Armstrong G2D95CT - Operation

506303-01 Page 27 of 40Issue 0938
Fan On
A call for fan from the thermostat closes R to G on the control
board. The control waits for a 1-second thermostat
debounce delay before responding by energizing the low
heat speed blower. When the call for fan is turned off, the
control de-energizes the blower.
Cooling
During a call for cooling, the thermostat energizes the R-Y
circuit of the control board. After a 1-second cooling “on” delay,
the control energizes the cooling fan speed. If the fan is already
energized, it remains running and does not de-energize for
the 1-second cooling fan “on” delay.
The call for cooling has priority over continuous fan operation
while a call for heating has priority over both a call for cooling
or continuous fan. Ignition lockouts for any reason do not
affect cooling operation.
As cooling demand is met, the thermostat de-energizes
the R-Y circuit of the control board. After a 60-second cooling
“off” delay, the control de-energizes the cooling speed fan.
At the end of the cooling “off” delay period, the control returns
to the standby mode.
Sequence of Operation (G2D95CT/U Models)
Heating (see Figures 37 and 38 on page 28)
On a call for heat from the room thermostat, the ignition
control performs a 1-second self-check. Upon confirmation
that the pressure switch contacts are in an open position,
the ignition control energizes the combustion blower on high
speed. The control then checks for adequate combustion
air by making sure the low-fire pressure switch contacts
are closed.
The igniter energizes and is allowed to warm up for
7 seconds before the gas valve energizes on 1
st
stage and
burners ignite. 45 seconds after the control confirms ignition
has occurred, the control drops the combustion blower to
low speed.
The circulating blower ramps up to 50% of 1
st
stage heat
speed and operates at that speed for one minute (including
ramp up time), then at 75% of 1
st
stage heat speed for an
additional minute. After that, the circulating blower operates
at full 1
st
stage heat speed until either the heat call is satisfied
or the thermostat initiates a call for 2
nd
stage heat. On a call
for 2
nd
stage heat, the control energizes the circulating air
blower on full CFM 2
nd
stage heat.
If the automatic heat staging option is being used (see
Single Stage Thermostat Operation on page 22), the
furnace does not switch to 2
nd
stage heat in response to a
call from the thermostat but instead operates at 1
st
stage
heat for the duration of the selected time before
automatically switching to 2
nd
stage heat.
OPERATION
Sequence of Operation (G2D93CT/U Models)
Heating
When the control is in standby mode, it continually monitors
thermostat input, rollout switch, and flame sense.
On a call for 1
st
stage heat, W1 and R are connected. The
control verifies the limit switch is closed and both low and
high pressure switches are open. The control always starts
on low-fire and ignores 2
nd
stage call for 30 seconds.
The induced draft blower is energized on high speed and
waits for the low pressure switch to close. The inducer
remains on high speed and begins a pre-purge period when
the low pressure switch closes. After a 15-second pre-purge,
the control energizes the hot surface igniter for a period of
7 seconds (15 seconds on retries) before the gas valve is
energized on 1
st
stage and burners ignite. After a 30-second
blower “on” delay, the control energizes the low heat
circulating air blower. Fifteen seconds after the circulating
air blower energizes, the combustion air blower switches
to low speed and runs at that speed until the thermostat is
satisfied.
If the 1
st
stage is not sufficient to satisfy the heating demand,
the room thermostat initiates call for the 2
nd
stage heat (W1
and W2). The control energizes the combustion blower on
high speed and confirms the high-fire pressure switch
contacts are closed. If the contacts are closed, the gas valve
energizes on the 2
nd
stage and the high-heat circulating air
blower is energized.
If the automatic heat staging option is being used (see
Single Stage Thermostat Operation on page 22), the
furnace does not switch to 2
nd
stage heat in response to a
call from the thermostat but instead operates at 1
st
stage
heat for the duration of the selected time before
automatically switching to 2
nd
stage heat.
When the 2
nd
stage heat is satisfied, the control shuts off
the gas valve and runs the combustion blower for a post-
purge period of 15 seconds. The circulating air blower
continues to run according until the selected blower “off”
delay time (factory shipped at 120 seconds) ends.
In the event the unit loses ignition, the control will attempt
to recycle up to five times before it goes into a 1-hour lockout.
Lockout may be manually reset by removing power from
the control for more than 1 second or removing the
thermostat call for heat for more than 3 seconds.
If during a heating cycle the limit control senses an
abnormally high temperature and opens, the circulating air
blower is energized on high speed and continues to run
while the limit is still open.