Connect Gas Piping; High Altitude Derate; Pressure Test Gas Piping; Two-Stage Gas Valve Adjustment - Lennox LGH420 Installation Instructions Manual

Connect Gas Piping

Two grommets are packaged with the flue exhaust and
air intake hoods located in the gas heat section.
Grommets are installed in the unit entry and gas heat
division panel knockouts. An additional grommet is
required when routing gas piping through the bottom of
the unit.
Before connecting piping, check with gas company or
authorities having jurisdiction
requirements. When installing gas supply piping, length
of run from gas meter must be considered in determining
pipe size for 0.5" w.c. (.12kPa) maximum pressure drop.
Do not use supply pipe smaller than unit gas connection.
For natural gas units, operating pressure at the unit gas
connection must be a minimum of 6.0" w.c. (1.5kPa) and
a maximum of 14" (3.50kPa) w.c. For LP/propane gas
units, operating pressure at the unit gas connection must
be a minimum of 11" w.c. (2.74kPa) and a maximum of
13.5" w.c. (3.36kPa).
When making piping connections a drip leg should be
installed on vertical pipe runs to serve as a trap for
sediment or condensate. A 1/8" N.P.T. plugged tap is
located on gas valve for test gauge connection. Refer to
Heating Start-Up section for tap location. Install a ground
joint union between the gas control manifold and the
main manual shut-off valve. See figure 2 for side entry
gas supply piping.
Compounds used on threaded joints of gas piping shall be
resistant to the action of liquified petroleum gases.
SIDE ENTRY OF GAS SUPPLY PIPING
GROUND
JOINT UNION
MANUAL MAIN
SHUT-OFF VALVE
TO GAS
SUPPLY
GAS PIPING
SUPPORT
FIGURE 2
for local code
(REFER TO
LOCAL CODES)
TO GAS
VALVE
DRIP LEG
Page 3

High Altitude Derate

Locate the high altitude conversion sticker in the unit
literature bag. Fill out the conversion sticker and affix next
to the unit nameplate.
Refer to table 1 for high altitude adjustments.
TABLE 1
HIGH ALTITUDE DERATE
Altitude Ft.*
2000-4500
4500 And Above Derate 4% / 1000 Ft. Above Sea Level
*Units installed at 0-2000 feet do not need to be modified.
NOTE ‐ This is the only permissible derate for these units.

Pressure Test Gas Piping

For natural gas units, operating pressure at the unit gas
connection must be a minimum of 6.0" w.c. (1.5kPa) and
a maximum of 14.0" (3.5kPa) w.c. For LP/propane gas
units, operating pressure at the unit gas connection must
be a minimum of 11" w.c. (2.74kPa) and a maximum of
13.5" w.c. (3.36kPa).
When testing the pressure of gas lines, the gas valve
must be disconnected and isolated. Gas valves can be
damaged if subjected to more than 0.5 psig (3.5 kPa).
WARNING
FIRE OR EXPLOSION HAZARD
Failure to follow the safety warnings exactly could
result in serious injury, death or property damage.
Never test for gas leaks with an open flame. Use a
commercially available soap solution made specifi­
cally for the detection of leaks to check all connec­
tions. A fire or explosion may result causing prop­
erty damage, personal injury or loss of life.

Two-Stage Gas Valve Adjustment

Gas manifold pressures should match pressures shown in
table 2. On two stage gas valves, initiate a W2 thermostat
demand to check high fire pressure before low fire
pressure. See figure 3 or 4. With high fire operating, reduce
the thermostat demand to W1 and check the low fire
pressure. Refer to the Unit Controller manual to initiate a
thermostat demand.
See Optional Modulating Gas Valve (MGV) section on
units equipped with MGVs.
IMPORTANT - On 2-stage gas valves do not set low
fire pressure lower than the certified minimum input
rating listed in table 2.
Gas Manifold Pressure
See Unit Nameplate