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Primary Limit Control (S10)
Figure 15 shows the primary limit (S10) used on A97USMV
units located in the heating vestibule panel. When excess
heat is sensed in the heat exchanger, the limit will open.
Once the limit opens, the furnace control energizes the
supply air blower and de-energizes the gas valve. The
limit automatically resets when unit temperature returns to
normal. The switch is factory set and cannot be adjusted.
In the event of restricted air flow, the integrated control
will reduce firing rate and indoor blower airflow in 10%
increments until a sustainable air flow is reached. If the
furnace reaches 35% firing rate, and adequate air flow is
not available, the furnace will shutdown and enter one hour
watchguard. For limit replacement remove wires from limit
terminals and rotate limit switch 90 degrees. Slowly remove
from the vestibule panel.
install limit face down
Figure 15.
Gas Valve (GV1)
The NF97UM uses a variable capacity gas valve (Figure 16)
and is applicable for two-stage or variable capacity settings.
See "Thermostat selection modes" in the integrated control
section for more details The valve is internally redundant to
assure safety shut-off. If the gas valve must be replaced, the
same type valve must be used.
A 24VAC 2-pin plug and gas control switch are located on
the valve. 24V applied to the pins enables valve operation.
Inlet and outlet pressure taps are located on the valve.
LPG change over kits are available from GE Appliances. Kits
include burner orifices and an LP gas valve.
31-5000723 Rev. 0
Danger of explosion.
There are circumstances in which odorant
used with LP/propane gas can lose its scent.
In case of a leak, LP/propane gas will settle
close to the floor and may be difficult to
smell. An LP/propane leak detector should
be installed in all LP applications.
The burner box is sealed and operates under a negative
pressure. A pressure hose is connected from the burner
box to the gas valve. The gas valve senses the pressure in
the burner box and uses that to set the maximum manifold
pressure while the pressure switch with pressure conditioning
device adjusts the gas flow. This will compensate for different
vent configurations which can greatly affect the rate of the
unit.
Combustion Air Inducer (B6) & Pressure Switch (S18)
All NF97UM units are equipped with a combustion air inducer
(B6) and dual pressure switch assembly (high fire and low
fire). The pressure switch (Figure 17) serves four functions.
First it establishes calibration points for the vent calibration
routine. The combustion air inducer's speed at a given
firing rate is a function of the vent system resistance. The
calibration routine establishes the inducer speed required
to make low and high fire switches for a given vent pipe
installation and interpolates the speeds required to achieve
all intermediate rates between these two points. The setting
for lowfire switch on the assembly is such that it does not
normally enter into the vent calibration routine.
Second, the switch proves combustion air inducer operation
by sensing a vacuum energizing the control circuit and
allowing ignition. The low fire pressure switch provides this
function.
Third, the switch interrupts the combustion process in the
event vent outlet or combustion air intake blockage.
Page 29
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Figure 16.
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