Gas Heat Components - Lennox LGA Series Unit Information

D−GAS HEAT COMPONENTS (all LGA units)
LGA300/360 production units to July 2004 are available
in 260,000 BTUH (76.2 kW) standard gas heat or
470,000 BTUH (137.7 kW) high gas heat sizes. LGA/
LGC248/300/360 units built August 2004 and later are
available in 260,000 Btuh (76.2 kW) standard gas heat,
360,000 Btuh (105.5 kW) medium gas heat and 480,000
Btuh (140.6 kW) high gas heat sizes. See unit name-
plate for capacities. All units are equipped with two iden-
tical gas heat sections (gas heat section one and gas
heat section two). Most units will have a flexible connec-
tion instead of cast iron pipe, though some earlier mod-
els will have cast iron pipe. Black steel pipe will feed the
supply gas to each gas valve. If for service the flexible
connection must broken, hand tighten, then using a
wrench turn additional 1/4 turn for metal to metal seal
(do not over tighten).
NOTE−Do not use thread sealing compound on flex pipe
flare connections.
1−Control Box Components
A3, A12, A55, A58, T3, T13, K13 and K19
The main control box (see figure 3) houses the burner controls
A3 and A12, main control module A55, gas valve (burner) con-
trol module A58, combustion air inducertransformers T3 and
T13, combustion air inducerrelay K13, and second heat sec-
tion relay K19. For a description of the components see sec-
tion I-A. A more detailed description of burner controls A3 and
A12 is given below.
Burner Ignition Control A3 and A12
The ignition controls are located in the control box. Three dif-
ferent manufacturers' (Fenwal, Johnson Controls, and RAM)
controls are used in the LGA/LGC units. All three ignition con-
trols operate the same.
The ignition control provides three main functions: gas
valve control, ignition, and flame sensing. The unit will
usually ignite on the first attempt; however, the ignition at-
tempt sequence provides three trials for ignition before
locking out. The lockout time for the Johnson control is 5 min-
utes. The lockout time for the Fenwall and RAM control is 1
hour. After lockout, the ignition control automatically resets
and provides three more attempts at ignition. Manual reset
after lockout requires breaking and remaking power to the
ignition control. See figure 18 for a normal ignition sequence
and figure 19 for the ignition attempt sequence with re-
trials (nominal timings given for simplicity). Specific timings
for the ignition controls are shown in figure 20.
Flame rectification sensing is used on all LGA/LGC
units. Loss of flame during a heating cycle is indicated by
an absence of flame signal (0 microamps). If this happens, the
control will immediately restart the ignition sequence and then
lock out if ignition is not gained after the third trial. See table 21
for microamp signal values .
The control shuts off gas flow immediately in the event of a
power failure. Upon restoration of gas and power, the control
will restart the ignition sequence and continue until flame is
established or system locks out.
On a heating demand, the ignition control is energized by the
main control module A55. The ignition control then allows 30
to 40 seconds for the combustion air inducerto vent exhaust
gases from the burners. When the combustion air induceris
purging the exhaust gases, the combustion air prove switch is
closing proving that the combustion air induceris operating
before allowing the ignition control to energize. When the
combustion air prove switch is closed and the delay is over,
the ignition control activates gas valve, the spark electrode
and the flame sensing electrode. Sparking stops immediate-
ly after flame is sensed. The combustion air inducercontinues
to operate throughout the heating demand. If the flame fails or
if the burners do not ignite, the ignition control will attempt to
ignite the burners up to two more times. If ignition cannot be
obtained after the third attempt, the control will lock out. The
ignition control is not adjustable.
The Johnson control is illustrated in figure 17. The spade
connections are used to connect the control to unit. Each of
the spade terminals are identified by function. The spark
electrode wire connects to the spark-plug-type connector on
top of the control.
JOHNSON IGNITION CONTROL
Page 44
FIGURE 17