A low water cutoff (LWCO) is a safety device, typically a float switch, on steam boilers designed to shut the boiler off if it runs low on water. If a boiler is “dry-fired” as it runs low on water, it can suffer rupture or catastrophic failure. A “dry fire” is when no water is in the boiler to regulate the temperature. As a result, this can cause the boiler to overheat, warp or even crack, resulting in costly repairs or even a complete boiler replacement.
What Causes a Low Water Condition?
As an open system, some evaporative water loss is normal for a steam system. How much depends on the size and condition of the system. However, it’s time to troubleshoot if too much water is lost. Possible causes:
- Dirty air vents are not seating correctly and passing steam to the atmosphere
- The boiler blowdown valve has been left partially open
- Hot water is being drawn from the boiler
- The relief valve has discharged
- Malfunctioning condensate pump
- Improper near-boiler piping
- Leaking wet returns
- A closed or partially closed check valve
- The boiler may be foaming and priming
- Improperly pitched pipes
- Malfunctioning automatic feeder
- All of the condensate may not be returning from the system
- Corroded boiler metal
How Does a Low Water Cutoff Work?
There are several types of low water cutoffs that work in different ways for different applications.
Types of Low Water Cutoffs
Mechanical Float-Operated Low Water Cutoffs
Float-operated low-water cutoffs have been around since the 1920s and have earned a reputation worldwide for reliability. They are usually mounted directly in the boiler’s gauge glass tappings. Quick hook-up fittings are available to simplify installation. The chamber’s water level will mimic the boiler’s water level as the water level drops in the boiler, the level in the chamber, and the float drop. If the float sinks to the boiler’s critical low water cutoff point, the float will trip an electrical switch wired in series with the burner. The burner instantly stops firing. It will stay off until the water level rises to a safe operating point. This happens when the condensate returns from the system or when an automatic water feeder or a boiler attendant adds water to the boiler. When the level reaches a safe position, the low water cutoff will make its electrical connection, and the burner will restart.
Electronic Low Water Cutoffs
Electronic low water cutoffs continuously protect against high water conditions in steam boilers and other water level applications. Electronic low water cutoffs typically feature a probe. A manual reset function will require the unit to be reset after the water has risen above the level of the probe. If the control is in a low water condition when there is an interruption of power, the control will remain in a low water condition when power is restored. The reset button will need to be pressed when the water level is restored to a level above the probe to allow the burner to fire.
Probe & Float-Type Built-In Low Water Cutoffs
Some jacketed boilers don’t readily accept quick hook-up fittings. These boilers will often have a tapping for a built-in low water cutoff. These built-in units do the same thing as external units, but instead of being in a chamber, the “built-ins” are right inside the boiler, where they can sense the water level directly. There are two types of built-in low-water cutoffs:
Probe-Type Built-In LWCOs
The boiler manufacturer will specify where this type of low water cutoff should be inserted, usually below the water line but above the boiler’s crown. A probe uses the boiler’s water to complete an electrical circuit past an insulator back to a ground. An electronic “go” signal will travel to the burner if water covers the probe. When water drops off the probe for ten seconds, an electronic “stop” signal goes to the burner, shutting it down and protecting the boiler from a low water condition.
Float-Type Built-In LWCOs
These units sense the level directly inside the boiler.
Combination Low Water Cutoff/Automatic Water Feeders
These are ideal for use in residential or small commercial applications. They are compact and easily installed with either a probe or float-type low water cutoff. These feeders are always ready to add water when given the signal from the low water cutoff. They offer the convenience of not adding water manually. They will protect the boiler from a dry fire condition by maintaining a safe minimum water level in the boiler should a system leak occur. Using a mechanical automatic water feeder can keep your burner operating even during a power failure.
A mechanical feeder can also protect a boiler should a fuel-regulating device malfunction, causing the burner to lock in and stay there. A mechanical automatic water feeder will continue to feed the boiler whenever the level drops to the “feed” point. Under normal circumstances, the electrical low water cutoff (the second part of the feeder/cutoff combination) is always standing by, ready to shut off the burner should something go wrong with the automatic feeder. An automatic water feeder doesn’t feed at the two-thirds full point on the gauge glass. This is set by hand when the system is first started. The “normal” level will vary up and down as the system operates. An automatic feeder will maintain a safe minimum water line only. Doing this will lessen the possibility of human error.
Where to Mount a Low Water Cutoff
A low water cutoff sensor must be at or above the minimum safe water level for the boiler as specified by the boiler manufacturer. While float-type low water cutoffs must be installed on the boiler at an appropriate tapping or in conjunction with gauge glass, probe-type LWCOs can be installed either into a side tapping on the boiler or for some equipment. An electronic sensor probe-type LWCO can be mounted in a tee on the supply or return piping from and to the boiler.
Low Water Cutoff Maintenance
The leading cause of LWCO failure is lack of maintenance, incorrect maintenance, or improper modification. Depending on your boiler’s low water cutoff device, it may need to be blown down and tested daily or weekly. Aside from performing daily/weekly blowdown testing, low water cutoffs should be inspected annually to ensure safe and reliable operations of your boiler. Debris and sediment should be removed, and the LWCO should be replaced if worn or corroded. Having low water cutoff maintenance performed annually will help ensure that your boiler safety devices are functioning correctly.
Mechanical-float LWCOs are subject to jamming due to sludge that forms in the steam boiler as water is lost and mineral debris is left behind.
LWCOs by Manufacturer
Taco Low Water Cutoffs
Taco Low Water Cutoffs provide accurate and dependable protection for all boilers, from small residential hot water boilers to large industrial steam units. This probe-style Low Water Cutoff features patented DualVision Technology, external LEDs, simplified wiring for easy installation and testing, and automatic, manual reset mode and is available in 24VAC or 120VAC models. Taco Low Water Cutoffs can act as either the primary or secondary low water cutoff on low-pressure steam boilers, the primary limit control on hot water boilers, or to protect pumps from running dry in condensate receiver tank applications. It can also activate alarm systems or automatic water feeders.
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McDonnell & Miller Low Water Cutoffs
McDonnell & Miller offers a wide range of low-water cutoffs for hot water and steam boilers in electronic and float-type models. Additionally, they offer combination low water cutoffs/pump controllers. McDonnell & Miller Low Water cutoffs protect steam and hot water boilers from the hazards of a low water condition. They will interrupt the electrical current to the firing device if the water in the system drops below the boiler manufacturer’s minimum safe water level. They also provide an additional circuit for a low water alarm, if desired, for extra protection.
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Hydrolevel Low Water Cutoffs
Hydrolevel offers temperature limit/low water cut-off combinations, low water cut-offs for steam and hot water boilers, multi-purpose tank level controls, and condensate pump controller for residential and commercial hot water boilers.
