Hot water (hydronic) boilers rely on water to safely transfer heat. When water levels drop too low, the boiler can overheat and suffer serious damage. This guide explains how a Low Water Cut Off (LWCO) protects your system.
Why a Low Water Cutoff (LWCO) Is Needed
A boiler operating with insufficient water can overheat very quickly. Unlike comfort-related issues, low-water conditions create serious equipment and safety risks. While boilers are designed to circulate hot water to heat a space, that same water also protects the boiler itself by absorbing heat and preventing metal components from overheating or burning.
When water levels drop too low, internal temperatures can rise rapidly, leading to anything from nuisance shutdowns to major mechanical failure. In extreme cases, continued operation without adequate water can trigger pressure relief valve discharge, crack cast iron or steel heat exchangers, or cause catastrophic damage. Documented incidents—including a 2013 daycare boiler explosion in Minnesota—highlight just how dangerous uncontrolled low-water conditions can be.
- Dry firing: The boiler fires with little or no water present.
- Cracked heat exchangers: Cast iron and steel sections fracture from overheating.
- Circulator pump damage: Pumps overheat, seize, or fail prematurely.
- System lockout: Modern controls shut down the system to prevent further damage.
These risks are why low water cutoff devices are increasingly considered essential safety controls in residential and light commercial hot water boiler systems. An LWCO acts as a last line of defense, shutting the boiler down before low-water conditions can escalate into costly or dangerous failures.
What Is a Low Water Cut Off (LWCO)?
Many people overcomplicate the concept of a Low Water Cut Off (LWCO). To make it clearer, we often expand the name to Low Water Cut Off Safety Switch (LWCOS). Adding the word "switch" helps explain its purpose: the LWCOS is a smart safety device that interrupts power to the boiler, preventing it from firing if water levels fall below a safe threshold.
Most hot water boilers operate with both line voltage (120V AC) and control or "low" voltage (24V AC). Understanding the system architecture is important. Some manufacturers recommend wiring the LWCO to shut power to the aquastat, which stops all boiler components from running. Others wire it to interrupt the 24V gas valve circuit, preventing the burner from firing while leaving other controls powered. Our preferred method is to cut line voltage entirely, which simplifies wiring for experienced HVAC technicians while maintaining safety.
Although wiring the LWCO itself is straightforward, installing it is more involved. The process requires draining the boiler, installing the switch, refilling to the proper water level, and purging any trapped air to ensure safe operation.
LWCO to Aquastat (120V)
This schematic shows the LWCO interrupting line voltage to the aquastat, stopping all boiler components if water is low.
LWCO to Ignition Module (24V)
This schematic shows the LWCO wired to the 24V gas valve circuit, preventing the burner from firing while other controls remain active.
Installing and wiring a LWCO is a critical safety step in any hot water boiler system. Proper placement ensures the boiler will not fire under low-water conditions, protecting the system, components, and anyone around the equipment.
How LWCO Works in Hot Water Boilers
In hot water boilers, there are 2 main technologies for LWCO devices typically use probe-type or float-style sensors. These sensors monitor water conditions indirectly. Often asked which technolgy is better? And there is no way to definitevely answer that question but here are some pros and cons of each technology.
| Feature / Concern | Probe-Type LWCO | Float-Style LWCO |
|---|---|---|
| Typical Device Example |
|
|
| Sensitivity to water quality | ❌ High | ✅ Low |
| Mechanical wear points | ✅ Minimal | ❌ Moderate |
| Tolerance to debris / sediment | ❌ Needs clean water | ✅ Better tolerance |
| Ease of manual testing | ❌ Harder | ✅ Very easy |
| False trips risk | ❌ Higher with conductivity swings | ✅ Lower (mechanical) |
| Ease of installation | ✅ Simple wiring | ❌ Chamber & plumbing |
| Maintenance interval | ❌ Need probe cleaning | ❌ Need float chamber cleaning |
| Best for low conductivity water | ❌ | ✅ |
| Best for cost-sensitive installs | ✅ | ❌ Generally pricier |
Practical real-world conditions matter more than theory. Water quality—especially scale, hardness, and mineral content—plays a major role in how low water cutoffs perform. Probe-type LWCOs can accumulate scale over time, which insulates the probe and leads to nuisance shutdowns or false low-water readings. While it’s often said that float-style LWCOs are unaffected by water conductivity, they are not immune to problems either. Sludge, rust, and debris can cause floats to stick or bind, which is why regular inspection and maintenance are critical for both designs.
