Page 13
senses rollout, the ignition control immediately stops igni-
tion and closes the gas valve. If unit is running and flame
rollout is detected, the gas valve will close and ignition con-
trol will be disabled. Rollout can be caused by a blocked
flue or lack of combustion air. The switch is factory set and
cannot be adjusted. The setpoint will be printed on the side of
the limit. The switch can be manually reset. To manually re-
set a tripped switch, push the reset button located on the
control.
FIGURE 12
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
9−Primary Limit Control (S10)
FIGURE 13
80UHG SERIES UNITS
LIMIT CONTROL FOR
THIS TYPE AUTO-RESET LIMIT
IS USED FOR THE PRIMARY LIMIT (S10) AND
RIGHT SECONDARY LIMIT (S21)
(see FIGURE 11)
The primary limit (S10) on 80UHG units is located in the
middle of the heating vestibule panel. When excess heat is
sensed in the heat exchanger, the limit will open. If the limit is
tripped, the furnace control energizes the supply air blower
and closes the gas valve. The limit automatically resets when
unit temperature returns to normal. The switch is factory set
and cannot be adjusted. The switch may have a different
setpoint for each unit model number. However, the setpoint
will be printed on the side of the limit.
10−Secondary Limit Controls (S21)
The secondary limit (S21) on 80UHG units is located in the
blower compartment in the back side of the blower housing.
When excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the furnace control ener-
gizes the supply air blower and closes the gas valve. The limit
automatically resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted. The set-
point will be printed on the limit.
Two limits are supplied in each furnace. 80UHG3, 80UHG4
and 80UHG4/5 model units
built prior to Apri 1999, will
have one each of style shown in figures 13 and 14. If stick
limit (figure 14) suffers frm nuisance trips on the above
model units and the furnace is in the horizontal position, re-
place with limit kit no. 50L98. All 80UHG2 model units will
have one of each style shown in figures 13 and 14.
INSULATING COVER (s)
FIGURE 14
SECONDARY LIMIT CONTROL (S21)
FOR 80UHG SERIES UNITS
SP
ADE CONNECT
ORS
INSULA
TING COVER
LIMIT
LIMIT
THIS TYPE AUTO-RESET LIMIT IS
USED FOR THE LEFT SECONDARY
LIMIT (S21) (see FIGURE 11)
11−Spark Electrode and Flame Sensor
80UHG−1 Models
Figure 17 shows the arrangement of flame sensor, spark
electrode and burners. The ignition control uses direct
spark to ignite the rightmost burner and the burners cross-
light to the left. The flame sensor uses flame rectification to
sense combustion. A flame retention ring in the end of
each burner is used to maintain correct flame length and
shape and to keep the flame from lifting off the burner
head.
Figure 18 shows the gap between tip of the electrodes and
the burner surface, and the gap between the sensor and
burner surface.
12−Gas Valve
The 80UHG uses a gas valve manufactured by Honey-
well or White Rodgers. The valve is internally redundant to
assure safety shut−off. If the gas valve must be replaced,
the same type valve must be used.
24VAC terminals and gas control knob are located on top of
the valve. All terminals on the gas valve are connected to
wires from the electronic ignition control. 24V applied to the
terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A regu-
lator adjustment screw (figures 15 and 16) is located on the
valve.
LPG changeover kits are available from Lennox. Kit s include
burner orifices and a gas valve regulator conversion kit.
