Lennox SLP99UHV Series Unit Information page 74

NOTE - The thermostat selection DIP switch on the con-
trol is factory-set in the "TWO-STAGE" position.
Applications Using a Two-Stage Thermostat
A - Heating Sequence -- Control Thermostat Selection
DIP switch in "Variable Capacity" Position
1 - On a call for heat, thermostat first-stage contacts
close sending a signal to the integrated control. The
integrated control runs a self-diagnostic program
and checks high temperature limit switches for
normally closed contacts and pressure switches
for normally open contacts. The combustion air
inducer is energized at ignition speed, which is
approximately the same as the inducer speed at 70
percent firing rate.
2 - Once the control receives a signal that the low-fire
pressure switch has closed, the combustion air
inducer begins a 15-second pre-purge in ignition
speed.
FIGURE 62
3 - After the pre-purge is complete, a 20-second initial
ignitor warm-up period begins. The combustion air
inducer continues to operate at the ignition speed.
4 - After the 20-second warm-up period has ended,
the gas valve is energized and ignition occurs. At
the same time, the control module begins an indoor
blower 45-second ON-delay. When the delay ends,
the indoor blower motor is energized at a speed that
matches the firing rate. After the 10-second ignition
stabilization delay expires, the inducer speed
is adjusted to the appropriate target rate. If the
furnace is operating in the initial heating cycle after
power-up, the initial firing rate will be approximately
35 percent. The firing rate on subsequent cycles will
be automatically adjusted by the integrated control
based on thermostat cycles. The firing rate will vary
and will range from 40 percent to 90 percent. The
furnace will continue this operation as long as the
thermostat has a first-stage heating demand.
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