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Vent Calibration
The vent calibration sequence establishes furnace operat
ing parameters in a specific installation. The integrated con
trol 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 condensate 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 cal
ibration 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).
Wait 7.5 seconds.
3- Check low pressure switch, if open, increase RPM by
250, wait 5 seconds. Repeat this step until low pressure
switch is closed.
4- Decrease RPM by 50, wait 5 seconds and look for the
low pressure switch to open. Repeat this step until it is
open.
5- Keep this RPM as RPM1.
6- Increase RPM by 1250. Wait 5 seconds.
7- Check high pressure switch, if open, increase RPM
by 250, wait 5 seconds. Repeat this step until high pres
sure switch is closed.
8- Decrease RPM by 50, check after 5 seconds. Repeat
this step until switch is open.
9- Keep this RPM as RPM2.
10- Calibration complete.
NOTE - If after a successful calibration and a heat call is
present the integrated control will by-pass the prepurge
state and go straight into ignitor warm up.
After calibration, the integrated control stores the RPM1
and RPM2 values. The low fire (35%) 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 35% 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 mar
gin. 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 18
Checks of pressure differential can aid in troubleshooting. Len
nox provides a kit (10L34) if necessary. When measuring the
pressure differential, readings should be taken at the pressure
switch. Lack of differential usually indicates problems in the in
take or exhaust piping, but may indicate problems in the
heat exchanger, condensing coil, header boxes, com
bustion inducer or other components.
The differential pressure is the difference in pressure mea
sured across the cold end header box orifice.
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