Low Water Cutoff (Is Your Boiler Safe?)

Two of the most important safety devices on a boiler are the primary and auxiliary low water cutoff (LWCO) switches. Water in the boiler must be maintained at the normal operating water level (NOWL). Low water cutoff switches are generally 2-6 inches below the NOWL. The LWCO uses an electric conductivity probe or a float to detect the water level inside the boiler. When water drops below this level, LWCOs prevent the burner from firing. 

 Mcdonald miller low water cut off

If only the primary LWCO was tripped than as the water level returns to normal, the burner can begin firing again. If the auxiliary low water cut off trips, it will require an operator reset. Some systems however, may lock out a burner following a primary low water shut-down. This would require a boiler operator to manually reset the switch or controls to get the boiler back up and running.


Failures can result in boiler tube damage or catastrophic failure of your boiler! When heat from hot combustion gasses flows into the boiler during low water conditions (aka dry firing) the following may occur:

  • Hot spots and metal discoloration
  • Warping of metal walls and or boiler tubes
  • In extreme cases your entire boiler can explode.

These are all indications that the steel (typically carbon steel) is overheating, and the properties of the metal are altered. Do not run a boiler with these signs, until someone can inspect it. In some jurisdictions, this is a reportable failure of both low water devices and your state should be notified.

Prevent LWCO Failure

Low Water Cut off with sludge build up

Sludge build up around a LWCO float switch.


Perform a blowdown of the level controls, LWCO bridle, and auxiliary low water cutoff bridle every day. This will help to remove solids. This usually involves turning a valve open and then closed a series of times to remove sediment.

Weekly or daily quick drain tests on the LWCOs are typical on high pressure steam boilers to ensure safe operation. This blowdown cleans the column and the piping surrounding the switch. This blowdown will either shut the boiler off, or produce an audible alarm or indicator light.

Boiler controls equipped with a shunt button should allow verification the low water switches are working, even if the shunt prevents a burner trip. Without a shunt button, the boiler will need to be tripped to ensure these lower water switches are working. Slow drain tests should be performed semi-annually or as recommended by the boiler manufacturer, your insurance provider, or the AHJ. 


These switches have a 10-year lifespan when maintained, but can fail sooner when neglected. By understanding the average age you can better plan for repairs and replacements.


All low water cutoffs are susceptible to deposit build up inside the bridle piping and the float or conductivity probe chamber. This build up can make render the switch used for detecting water level useless. This creates all sorts of problems! By blowing down these lines you can reduce the risk of such a malfunction. Monitor buildup by opening up floats, switches, and bridle piping crosses at every internal inspection.


Performing these tests more accurately reflects a real low water scenario. In an article written by Ware they lay out the best way to perform a slow low cutoff test.

“To perform the test, your burner needs to be set at 50% or lower. Instead of opening the blowdown valve to force a rapid low-water situation, you are going to turn off the feedwater pump. Once the supply of feedwater stops, the boiler will begin converting all the remaining water to steam, and the water level will slowly drop…

Watch the water level at the sight glass. When the water level gets low, the burner should shut off. If during this test the water level falls below the level of the sight glass and the burner is still firing, your system has not passed the test.” – Jude Wolf

Blowing Down A LWCO

Begin by gradually opening the blowdown valves. This gradual opening allows the piping to warm up and removes stagnant water from the drain piping. Avoid abruptly opening the valves, as this can cause steam to condense and result in water hammer. Improper blowdown piping can lead to damage to components. To address these concerns, McDonnell & Miller recommends employing a dual valve blow-down system for each control.

Here’s how to conduct the blowdown using the dual valve system:

  1. Preparation: Ensure the water level in the boiler is at its normal level and the burner is active.
  2. Opening Valves: Open the upper valve (#1) first, followed by slowly opening the lower valve (#2). As you do this, observe the water level in the sight glass – it should begin to lower.
  3. Pump Activation: As the water level in the sight glass starts to lower, the pump should activate.
  4. Burner Deactivation: As the water level continues to decrease in the sight glass, the burner should deactivate.
  5. Closing Valves: Gradually close the lower valve (#2), followed by closing the upper valve (#1).
  6. Water Level Restoration: The water level in the sight glass should begin to rise. Initially, this should trigger the burner to turn on, and subsequently, the pump should turn off.

If your system has manual resets, you’ll need to press the reset button after the water level has been restored before the burner can operate.

Low Pressure Steam

These are systems where the pressure of a boiler is below 15 PSIG. They are able to use a different system to regulate low water. Oftentimes we see built-in cutoffs from the boiler OEM. These systems will commonly have a feedwater cut-off combination float device. When they detect low water, they supply feedwater to the boiler. If the water continues to lower, the boiler will trip.

Low water cut off system provided by mcdonald miller

Picture courtesy of MacDonald Miller 

NOTE: Never use untreated water unless it is an emergency situation. This can create scale and corrosion in the boiler and the system!

Low Water Cut Off Questions?

Talk to the experts. We can help with installation, service and parts. Call us at 1-800-237-3141, email sales@rasmech.com or contact us online.

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