All Topics
Low Water Cutoff (LWCO)
Critical safety device that shuts off fuel when water drops below a safe level.
In this topic:
Purpose
The LWCO automatically shuts off the fuel supply to the burner when the water level drops below a safe minimum. It is a critical safety device that prevents boiler damage and explosions.
- Stops the burner when water level is too low
- Prevents firing on exposed heating surfaces
- Must NEVER be bypassed, jumpered, or disabled
- Required on all automatically fired boilers
A low-water condition with continued firing is the most common cause of boiler explosions.
Types of LWCO
Two primary types of LWCO are used in boiler applications.
- Float-type: a float on an arm rises and falls with water level; drops below cutoff point to trip
- Probe-type (electronic): conductivity probes sense whether water or steam is present
- Float-type requires regular blowdown to prevent stuck floats
- Probe-type requires clean probes — scale buildup can cause false readings
Testing the LWCO
Regular testing verifies the LWCO will function when needed. Testing should be documented in the boiler logbook.
- Blowdown test: open the LWCO drain valve to drop the water level and verify the burner shuts off
- Evaporation test: let the water level drop naturally by not feeding water, verify cutoff
- The blowdown test is the most common daily test method
- The burner must shut off BEFORE the water level drops below the lowest visible level in the gauge glass
- If the LWCO does not trip, shut down the boiler immediately and repair
Test the LWCO at least once per day on operating boilers.