Lennox LGH094U Unit Information page 43

Flame rectification sensing is used on all LGH 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 System Ser­
vice Checks section for flame current measurement.
The control shuts off gas flow immediately in the event of a
power failure. Upon restoration of gas and power, the con­
trol 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 A55 Unit Controller. The ignition control then allows 30
to 40 seconds for the combustion air blower to vent exhaust
gases from the burners. When the combustion air blower is
purging the exhaust gases, the combustion air prove switch
is closing proving that the combustion air blower is operat­
ing 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 elec­
trode and the flame sensing electrode. Sparking stops im­
mediately after flame is sensed. The combustion air blower
continues 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 ig­
nition cannot be obtained after the third attempt, the control
will lock out. The ignition control is not adjustable.
2-Heat Exchanger (Figures 25 and 26)
The LGH units use aluminized steel inshot burners with
matching tubular aluminized (stainless steel is an option)
steel heat exchangers and two‐stage redundant gas
valves. Units are equipped with one eleven tube/burner for
high heat and one six tube/burner for standard heat. Burn­
ers use a burner venturi to mix gas and air for proper com­
bustion. Combustion takes place at each tube entrance. As
hot combustion gases are drawn upward through each
tube by the combustion air blower, exhaust gases are
drawn out the top and fresh air/gas mixture is drawn in at
the bottom. Heat is transferred to the air stream from all sur­
faces of the heat exchanger tubes. The supply air blowers,
controlled by the A55 Unit Controller, force air across all
surfaces of the tubes to extract the heat of combustion. The
shape of the tubes ensures maximum heat exchange.
The gas valves accomplish staging by allowing more or
less gas to the burners as called for by heating demand.
3-Gas Heat Exchanger Inserts - Direct Drive
Only
Inserts are installed on standard (130,000Btuh) and high
(240,000Btuh) heat exchangers. Medium heat exchangers
do not require inserts. See figure 27. Inserts are used to
maintain even temperature distribution through the heat
exchanger. Temperature distribution can vary depending
on supply air flow, number of heat exchanger tubes and the
blower deck opening.
HEAT EXCHANGER ASSEMBLY
HEAT
EXCHANGER
TUBE
COMBUSTION
AIR INDUCER
GAS VALVE
BURNER
BURNER BOX ASSEMBLY
GAS
VALVE
GAS MANIFOLD
4-Burner Assembly (Figure 28)
The burners are controlled by the spark electrode, flame sens­
ing electrode, gas valve and combustion air blower. The spark
electrode, flame sensing electrode and gas valve are directly
controlled by ignition control. Ignition control and combustion
air blower is controlled by A55 Unit Controller.
Burners
All units use inshot burners (see figure 28). Burners are
factory set and do not require adjustment. A peep hole
with cover is furnished in the heating access panel
for flame viewing. Always operate the unit with the
access panel in place.
Burners can be removed individually for service.
Burner maintenance and service is detailed in the
SERVICE CHECKS section of this manual.
Page 43
VENT
CONNECTOR
FIGURE 25
BURNERS
FLAME
SENSOR
FIGURE 26