50A52-100/101/102 Sequence Of Operation - American Standard *UD Series Operation Manual

White-Rodgers 50A52-100/101/102 Sequence of Operation
Version 4 (E93, E1, E2, E3) 5 Relays,
3 Wire Flame Switch Radiant Sensor
This control contains a micro-processor allowing a
single re-try upon ignition failure or loss of flame. It is a
"two try" board. It also provides an automatic reset
after three hours following a system lock-out condition.
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 9
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. In
this model, the K1 indoor fan cooling relay will only be
energized when power is applied to the "G" terminal l
on the furnace control. In the event of a limit trip o, the
indoor fan motor 3 will run at the chosen heating speed
Typical Start Up
On a call for heat, the indoor thermostat completes
the circuit from the "R" terminal to "W" to the common
contact of the combustion air pressure switch u through
the limit switch and roll-out fuse link o. (On downflow
and horizontal models, an additional limit switch and
fuse link may be used.) It also starts the micro-processor.
As the control micro-processor is energized, the K 3
normally open relay contacts closes 7, and starts the
Induced Draft Motor 0. When the Induced Draft Motor
comes up to speed, the combustion air pressure switch
u trips supplying power through the normally closed
(cold) radiant sensor contacts a to the redundant h
and EQSO gas valve terminals f. The hot surface
ignitor 0 is also energized when the K 4 relay contacts
8 close. The flame switch radiant sensor normally closed
(cold) contacts a will transfer to the hot position when
sufficient radiant energy is sensed.
The microprocessor k initiates a timing cycle for
ignition. After 17 seconds (see note), the normally open
K 5 contacts j (main gas valve) will close. The control
will allow the ignitor to remain on until a total of 90
seconds (see note) have passed. If at the end of 90
seconds (see note), the control has not sensed the
flame switch radiant sensor bi-metal trip, it will lockout.
If the flame switch radiant sensor bi-metal has tripped
normally within time allowed, the gas valve will be
energized through K 5 j. Two things then happen, the
45 second indoor fan heat-on time delay will start and a
90 second flame proving cycle will be started.
After the gas valve has been energized for one second,
the ignitor is de-energized.
Providing the furnace fires normally, the cycle will
continue until a normal shut-down.
If the flame switch radiant sensor bi-metal resets to
the normally closed (cold) position a, indicating no flame
is present, within the 90 second flame proving time, one
re-try will be initiated after a 60 second purge by the com-
bustion blower. If this re-try fails, the control will lock out.
After the furnace is in normal operation for more than
90 seconds, any fault causing a burner shut-down will
be followed by two tries for re-ignition. This is called
a recycle.
Typical Shut Down
When the thermostat is satisfied and power is
removed from the "W" terminal l, the gas valve is
de-energized. The indoor fan time delay circuit
keeps contacts K 2 3 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, unless constant fan was selected.
Circuit Safety Features
The circuit is arranged to prove that the combustion
blower 0 is operating. The pressure switch normally
open contacts u must always start in the normally open
position and trip to a closed position, 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 signals its
tripped or "hot" position a to the micro-processor s.
This circuit maintains safe operation of the gas valve.
If the flame switch radiant sensor bi-metal is "hot" at
the start of a cycle, the redundant valve h cannot be
powered since the normally closed contacts would be
open. Resistor EQSO (Electrical Quick Shut Off) provides
current flow enough to hold the redundant valve open,
but will not allow it to open from a closed position f.
If either limit device opens o, (due to high heat
exchanger temperature or roll-out), relay K 2 3 will
power the indoor blower on the selected heating speed
and relay K 3 7 will power the combustion blower.
In both of these cases, the indoor blower and the
combustion blower will be powered until the limit
re-sets. The combustion blower will then stop immedi-
ately but the indoor blower will continue with the normal
blower shut-down sequence. E93 and E1 controls will
then energize the ignitor and begin another ignition
sequence. E2 controls will not energize the ignitor until
the radiant sensor trips to the cold position.
A momentary power interruption will cause the micro-
processor k to de-energize and break the holding
circuit. This will cause the system to enable a re-start only
after the pressure switch opens and then closes again,
proving its operation i.
A momentary gas interruption will cause the flame
switch radiant sensor 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. The
normal ignition start-up cycle will begin if the thermostat
is calling for heat.
Note: 50A52-101 (E93, E3) and 50A52-102 (E90, E4) have a
mimimum warm up time of 12 seconds and a maximum
time of 30 seconds
48