6. The circulating fan begins 30 seconds after flame is
sensed.
7. Furnace continues to run in this state until 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 high heat. The second stage pressure
switch contacts will close and energize the second
stage gas valve (high fire).
8. 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. 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.
With the thermostat heating contacts open, the burner
flames extinguish immediately, the induced blower
stops after a 5 second postpurge period. The pressure
switch contacts open.
9. The circulating fan continues to run until timed out,
depending on the program settings.
Sequence if the furnace does not light up or
detect flame:
1. If flame is not detected during the trial-for-ignition
period, the gas valve is de-energized, the ignitor is
turned off, and the control goes into the "retry"
sequence.
The "retry" sequence provides a 30-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 the flame is established for more than ten 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.
shorted or open condition 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, of 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.
Room temperature drops causing the room thermostat
heating contacts to close.
2. If pressure switch contacts are closed; and have failed
to open since the last cycle, all subsequent steps will
fail to occur.
3. The induced blower begins a prepurge cycle of 15
seconds.
A momentary loss of gas supply, flame blowout, or a
4. The pressure switch contacts close. The pressure switch
contacts must close within 30 seconds. If they fail to do
so, the induced blower will shut off for a five minute
period, then the sequence will begin again at Step 2.
regulator more than ± 0.30 inches water column.
magnehelic gauge. Do not adjust the gas valve pressure
carried out with the use of a manometer or calibrated
Any adjustments to the burner manifold pressure should be
during the test.
all gas fired appliances other than the furnace are off
If using a gas meter to check the furnace input, be sure that
supplier for specific gas heating content values.
Setting the Gas Pressure
section, and contact the fuel
also to
.c. specification to bring the input into compliance. Refer
w
reduced .3” w.c. to 3.4’ w.c. which is within the ±0.30 inches
, the burner manifold pressure can be
2
is 1100 BTU/ ft
In the previous example where the heating value of the gas
pressure or by changing the burner orifice size.
be made by minor adjustments to the burner manifold
problem; however, adjustments to compensate for this can
approximately 103% of the rated input. This is not usually a
heating value will increase the input to
2
with a 1100 BTU/ ft
will reduce the input to 93% of the rated input. Natural gas
heating value
2
For example, natural gas with a 1000 BTU/ ft
actual furnace input and output will vary accordingly.
Since heating values for the gas vary geographically, the
gas with a specific gravity of 0.60
3
• The input rating of the furnace is based on 1075 BTU/ ft
for high fire input, 1.8 inches for low fire input
• The burner manifold pressure is normally set to 3.5 inches
10.5 inches w.c. and minimum of 5 inches w.c.
• The natural gas supply pressure should be a maximum of
Checking Furnace Input
manual.
Additional information is available in the
retry.
5 seconds on a first try, 10 seconds on a subsequent
contacts have closed, the hot surface igniter heats up;
5. After the prepurge period, and after pressure switch
(See next page for formula.)
Troubleshooting section that starts on page 38 of this
34
Summary of Contents for operation and
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