65
quence will be executed on the next heat cycle.
The IFC continues the pressure switch proving sequence
by energizing the inducer at the rpm determined for the
high pressure switch during cold calibration and waits for
both the high and low pressure switches to close. The IFC
shall wait for 60 seconds for the switches to close. If the
low and high pressure switches are not both closed within
10 seconds, the IFC will set and display the low-level fault
“
45
” and/or “
57
” at the SSD and continue trying to close
the switches (at the same RPM) for the balance of one
minute. A re-learn cycle will not be required provided the
switches are both closed within the minute. If the offending
pressure switch(es) does/do not close within the 60 sec-
ond period, the control will initiate the five minute
wait/lockout period. After the five minute wait period, the
control shall initiate a new calibration sequence provided a
call for gas heat still exists.
If the mid pressure switch fails to close during the pres-
sure switch proving sequence, the fault code
“67”
will be
set and displayed simultaneously with the
“H”
or
“h”
(as
appropriate) and the proving and pre-purge sequences will
continue as long as all other requirements are met. How-
ever, furnace operation will fix to 70% any heating rate
which is between 50% and 65% regardless of the demand
of the thermostat until a successful calibration of the mid
pressure switch is completed. Also, a new calibration se-
quence will be executed on the next heat cycle.
Pre-Purge
The gas valve 100% rate demand command is to be
transmitted to the valve during the pre-purge period and
before ignition. The command must be sent and verified in
a timely manner so as not to interfere with or lengthen the
pre-purge cycle.
The furnace control begins a 30 second pre-purge period
at the cold high pressure switch inducer speed. Pre-Purge
is the amount of time the inducer runs before an attempt
for ignition.
If either or both the high and low pressure switch(es)
open(s) for longer than 2 seconds but less than 60 sec-
onds during pre-purge, the IFC will reset the 30 pre-purge
start time to 0 seconds after the high and low pressure
switch(es) has/have re-closed. If either the high and/or low
the pressure switch(es) is/are open for longer than 60 sec-
onds, the IFC de-energizes the inducer and will enter a 5-
minute lockout period. The fault will be set to level 1 until
the end of the lockout period. After the lockout period, the
control will return to calibration for the next heat attempt.
Trial for Ignition:
I.F.C. energizes the gas valve output and spark output si-
multaneously and begins to look for flame immediately.
When flame is detected for more than 1 second, the I.F.C.
immediately de-energizes the spark output. The spark out-
put is energized for a maximum of seven seconds.
Ignition Activation Period (IAP)
= 7 seconds (IAP = Pe-
riod of time between energizing the main gas valve and
deactivation of the ignition means during a trial for ignition
period (per ANSI Z21.47, definition section)).
Trial for Ignition Period (TFI)
= 8 seconds (TFI = The pe-
riod of time between energizing and de-energizing the gas
valve if the supervised flame is not established (per ANSI
Z21.47, definition section)).
After the spark output is de-energized due to the expira-
tion of the 7 second period and flame is not detected, the
I.F.C. continues to look for flame for an additional 1 sec-
ond. If flame is not detected by the I.F.C. within 8 seconds
of the gas valve and igniter becoming energized, the I.F.C.
will de-energize the gas valve and initiate ignition retry.
Directly after ignition, if a valid heat demand still exists, the
furnace will operate at the 100% heat rate for 2 minutes if
cold calibration preceded, otherwise for the period of heat
blower on delay.
Warm Calibration
If the heat call is canceled prior to start of warm calibra-
tion, the warm calibration cycle will always be completed
before shutting down the gas heat.
After the 2 minute warm-up period (or 2 minutes after igni-
tion), if the calibration cycle has taken place prior to pre-
purge, the furnace control will continue calibration by
attempting to determine the necessary inducer RPM’s cor-
responding to the open and closed points for the mid and
high pressures switches. Directly after ignition, the furnace
will operate for 2 minutes at the 100% heat rate provided.
The inducer RPM for this rate shall be that which was de-
termined during the cold calibration of the high pressure
switch. Next, the inducer RPM will be dropped steadily
and the furnace control waits for the high pressure switch
to open. This cycle repeats until the high pressure switch
opens. If the high pressure switch fails to open once the
inducer RPM has reached a pre-determined minimum, the
furnace will exit the heat sequence and set, display and
log a fault
“55”
. A new calibration (both cold and hot) will
take place upon entry into the next heat sequence.
Once the high pressure switch opens, the inducer RPM is
then increased steadily and the IFC waits for the high
pressure switch to close. The cycle repeats until the high
pressure switch closes. The speed at which the inducer is
operating when the high pressure switch closes shall be
added to a pre-determined value. The resultant sum is to
be used as the target RPM for the 70% to 100% hot gas
heating rates (two minutes or longer after ignition). This
value is only to be used during steady-state heat and after
the ignition, blower on delay and two minute warm-up pe-
riod.
If the high pressure switch fails to close after reaching a
pre-determined maximum value, the calibration and heat
sequence will be cancelled and the furnace will exit the
heat sequence normally (with post-purge and blower off
delay) and set, display and log a fault
“57”
at level 2. A
new calibration will take place upon entry into the next
heat sequence.
Once the high pressure switch has re-closed, the gas
valve output is reduced to 65% for the hot mid pressure
START-UP AND SEQUENCE OF OPERATIONS
Int
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