Sequence Of Operation - Bosch BGH96M060B3B Installation Instructions Manual

Furnace shutdown
In case of furnace malfunction, use the following steps to shut the unit down:
1. Set the thermostat to OFF or it's lowest temperature setting.
2. Turn off the electrical power supply to the furnace.
3. Remove the burner access panel.
4. Close manual gas shutoff valve external to the furnace.
5. Push the gas valve switch to the OFF position.
6. Replace burner access panel.

11.4 Sequence of Operation

Furnace control must be grounded for proper operation or control will
lock out. Control is grounded through green/yellow wire routed to gas
valve and manifold bracket screw. Follow the sequence of operation
through the heating, cooling, and fan only modes.
Read and follow the wiring diagram (Fig. 36) very carefully.
The blower access panel must be installed for power to be conducted
through all electrical components.
1. Heating mode
In a typical system, a call for first stage heat is initiated by closing the W1
thermostat contacts. The inducer blower is energized at high speed and the control
waits for the low pressure switch contacts to close. The humidifier (optional) is also
energized at this time. Once the low pressure switch contacts close, a 15-second
pre-purge is initiated. Then the inducer changes to low speed and the 120V ignitor
is powered. At the end of the ignitor warm-up time, the first stage of the two-
stage manifold gas valve is energized (low fire). Flame must be detected within 4
seconds. If flame is detected, the 45-second HEAT delay-to-fan-on period begins.
After the delay-to-fan-on period ends, the control will energize the circulator fan
at low heat speed. The electronic air cleaner (optional) will also energize at this
time. For a two-stage thermostat, a call for second stage heat (W1 and W2) after
a call for first stage heat will energize the inducer at high speed and the circulator
at high heat speed. The second stage pressure switch contacts will close and
energize the second stage gas valve (high fire). For a single-stage thermostat, when
a call for heat occurs (W1), a 10, 20 minute or auto mode heat staging timer will
be activated (timing is selectable with option switches S1-1 and S1-2 positions).
Following this delay, the second stage heat is energized as above.
When the second stage of the thermostat is satisfied, the inducer motor is reduced
to low speed and the second stage gas valve is de-energized. On the control, the
circulator will remain at high heat speed for 30 seconds following the opening of
the second stage gas valve and then is reduced to low heat speed. When the first
stage of the thermostat is satisfied, the first stage gas valve is de-energized and the
HEAT delay-to-fan-off begins timing. The inducer will postpurge for an additional 15
seconds, then the inducer and humidifier will turn off. Upon completion of the HEAT
delay-to-fan-off period, the circulator is turned off. The electronic air cleaner on the
control is also de-energized at this time.
If flame is not detected during the trial-for-ignition period or if the flame is
detected/sensed and then lost before completion of 10 seconds of establishment,
the gas valve is de-energized, the ignitor is turned off, and the control goes into the
"retry" sequence. The "retry" sequence provides a 60-second wait with the inducer
interpurge following an unsuccessful ignition attempt (flame not detected). After
this wait, the ignition attempt is restarted. Two retries will be attempted before the
control goes into system lockout. If flame is established for more than 10 seconds
after ignition, the controller will clear the ignition attempt (or retry) counter. If
flame is lost after 10 seconds, the control will restart the ignition sequence.
A momentary loss of gas supply, flame blowout, or a shorted or open condition
Bosch 96% AFUE Condensing Gas Furnace - BTC 770503101 J (06.2023)
in the flame probe circuit will be sensed within 2 seconds. The gas valve will de-
energize and the control will restart the ignition sequence. Recycles will begin and
the burner will operate normally if the gas supply returns, or the fault condition
is corrected, before the last ignition attempt. Otherwise, the control will go into
system lockout. If the control has gone into system lockout, it may be possible to
reset the control by a momentary power interruption of 10 seconds or longer.
Timing specifications
(All times are in seconds, unless noted otherwise)
Event Definition
The period of time intended to allow for the
dissipation of any unburned gas or residual
Pre-purge
products of combustion at the beginning of
Time
a furnace operating cycle prior to initiating
ignition
Ignitor
The length of time allowed for the ignitor to
Warm-up
heat up prior to the ignition of gas flow.
Time
The period of time between initiation of gas
Trial for flow and the action to shut off the gas flow in
ignition the event of failure to establish proof of the
Period(TFI) supervised ignition source or the supervised
main burner flame.
Ignition The period of time between energizing the
Activation main gas vale and deactivation of the ignition
Period(IAP) means prior to the end of TFI
The additional attempts within the same
thermostat cycle for ignition when the
Retries
supervised main burner flame is not proven
within the first trial for ignition period.
Valve Value sequence period equals 4 seconds
Sequence trial for ignition period x (1 initial try + 2
period retries)+12 seconds.
The period of time intended to allow for the
dissipation of any unburned gas or residual
Inter-purge
products of combustion between the failed trial
for ignition and the retry period.
The period of time intended to allow for the
dissipation of any unburned gas or residual
Post-purge
products of combustion at the end of a furnace
Time
burner operating cycle, Post-purge begins at
the loss of flame sense.
Lock-Out ANSI standard rated module timing.
The period of time between proof of the
Heat Delay-
supervised main burner flame and the
To-Fan-On
activation of the blower motor at heat speed.
The period of time between the loss of a call for
Heat Delay-
heat and the deactivation of the blower motor
To-Fan-Off*
at Heat speed.
The period of time after a thermostat demand
Cool Delay-
for cool before energizing the circulator blower
To-Fan-On
motor at cool speed.
The period of time between the loss of a call for
Cool Delay-
cool and the deactivation of the blower motor
To-Fan-Off
at cool speed.
After one (1) hour of internal or external
Automatic lockout, the control will automatically reset
Reset Time itself and go into an auto restart purge for 60
seconds.
Table 22
* These times will vary depending on option switch position.
Installation Instructions
Time (Seconds)
15
17
4
3
2 times
12
60
15
300
30
*90/120/
150/180
1
60/*90/
120/150
60 minutes
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