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

White-Rodgers 50A52-100 Sequence of Operation
Version 1 (E90) 7 Relays, 2 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 6 and out of the
control to the primary side of the control transformer
"XFMR" 7. The low voltage side of the transformer 0
supplies 24 Volts to the control through terminals "TH"
and "TR" ;. Control terminal "R" ; 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
on the furnace control, relay K 5 k will be energized
through the flame roll-out fuse link and limit
switch p. (On horizontal and downflow models, an
additional limit switch and fuse link may be used.)
The normally closed K 5A contacts open ; and the
normally open K 5B contacts close z. This prevents the
K 1 cooling relay z from being energized until power
is applied to the "G" terminal ; on the furnace control.
Note: If the limit switch or the flame roll-out fuse
link opens, relay K 5 k will be de-energized and the
normally closed K 5A contacts ; will complete a
circuit to the K 1 cooling relay z. The indoor blower
motor will run on the cooling speed selected.
Typical Start Up
On a call for heat, the indoor thermostat completes
the circuit from the R terminal to W then through the
limit switch and roll-out fuse link to the common
contact of the combustion air pressure switch i and
thus to relays K 2 and K 7 i.
When relay K 2 i is energized, its K 2A contacts
close u, providing a holding circuit for itself. Contacts
K 7 i also close, and start the Induced Draft Motor q.
When the Induced Draft Blower comes up to speed, the
combustion pressure switch trips, connecting power
(through the flame switch NC radiant sensor bi-metal) a,
to relays K 4 and K 6 d.
When relay K 6 d is energized, its contacts 9 connect
line power to the ignitor q. Also, normally open contacts
K 4B h close, bypassing the EQSO (Electrical Quick
Shut Off) resistor f and allowing the redundant gas
valve h to pull in. Normally closed contacts K 4A j
open, removing power from the K 3 l fan time delay
circuit and the main gas valve. As the ignitor heats up,
its' radiant energy is sensed by the flame switch radiant
sensor bi-metal which will cause the contacts to open.
When the contacts open, relays K 6 and K 4 d
are de-energized allowing contacts K 6 to open 9,
de-energizing the ignitor. At the same time, normally
open contacts K 4B h open, but the redundant valve
is held open due to the EQSO resistor in series with the
RV coil. Normally closed contact K 4A j closes, allow-
ing the main valve to open (thus gas flows and ignition
takes place) and the K 3 fan time delay circuit starts
the time delay to start the indoor blower 4 (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 ;, the gas valve is de-energized.
The time delay circuit keeps contacts K 3 4 closed for
approximately 90 seconds to allow all of the heat to be
extracted from the furnace. When the contacts open, the
indoor blower stops.
Circuit Safety Features
The circuit is arranged to prove that the combustion
blower is operating. The pressure switch i must always
start in the normally closed position and then trip to an
open position, proving that the switch is functioning. This
also proves that the Induced Draft Motor q is circulating
air through the furnace.
The flame switch radiant sensor bi-metal a controls relay
K 4. 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 K 4B h. (The EQSO resistor provides
current flow enough to hold the valve open, but will
not allow it to open from a closed position).
If the limit switch opens, (due to high heat exchanger
temperature), it will power the indoor blower by de-
energizing relay K 5 k, thus energizing relay K 1 z and
turning on the blower on cooling speed 5.
A momentary power interruption will cause relay K 2 i
to de-energize and break its holding circuit u. This will
cause the circuit to restart only after the pressure switch
closes again, proving its operation.
A momentary gas interruption will cause the flame switch
radiant sensor bimetal a to start cooling. In about 30
seconds, the switch will close, energizing relay K 4, d
thus opening the normally closed contact K 4A j
closing the main gas valve j and stopping gas
flow. Relay K 6 and K 4 d 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 or 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 cooling speed.
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