Low-Water Cutoff

Definition - What does Low-Water Cutoff mean?

A low-water cutoff is a mechanical device (usually a float switch) that is used to turn off the burner or shut off fuel to a boiler to prevent it from running once the water drops below a certain point. If a boiler is "dry-fired" (burned without water in it) it can suffer rupture or catastrophic failure.

The cause of low-water cutoff device failure is usually lack of maintenance, incorrect maintenance or improper modification.

Corrosionpedia explains Low-Water Cutoff

Low-water cutoff devices for boilers are one of the most important safety devices. These devices are installed on most steam heating boilers and also on many hydronic or "hot water" heating boilers as a safety device to shut down the boiler in the event of loss of water in the system.

The device's function is simply to prevent boiler operation in low-water conditions once it detects liquid levels dropping. These are relatively simple mechanisms; their purpose is to shut off fuel to the fire when the water level drops below a safe operating level. This is accomplished either mechanically or electronically. Without a properly functioning low-water cutoff, the burner can continue to fire, allowing for intense heat to build. The results can be catastrophic — explosions, fires and the possibility of severe injuries to personnel and property.

Low-water cutoff device failures are easy to prevent. Indicators of failure include:

  • Physical signs of poor maintenance
  • Very low blowdown frequency
  • Sediment buildup
  • Rough or erratic function in boiler components

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