GAS HEAT MODELS
Pre-Start Checks:
•When starting up these units, it is imperative that ALL gas fittings within the unit (in addition to the field installed gas connections) are leak checked with a soap solution as part of the unit commissioning process. The heat section must be fired 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, flame 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
•The furnace ignition control uses flame rectifi- cation as verification of burner operation. The minimum allowable flame current for operation is 0.7 dc microamps.
•If the furnace ignition control does not prove flame 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 flame 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 flame recti- fication is not proven on the third try, the ignition control will lock out.
FORM
•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 flame roll out, flame burning outside the heat exchanger. A restriction in the heat exchanger or breach in the flue 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- | 3 |
changer return bend. These limits are automatic |
reset devices. If the limit opens the ignition control will
•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 flame 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.
JOHNSON CONTROLS | 53 |
