Duralock Heat Exchanger; Flame Rollout Switches; Primary Limit Control - Lennox G71MPP SERIES Unit Information

MEASUREMENT IS TO
5/8
I.D. OF RETENTION
RING
"
7/32'
IGNITOR
BURNERS FRONT VIEW

3. Duralock Heat Exchanger

G71MPP units use an aluminized steel primary and
stainless steel secondary heat exchanger assembly.
Heat is transferred to the air stream from all surfaces of
the heat exchanger. The shape of the heat exchanger en-
sures maximum efficiency.
The combustion air inducer pulls fresh air through the burn-
er box. This air is mixed with gas in the burner venturi and
at the corbel orifices. The gas / air mixture is then burned
at the entrance of each clamshell. Combustion gases are
then pulled through the primary and secondary heat exchang-
ers and exhausted out the exhaust vent pipe.
4. Flame Rollout Switches (S47)
Flame rollout switch S47 is a high temperature limit located
on each side of the burner box. Each furnace is equipped
with two identical switches. The limit is a N.C. SPST manu-
al-reset limit connected in series with the primary limit S10.
When S47 senses rollout, the circuit breaks and the ignition
control immediately stops ignition and closes the gas
valve. If unit is running and flame rollout is detected, the gas
valve will close and ignition control will be disabled. Rollout
can be caused by a blocked heat exchanger, flue or lack of
combustion air. The switch is factory set to trip (open) at
250°F and cannot be adjusted. The switch can be manually
reset. To manually reset a tripped switch, push the reset but-
ton located on the control.
Ignitor Location
21/64"
BRACKET
FIGURE 12
5. Primary Limit Control (S10)
Figure 13 shows the primary limit (S10) used on G71MPP
units located in the heating vestibule panel. S10 is provided
with a shield on some models (figure 13) and must not be re-
moved. Note orientation of shield and limit if limit is replaced.
When excess heat is sensed in the heat exchanger, the limit
will open. Once the limit opens, the furnace control ener-
gizes the supply air blower and de−energizes the gas valve.
The limit automatically resets when unit temperature re-
turns to normal. The switch is factory set and cannot be ad-
justed. In the event of restricted air flow, the ignition control
board will reduce firing rate and indoor blower airflow in
10% increments until a sustainable air flow is reached. If
the furnace reaches 40% firing rate, and adequate air flow
is not available, the furnace will shutdown and enter one
hour watchguard.
Page 32
BURNERS TOP VIEW
PRIMARY LIMIT LOCATION
limit shield
090, −110 and
−135 only
limit faces shield
FIGURE 13