Vent Calibration; Measuring Pressure Differential - Lennox G71MPP SERIES Unit Information

Vent Calibration

The vent calibration sequence establishes furnace operat-
ing parameters in a specific installation. The integrated
control board runs the calibration and may be repeated as
necessary to maintain proper furnace operation. Prior to
calibration, all duct work (and returns) vent pipe and con-
densate trap (primed) must be connected.
If calibration is successful the data is stored in memory and
will be used to determine furnace operation and maintain
parameters during heat call. If calibration is not successful,
the integrated control will proceed to a 5 minute delay and
signal the appropriate code. After the 5 minute delay the
calibration will be repeated 4 more times with a 5 minute
delay in between. If still unsuccessful after the 4 trials (total
5) the integrated control will go into a 1 hour soft lockout.
Calibration may be initiated by:
S Initial call for heat
S Cycling main power off / on and then call for heat
S Venting conditions change (affecting high and low
pressure switch operation)
S Ramp down low fire switch check failed (calibration
will follow next call for heat)
S The service technician (by pressing the push button
found on the integrated control until the control cycles
through to Field Test Mode")
The integrated control will do the following during
calibration:
1− Verify both low pressure switch and high pressure
switch are open. If either are closed log error and end
calibration.
2− Start inducer at a predetermined low RPM (1600).
3− After a 7.5 second delay and if the low pressure
switch is still open, increase the RPM by 250. If after a 5
second delay the low pressure switch is still open, re-
peat steps 1, 2 and 3.
4− Decrease RPM by 50, wait 5 seconds and look for
the low pressure switch to open. Repeat this step until
it is closed.
5− Keep this RPM as RPM1.
6− Increase RPM to 1250. Wait 5 seconds.
7− Check high pressure switch, if open, increase RPM
by 250. Repeat this step until closed.
8− Decrease RPM by 50, check after 5 seconds. Re-
peat this step until switch is open.
9− Keep this RPM as RPM2.
10− Set RPM to 0. End calibration.
NOTE − If after a successful calibration and a heat call
is present the integrated control will by−pass the pre-
purge state and go straight into ignitor warm up.
After calibration, the integrated control stores the RPM1
and RPM2 values. The low fire (40%) and high fire (100%)
RPM points are calculated by adding margin values to the
RPM1 and RPM2 values.
The integrated control also initiates a low fire switch check
at the end of a normal heating cycle described below. If this
check fails the pressure switch calibration will follow on the
next call for heat.
1− The inducer runs 15 seconds at the last firing rate
before the heat call ended.
2− Inducer runs at 40% firing rate RPM (RPM1 + low
pressure switch open RPM margin value).
3− If low pressure switch is open, set flag for calibration
on next call for heat. Turn inducer off until next call for
heat.
4− If low pressure switch is closed move inducer speed
to RPM1. Allow 5 seconds for stabilization.
5− If low pressure switch opens turn off inducer. No fur-
ther action.
6− If low pressure switch is still closed, decrease induc-
er speed 1/2 of the low pressure switch open RPM
margin. Allow 5 seconds to stabilize.
7− If low pressure switch is open turn off inducer. No fur-
ther action.
8− If low pressure switch is still closed, set flag for cal-
ibration on next call for heat and turn off inducer.
Measuring pressure differential Figure 15
Checks of pressure differential can aid in troubleshooting.
When measuring the pressure differential, readings should be
taken at the pressure switch. Lack of differential usually indi-
cates problems in the intake or exhaust piping, but may indi-
cate problems in the heat exchanger, condensing coil,
header boxes, combustion inducer or other compo-
nents.
The differential pressure is the difference in pressure mea-
sured across the cold end header box orifice.
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