Clamshell Heat Exchanger; Backup Secondary Limit Control; Flame Rollout Switches; Gas Valve - Lennox G43UF SERIES Unit Information

BURNER DETAIL TOP VIEW
5/16"
ignitor
FIGURE 12

5. Clamshell Heat Exchanger

G43UF 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
intake 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
exchangers and exhausted out the exhaust vent pipe.
6. Backup Secondary Limit Control (S113)
(G43UF−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. 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
figure13). S47 is wired to the burner ignition control A92.
When either of the switches sense flame rollout (indicat-
ing a blockage in the combustion passages), the flame
rollout switch trips, and the ignition control immediately
closes the gas valve. Switch S47 in all G43UF units is fac-
tory preset to open at 250_F + 12_F (121_C + 6.7_C) on a
temperature rise. All flame rollout switches are manual reset.
BURNER BOX ASSEMBLY
8. Gas Valve (GV1)
The G43UF uses a gas valve manufactured by Honeywell or
White Rodgers (see figure 14 ). The valves are internally re-
dundant 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 con-
nected 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 20. 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)
pressure based on changes in the burner box pressure.
The intent is to compensate for different vent configura-
tions which can greatly affect the rate of the unit.
IMPORTANT
The White Rodgers 36G gas valve (figure 14) is
equipped with pressure posts for measuring supply
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.
Page 15
ROLLOUT SWITCH
(each side)
FIGURE 13