Testing Steam Boiler LWCO's

Testing Float Style LWCO

I have been asked many times through the years what the proper way to test a float type low water cut-off (LWCO). Early in my career which has been 43+ years now, I was taught the improper way of testing float type LWCO's. A float type LWCO usually has a valve on the bottom to flush the valve that allows cleaning and proper operation of the float. When flushing the LWCO, the rapid release of water flushed dirt from the float chamber of the LWCO.

Below are two distinct types of float type LWCO's manufactured by McDonnell Miller.

#47 - Mechanical LWCO & Feeder

#67 Mechanical LWCO, would require a separate feeder

This is required every week or two dependent on the manufacturer's directions. This would be done by opening the yellow handle until the water runs clear. This usually is about 5 - 10 seconds or slightly longer. If you do this while the boiler is running it should shut down as the float drops when the water is quickly evacuated.

While this will shut the boiler down and many technicians believe this is the proper way to test a float type LWCO, it is not.

The reason is, as the water evacuates quickly it is not how the boiler will operate during normal operation.

During the boiler operation the water level will drop slowly, not quickly. Thus, the float could hang up when the level drops slowly. The proper way to test the float type LWCO is slowly drain water slowly from the bottom of the boiler. As the water drops slowly instead of quickly from the float chamber, there is a better chance to determine if LWCO will shut the boiler down. It simulate's the normal operation of the boiler.

In the above picture you can see a float type LWCO and a hose on a drain to lower the water level slowly to test the LWCO.

Testing Probe Style LWCO

The float type LWCO is still in use but the more standard is the probe type electronic LWCO's. These do not require flushing and there is no way to flush it. Contrary to belief these also require annual maintenance. Removing the probe annually and wiping it off or clean it per the directions from the manufacturer.

McDonnell Miller PSE-801          Hydrolevel 1100                Hydrolevel CG450


These are tested the same way, lowering the water level slowly from a boiler drain with a hose connected to the bottom of the boiler.

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