Clamshell Heat Exchanger; Backup Secondary Limit Control (S113) (G51Mp−090, 110, 135 Only); Gas Valve (Gv1) - Lennox G51MP Unit Information

5. Clamshell Heat Exchanger

G51MP units use an aluminized steel primary and stain-
less 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 ensures
maximum efficiency.
The combustion air inducer pulls fresh air through the air in-
take 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.
6. Backup Secondary Limit Control (S113)
(G51MP−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 blower
failure. S113 contacts open when temperature on the CAI
reaches 142°.
7. Flame Rollout Switches (S47)
Flame rollout switches S47 are SPST N.C. high temperature
limits located on each side of the burner box assembly (see fig-
ure 13). S47 is wired to the ignition control A92. When ei-
ther of the switches sense flame rollout (indicating a
blockage in the combustion passages), the flame rollout
switch trips, and the ignition control immediately closes
the gas valve. Switch S47 in all G51MP units is factory pre-
set to open at 250_F + 12_F (121_C + 6.7_C) on a tempera-
ture rise. All flame rollout switches are manual reset.
ROLLOUT SWITCH
(each side)
FIGURE 13

8. Gas Valve (GV1)

The G51MP uses a gas valve manufactured by Honeywell or
White Rodgers (see figure 14 ). The valves are internally redun-
dant to assure safety shut-off. If the gas valve must be re-
placed, 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 ignition control. 24V applied to the terminals
opens the valve.
Inlet and outlet pressure taps are located on the valve. A
manifold adjustment screw is also located on the valve.
An LPG changeover kit is available. See table 22. For
units equipped with the Honeywell VR205 valve, the kit
includes a low pressure switch that must be installed in
the valve as shown in figure 14.
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 changes gas valve outlet (manifold) pres-
sure based on changes in the burner box pressure. The in-
tent is to compensate for different vent configurations which
can greatly affect the rate of the unit.
The White Rodgers 36G gas valve (figures 14 and 42)
is equipped with pressure posts for measuring sup-
ply and manifold pressures. The posts provide built−
in hose connections and have an integral 3/32" Allen−
head screw. Rotate the screw counterclockwise one
full turn to permit pressure measurement. Reseat the
screw (rotate one full turn clockwise) after measure-
ments have been taken to prevent gas leakage.
HONEYWELL VR8205 SERIES GAS VALVE
PRESSURE
SWITCH
(LP ONLY)
INLET
PRESSURE
PORT WITH
INLET TAP
FITTING
WHITE RODGERS 36G SERIES GAS VALVE
LINE PRESSURE
INLET POST
GAS VALVE SHOWN IN OFF POSITION
Page 16
IMPORTANT
BARBED
FITTING
ON
OFF
MANIFOLD
PRESSURE
OUTLET POST
MANIFOLD
PRESSURE
ADJUSTMENT
FIGURE 14
MANIFOLD
PRESSURE
OUTLET
REFERENCE
TO BURNER
BOX
SCREW