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2. Flame Sensor
(Figure 10 up flow position)
A flame sensor is located on the left side of the burner sup-
port. The sensor is mounted on the bottom burner box plate
and the tip protrudes into the flame envelope of the left−most
burner. The sensor can be removed for service without re-
moving any part of the burners. During operation, flame is
sensed by current passed through the flame and sensing
electrode. The ignition control allows the gas valve to re-
main open as long as flame signal is sensed. See table 20
for flame signal.
NOTE − The G51MP furnace contains electronic com-
ponents that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.
FIGURE
10
5/16"
3. Primary Limit Control (S10)
Figure 11 shows the primary limit (S10) used on G51MP units
located in the heating vestibule panel. S10 is provided with a
shield on some models (figure 11) and must not be removed.
Note orientation of shield and limit if limit is replaced. When ex-
cess heat is sensed in the heat exchanger, the limit will open.
Once the limit opens, the ignition control energizes the sup-
ply air blower and de−energizes the gas valve. The limit au-
tomatically resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted.
FIGURE 11
PRIMARY LIMIT LOCATION
limit faces shield
limit shield
090, −110 and
−135 only
4. Burners (Figure 12)
All units use inshot burners. Burners are factory set and do not
require adjustment. Burners can be removed as an assembly
for service. Burner maintenance and service is detailed in the
MAINTENANCE section of this manual. Each burner uses
an orifice which is precisely matched to the burner input. All
G51MP natural gas units are fitted with .089" sized orifices.
See table 22 or SPECIFICATIONS" tables for LP kits and
high altitude. The orifice is threaded into the burner manifold.
The burner is supported by the orifice and will easily slide off
for service. A flame retention ring in the end of each burner
maintains correct flame length and shape and keeps the flame
from lifting off the burner head. In addition, the burner entrance
to each clamshell is fitted with a corbel cup (orifice) used to di-
rect the flow of combustion products.
NOTE − Do not use thread-sealing compound on the ori-
fices. Thread-sealing compound may plug the orifices.
FIGURE 12
5/16"
BURNER DETAIL TOP VIEW
ignitor
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