Testing and failure diagnosis also differ by type. Float-style LWCOs are typically checked by manually lifting the float arm, while probe-type units often require more technical testing involving conductivity checks or simulated water conditions. Although these methods may sound simple, they usually require opening the water circuit and partially draining the boiler.
Our method is even simpler. This is strictly a diagnostic step performed by licensed technicians, we temporarily bypass the LWCOS. Since it is essentially a switch, testing involves momentarily jumping two wires to confirm whether the control is the cause of the shutdown. This is NOT a DIY procedure so do not try it at home and must be performed with extreme caution, as it involves live electrical components and critical safety controls.
Common failure modes include scale-coated probes causing false trips, loose wiring, or probe corrosion. Float-style LWCOs most often fail due to debris jamming the float, water-logged floats that no longer rise, or corroded pivots that prevent smooth movement. Because low water cutoffs are critical safety devices, once a failure is confirmed, full replacement is typically recommended rather than repair.
Is a Low Water Cutoff (LWCO) Required on Hot Water Boilers?
Many older hot water boilers were installed without a low water cutoff (LWCO), largely because earlier codes did not require them or because the technology was not widely adopted at the time. Today, however, modern safety standards in New Jersey increasingly require—or strongly recommend—LWCO protection on both residential and commercial boiler systems.
In many situations, an LWCO is required by code. The regulations referenced below are not suggestions—they are enforceable safety laws. Even in cases where a specific installation may fall under a limited exception, installing an LWCO is still strongly recommended. From a practical standpoint, adding this safety device helps protect the boiler, maintains manufacturer warranty coverage, and reduces the risk of insurance claim disputes. In short, installing an LWCO is a straightforward and sensible safety decision.
The absence of an LWCO on older homes does not mean the system is safe. Previous HVAC contractors may not have been required to install one, may not have had access to modern devices, or may have followed outdated standards. That does not change the risks associated with low-water conditions. Retrofitting an LWCO is an effective way to protect the boiler, the property, and the people inside the home.
Residential Boilers (New Jersey Residential Code / IRC M2002.4):
Most residential hot water boilers in one- and two-family dwellings require a low water cutoff unless they meet
the forced-circulation exception outlined in the International Residential Code (IRC), which is adopted by most
U.S. jurisdictions, including New Jersey.
Commercial and Larger Boilers (N.J.A.C. 12:90-4.5):
Automatically fired hot water boilers with a heat input exceeding 400,000 BTU per hour must be equipped with an
automatic low water cutoff specifically designed for hot water service.
- Most new boiler installations in New Jersey include LWCO devices to meet current safety standards.
- Retrofitting older boilers with an LWCO significantly improves safety and code compliance and is strongly recommended—especially in older homes with aging piping that may be more susceptible to corrosion or leaks.
- Many insurance providers expect modern safety controls to be present and may require them to maintain coverage.
Whether required by code or not, a low water cutoff is widely regarded as a critical safety upgrade for any hot water boiler system.
Always verify requirements with your local Authority Having Jurisdiction (AHJ).
Common Reasons a Low Water Cutoff (LWCO) Trips
When a low water cutoff switch trips, there’s no need to panic. In most cases, this means the device is doing exactly what it was designed to do—protect the boiler from operating under unsafe low-water conditions. You may try resetting the LWCO once, and only once. If the system restarts, continue to monitor it closely.
If the LWCO trips again or the boiler shuts down repeatedly, it typically indicates either a problem with the switch itself or an underlying system issue that needs attention. Common causes include:
- Slow system leaks: Small, unnoticed leaks can gradually lower the boiler’s water level.
- Air bleeding issues: Trapped air can displace water and cause false low-water readings.
- Faulty expansion tank: Loss of air charge or internal failure can affect system pressure and water volume.
- Improper boiler draining: Incomplete refilling after maintenance can leave the system underfilled.
Repeated LWCO trips should never be ignored. Continued shutdowns are a sign that the boiler is protecting itself from potential damage. Having a licensed technician perform a professional boiler safety inspection can identify the root cause early and help prevent costly repairs or unsafe operating conditions.
FAQ – Low Water Cut Off
- Can I bypass an LWCO? No. It is a critical safety device.
- Does adding water reset it? Sometimes, but the cause must be addressed.
- How often should it be tested? During routine boiler maintenance.