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3
JOHNSON CONTROLS
FORM 100.50-NOM6 (1207)
GAS HEAT MODELS
Pre-Start Checks:
• When starting up these units, it is imperative
that ALL gas fi ttings within the unit (in addition
to the fi eld installed gas connections) are leak
checked with a soap solution as part of the unit
commissioning process. The heat section must
be fi red when checking the joints on the manifold
side of the gas valve. If any leaks are detected,
the leaks must be repaired immediately and all
joints rechecked.
• Verify wiring inside the burner compartment to
insure the wiring/terminals are tight and securely
connected to the components, such as the ignition
control, fl ame sensor, gas valve, rollout switches
and igniter.
• The gas heat start up sequence begins with a 30
second prepurge. The next step in the sequence
is the closure of the air proving switch. The heat
section has a combustion air-proving switch.
This switch must close before the ignition se-
quence can initiate. If the air-proving switch is
closed after the 30 second prepurge the ignition
control will energize the spark igniter and open
the gas valve.
• The furnace ignition control uses fl ame rectifi -
cation as verifi cation of burner operation. The
minimum allowable fl ame current for operation
is 0.7 dc microamps.
• If the furnace ignition control does not prove
fl ame in 7 seconds, it will turn off the spark signal
and close the gas valve. It will wait 30 seconds
and then initiate a second ignition sequence. If
fl ame is not proven during the second 7 second
trial for ignition the control will turn off the spark
signal, close the gas valve, wait 30 seconds and
initiate a third ignition sequence. If fl ame recti-
fi cation is not proven on the third try, the ignition
control will lock out.
• The heat section has two roll out switches mount-
ed above the burners. The purpose of the roll
out switch is to protect the gas heat section from
fl ame roll out, fl ame burning outside the heat
exchanger. A restriction in the heat exchanger
or breach in the fl ue passages could result in a
roll out situation. The roll out switch is a manual
reset device.
• The unit has two high temperature limit switches.
One located at the heat exchanger vestibule panel
and the other located in the area of the heat ex-
changer return bend. These limits are automatic
reset devices. If the limit opens the ignition
control will de-energize the gas valve. On staged
gas heat, as soon as the limit closes the ignition
control will reinitiate the ignition sequence. If the
limit opens on a modulating gas heat section the
Unit Controller will lockout the heat section.
• The control circuit is tested in the factory to insure
that all of these steps are followed, however, natu-
ral gas is not actually introduced to the system in
the plant; nitrogen is used in its place.
Post Start Checks:
When a signal is received at the gas heat control mod-
ule from the Unit Controller, verify:
• Combustion blower starts and runs for 30 seconds
before the spark is initiated.
• Spark igniter sparks.
• Gas valve opens.
• Burners light from right to left, in a 2.5 second
time frame; that each one lights in sequential order
from right to left; and establishes stable fl ame im-
mediately upon ignition.
• Check for gas leaks in the unit piping as well as
the supply piping.
• Check for correct manifold gas pressures. See
“Manifold Gas Pressure Adjustment” in this Sec-
tion.