Backup Secondary Limit Control; Gas Valve; Variable Speed Combustion Air Inducer Pressure Switch - Lennox G71MPP SERIES Unit Information

6. Backup Secondary Limit Control (S113)
(G71MPP−090, 110, 135 only)
Backup secondary limit control S113 is a N.C. auto−reset
switch located on the combustion air inducer. S113 acts as
a backup to primary limit S10 in the event of an indoor blow-
er failure. S113 contacts open when temperature on the
CAI reaches 142°.
7. Gas Valve (GV1)
The G71MPP uses a variable capacity gas valve (figure 35)
and is applicable for single stage, two−stage or varaiable
capacity settings. See Thermostat selection modes" in the
ignition control section (section A− subsection 4.) 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 Lennox. Kits include
burner orifices and an LP gas valve.
DANGER
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 condi-
tioning device adjusts the gas flow. This will compensate
for different vent configurations which can greatly affect the
rate of the unit.
8. Variable Speed Combustion Air Inducer
Pressure Switch (S18)
All G71MPP units are equipped with two pressure switch as-
semblies (one dual switch and one single switch see figure
14) to allow for multiple unit installations without any hose
routing changes. The pressure switches serve four func-
DUAL COMBUSTION AIR PRESSURE SWITCH
high fire
low fire
tions. First they establish calibration points for the vent cal-
ibration routine. The combustion air inducer's speed at a giv-
en firing rate is a function of the vent system restriction. The
calibration routine establishes the inducer speed required to
make low and high fire switches for a given vent pipe instal-
lation and interpolates the speeds required to achieve all in-
termediate rates between these two points. The pressure
conditioning device (low fire) and the high fire switch are uti-
lized for this function. The setting for low−fire switch on dual
assembly is such that it does not normally enter into the vent
calibration routine.
Second, the switches prove combustion air inducer opera-
tion by sensing a vacuum energizing the control circuit and
allowing ignition. The low fire pressure switch provides this
function.
Thirdly the switches interrupt the combustion process in the
event vent outlet or combustion air intake blockage. Both
switches provide this function.
Finally the switches interrupt the combustion process if the
condensate drainage system becomes blocked to the point
the condensate level builds up in the cold end header box/
secondary coil or vent system. Both switches serve this
function.
If either switch assembly (dual or single) is to be replaced,
replace the entire assembly. Individual switch components
can not be replaced.
To troubleshoot the pressure switches, temporarily jumper
them. The unit will not fire with the switches jumpered. There-
fore, the pressure switches must be bypassed after the com-
bustion air inducer is activated. This will determine if the pres-
sure switches and furnace are operating properly. However,
this may not indicate if the sealed combustion system is oper-
ating properly.
SINGLE COMBUSTION AIR PRESSURE SWITCH
negative
pressure
positive
operating pressure
pressure
FIGURE 14
Page 33
WARNING
The pressure switch is a safety shut−down control
in the furnace and must not be jumpered for any
reason other than troubleshooting.
pressure conditioning device
to valve
system pressure across
heat exchanger