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SEQUENCE OF OPERATION
When the thermostat calls for heat, it energizes the cad cell primary control. The burner will initiate ignition after com-
pleting 15 second pre-purge cycle. The burner will operate until the thermostat is satised or the limit setting on the
high limit is reached. When the high limit control restores the circuit on a drop in pressure, the burner will start if the
thermostat is still calling for heat. The low water cut off will shut down the burner if the water level in the boiler drops
too low. The control resets and restarts the burner with a call for heat a few seconds after the water is returned to its
normal level.
The McDonnell Miller low water cut off incorporates a 10 second delay to prevent nuisance burner cut-off due to rapid
water level uctuations. The LWCO also has a time delay of 15 seconds which allows additional ll time after water
touches the probe. Red LED indicates a low water condition.
On burner start, if the cad cell does not see ame within approximately 45 seconds, primary control will lock out on
safety and must be reset before burner can be restarted.
When there is no demand for heat, the operating control will maintain the boiler water temperature at the selected set-
ting for proper operation of the domestic water heater, if equipped with optional tankless heater.
Figure 19: Wiring Diagram, Steam Boilers With or Without Tankless Heater,
McDonnell & Miller PS-801 Probe LWCO, Beckett AFG Burner