750-91 (revised 2009)
Model CB-LE Packaged Boiler Manual
6-21
6.18 — Low-Gas Pressure Switch
6.17.2 — High Turndown Burner Low-Fire Adjustment
The fuel input should be adjusted using the low-fire cam screw, to approximately 10% of that at high-fire (see Fig-
ure 6-9). At low-fire the O
2
flue gas reading should be between 9.5% and 11%. When adjusting a high turndown
burner equipped with IFGR, the O
2
readings should be 6% to 7%.
It may be necessary to readjust the setting of the low-fire stopscrew in order to obtain the proper air/fuel ratio at
low-fire. To ensure that the low-fire position of the gas butterfly valve is always the same, allow a half turn of the
stop screw for overtravel.
If the air damper must be reset to meet the low-fire air/fuel requirements, combustion at higher firing rates must be
rechecked.
The second cam adjusting screw may need to be adjusted in order to maintain a smooth cam profile. If all screws
are properly adjusted, none will deviate from the general overall contour of the cam face.
6.18 — Low-Gas Pressure Switch
Adjust the scale setting to slightly below the normal burning pressure. The control circuit will be broken when
pressure falls below this point. Since gas line distribution pressure may decrease under some conditions, shutdowns
may result if the setting is too close to normal. However, regulations require that the setting may not be less than
50% of the rated pressure downstream of the regulator.
Manual resetting is necessary after a pressure drop. Press the reset lever after pressure is restored. Be sure that the
mercury switch equipped control is level.
6.19 — High-Gas Pressure Switch
Adjust the scale setting to slightly above the normal burning pressure. The control circuit will be broken when
pressure exceeds the normal operating pressure. Unnecessary shutdowns may result if the setting is too close to
normal, however, regulations require that the setting may not be greater than 150% of rated pressure.
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