 13. Only
after the pilot flame has been ignited and the presence of pilot
flame detected at "IC2" by a signal sent back through the flame
sensor is sparking terminated and the main gas valve opened.
Gas flows into each of the second stage main burners and is
ignited from the carryover tube flame.
If "IC2" fails to detect a pilot flame, it will continue to try for a
maximum of 85 seconds to ignite the pilot tube. If the pilot flame
is not detected, then "IC2" will lock out furnace operation for 5
minutes, then retry ignition sequence. Note that the second
stage furnace can operate even if first stage has locked out.
W he n t he he at in g c y cl e i s c om pl et e, "T H2 " op en s
de-energizing the "RW2" then "TH1" opens de-energizing
"RW1" and "TDR", thus closing all gas valves. The blower
motor will continue to run (approximately 26 seconds after the
furnace is shut down) until "TDR" opens, de-energizing the "K5"
relay and "3M" contactor. The draft motor will continue to run
for a brief post-purge cycle.
REDUNDANT
VALVE
GAS MAIN
GAS
VALVE
TO PILOT BURNER
FIG. 11 - GAS VALVE PIPING
SAFETY CONTROLS
The control circuit includes the following safety controls:
1. Limit Control (LS). This control is located inside the heat
exchanger compartment and is set to open at the
temperature indicated in Table 12. It resets automatically.
The limit switch operates when a high temperature
condition, caused by inadequate supply air flow occurs,
thus shutting down the ignition control and closing the main
gas valves and energizing the blower.
2. Centrifugal Switch (CS). If the draft motor should fail, the
centrifugal switch attached to the shaft of the motor
prevents the ignition controls and gas valves from being
energized.
3. Redundant Gas Valve. There are two separate gas valves
in the furnace. Each valve contains a main and a redundant
valve. The redundant valves are located upstream of the
main gas valves. Should either or both of the main gas
valves fail in the open position the redundant valves serve
as back-ups and shuts off the flow of gas.
4. Flame Sensor Rod / 100% Ignition Control Lock-Out. The
flame rods and controls are located per Figure 14. If an
ignition control fails to detect a signal from the flame sensor
indicating the pilot flame is properly ignited, then the main
gas valve will not open. It will continue to try and ignite the
pilot for a maximum of 85 seconds, then if the pilot flame is
not detected, the ignition control will lock out furnace
operation until 24V power is removed from the module
either at the unit or by resetting the room thermostat.
5. Rollout Switch. This switch is located above the main
burners in the control compartment which in the event of a
sustained main burner rollout shuts off and locks out both
ignition controls closing both gas valves. The ignition
controls lock out furnace operation until 24V power is
removed from the controls either at the unit or by resetting
the room thermostat. Note the auto reset rollout switch must
reset before allowing furnace operation.
TABLE 12 - LIMIT CONTROL SETTING
Capacity, MBH
Input
300
400
MAIN VALVE
TO MAIN
BURNER
Limit Control
Opens, °F
Output
240
195
320
195
Unitary Products Group
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