The Level Control Board and the Low Water Cutoff boards detect water by sensing the current flow between a probe and the boiler vessel (shell) through the water. The water quality specifications for standard boilers call for 8 to 85 ppm TDS. The minimum value of 8 ppm ensures the water is capable of current flow. Pure water requires a level detection system that is not based on conductivity, such as a float switch.


There is a level control probe and a low water cutoff probe in the boiler pressure vessel. Each probe is connected to a circuit board. Each circuit board also has a ground terminal that must be connected to the boiler pressure vessel.

The wiring from the board probe terminal to the probe and the wiring from the GND terminal to the boiler pressure vessel are equally important. The boards are simply looking for current flow between the GND and Probe terminals to detect water. If no conductivity exists between these terminals, then no water is assumed. This function is similar to a typical ohm meter function.

The level control board toggles a 110 VAC signal, either to the feed pump circuit or to the heating contactor control circuit. When the level control is satisfied, voltage to the CONTACTOR terminal is present. When the level control is not satisfied, the voltage is switched to the PUMP terminal.

The PUMP terminal supplies current to the coil of the feed water pump relay and feed water solenoid valve when the board is calling for water. As the boiler fills and water reaches the level control probe, it toggles to CONTACTOR, thus de-energizing both the feed water pump and the feed water solenoid valve.

Boilers without Low Water Cut-off (LWCO)

If the boiler does not have an LWCO board, current from the CONTACTOR terminal goes directly to the High Limit Control pressure switch, the Operation Control pressure switch, and onto the coil of the heater contactor if the pressure switch is calling for steam.

Boilers with Low Water Cut-off (LWCO)

If the boiler does have an LWCO board, the CONTACTOR signal goes to the LWCO board. If the LWCO board senses water and is satisfied, it then relays the CONTACTOR current to the High Limit Control pressure switch, the Operation Control pressure switch, and onto the coil if the heater contactor of the pressure switch is calling for steam.

If the LWCO board does not sense water, it toggles from sending current to the heating contactor to sending current to the LWCO status light circuit. The boiler contactor will remain de-energized until a manual reset is performed. A manual reset will cause the contactor circuit to be energized again if the boiler is full of water.

Have a question or having difficulties? Talk to Dave Schall, our PRIMUS sterilizer expert who is here to assist you anytime at 877.679.7800 extension 1212 or email at dschall@spire-is.com.

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