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The LDU11 (or control unit) is used in a gas-fired combustion system with or without a vent valve in the gas train.

Installed with one or two pressure switches mounted between the gas valves, the LDU automatically initiates gas valve proving either

• prior to burner start-up
• immediately after the controlled shutdown, or
• both prior to burner start-up and immediately after shutdown

The valve leak test is completed in two stages:

• First test phase Test1, evacuates the pressure between the gas valves to test the upstream gas shutoff valve. The pipe between the gas valves is monitored by a leak detect pressure switch for no increase in pressure.
• Second test phase Test2, pressurizes the area between the gas valves to test the downstream safety shutoff valve. The pipe between the gas valves is monitored by the leak detect pressure switch for no decrease in pressure.

If the leak detect switch signals excessive gas pressure increase during the first test phase Test1, or decrease during the second test phase Test2, the LDU control will lockout and prevent burner start-up. The indicator dial on the front of the unit indicates the progress of the leak test. When a fault occurs, the indicator dial stops at the time of lockout, and indicates which of the valves is leaking. In the event of a lockout, the reset button on the front of the control will light orange to indicate a fault.