50A52-100 Sequence Of Operation - American Standard *UD Series Operation Manual

White-Rodgers 50A52-100 Sequence of Operation
Version 3 (E92) 6 Relays, 3 Wire Flame
Switch Radiant Sensor
This control allows a constant re-try for ignition after
a flame failure.
When the disconnect is in the "ON" position 1, power
is applied through the blower door interlock switch 2
to the control line voltage input terminal 5 and out
of the control to the primary side of the control
transformer "XFMR" 6. The low voltage side of the
transformer supplies 24 volts to the control through
terminals "TH" and "TR" l. Control terminal "R" l
supplies 24 volts to the "R" terminal on the room
thermostat.
24 Volt Power On
When 24 volt power is present at terminals TH and
TR l on the furnace control, no relays are activated. The
K 1 cooling relay k will only be energized when
power is applied to the "G" terminal l on the
furnace control. If a limit trips, the indoor fan motor 3
will run at the heating speed 3.
Typical Start Up
On a call for heat, the indoor thermostat completes
the circuit from the "R" terminal to "W" l then to the
common contact of the combustion air pressure
switch u through the limit switch and rollout fuse link
o, and thus to relay K 2 and K 7 u. (On downflow and
horizontal models, an additional limit switch and fuse
link may be used.) When relay K 2 is energized u, its
K 2A contacts close y, providing a holding circuit
for itself. Contacts K 7 also close 7, and start the
Induced Draft Motor 0.
When the Induced Draft blower 0 comes up to speed,
the combustion pressure switch u trips, connecting
power, (through the flame switch radiant sensor bi-metal)
a, to relays K 4 and K 6 ap.
When relay K 6 is energized, its contacts 8 connect
line power to the ignitor. Also, normally open contacts
K 4B close h, bypassing resistor EQSO f (Electrical
Quick Shut Off) and allowing the redundant gas
valve h to pull in. As the ignitor heats up, its radiant
energy is sensed by the flame switch radiant sensor's
bi-metal which will cause its contacts to trip. When
the contacts trip, relays K 6 and K 4 pa are de-
energized allowing contacts K 6 to open, de-energizing
the ignitor 8. At the same time, normally open contacts
K 4B open h, but the redundant valve is held open
due to the EQSO resistor in series with the RV coil. The
flame switch radiant sensor bi-metal closes a circuit
to the normally open hot side a, energizing the main
valve s and the K 3 fan time delay relay circuit.
Thus gas flows and ignition takes place. The fan time
delay circuit starts the time delay to start the indoor
blower (normally a 45 second delay), then warm air
circulation begins.
Typical Shut Down
When the thermostat is satisfied and power is
removed from the "W" terminal l, the gas valve is
de-energized. The time delay circuit keeps contacts
K 3 3 closed for approximately 90 seconds to allow
all of the heat to be extracted from the furnace. When
the K 3 contacts open, the indoor blower stops, unless
constant fan has been selected.
Circuit Safety Features
The circuit is arranged to prove that the combustion
blower 0 is operating. The pressure switch u must
always start in the normally closed position y and
then transfer to an open position i, proving that the
switch is functioning. This also proves that the Induced
Draft Motor is circulating air through the furnace.
The flame switch radiant sensor bi-metal a controls
relay K 4 a. Its contacts maintain safe operation of
the gas valve. If the flame switch is open at the start of
a cycle, the redundant valve h cannot open due to
the normally open contacts of relay K 4B h. Resistor
EQSO f provides current flow enough to hold the
valve open, but will not allow it to open from a
closed position).
If the limit switch o opens, due to high heat exchanger
temperature, relay K 3 s will power the indoor blower
on the selected heating speed.
A momentary power interruption will cause relay K 2
to de-energize u and break its holding circuit y. This
will cause the furnace to restart only after the pressure
switch closes again y, proving its operation.
A momentary gas interruption will cause the flame
switch radiant sensor bi-metal to start cooling. In about
30 seconds, the switch will transfer to the cold
position a, closing the main valve j and stopping
gas flow. Relay K 6 and K 4 will be energized, and a
new start up cycle will begin.
Note: This furnace has been certified to allow unburned
gas to flow for a stated "flame failure response time"
and then be ignited without excessive flame roll-out.
The roll-out fuse link will open the circuit and stop
gas flow if it is overheated due to a flame roll-out. (On
downflow and horizontal models, an additional limit
switch and fuse link may be used.) As an example, this
may occur with a blocked flue. At this time, the indoor
blower will be powered on the selected heating speed.
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