LARGER VIEW of a steam boiler low water cutoff controlHeating System Low Water Cutoff Safety Switches LWCOs
Installation, operation, location, repair of LWCOs

  • LOW WATER CUTOFF VALVE - CONTENTS: Guide to low water cutoff valves & safety switches. What is a low-water cutoff control? What is a LWCO and how does it work? Low Water Cutoff Valves: Guide to LWCOs on steam boilers. Low Water Cutoff Controls: Guide to LWCOs on hot water heating systems Troubleshooting & Repair Guide. How a low water cutoff control can avoid a boiler explosion or BLEVE.
  • POST a QUESTION or READ FAQs about diagnosing & repairing LWCOs low water cutoff switches & controls on heating boilers
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Boiler low water cutoff control service or repair: Here we explain Low Water Cutoff Controls: Guide to LWCOs on steam heating and hot water heating systems and we provide a low water cutoff switch Troubleshooting & Repair Guide.

LWCOs are installed on most steam heating boilers and also on many hydronic or "hot water" heating boilers as a safety device to shut down the boiler in the event of loss of water in the system.

This article series answers most questions about central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

Green links show where you are. © Copyright 2017, All Rights Reserved.

A Guide to Inspecting and Flushing Low Water Cutoff Valves on Steam Heating Boilers

Low water cutoff valve schematicWhat is a low water cutoff valve or LWCO?

In order to avoid a heating boiler explosion or damage from loss of water, the low water cutoff is a device intended to shut down unsafe heating equipment by turning off electrical power to the oil burner or gas burner should the water level or pressure in the heating system fall below a safe level.

The schematic at left, courtesy of Carson Dunlop, explains how the low water cutoff valve works.

[Click to enlarge any image]

In short, LWCOs are among multiple heating system safety controls that help prevent a boiler explosion or BLEVE

Low Water Cutoff - LWCO's on heating boilers: this steam heating boiler safety device contains a mechanical or electronic sensor to monitor water level in the boiler. LWCOs are installed on many modern residential hydronic (hot water) heating boilers and on virtually all steam boilers.

LWCOs are also installed on all commercial boilers of both types.

Low water cutoff valve on an older steam boiler (C) Daniel FriedmanLow water cutoff valves are installed on all steam boilers, most commercial heating boilers, and some home heating boilers (hydronic or hot water heating systems).

Original LWCO valve designs like the low water cutoff mounted on the face of the boiler shown at left used a mechanical float which operated not unlike the float arm in a toilet tank.

As water level drops the arm moves down and ultimately trips a mechanical switch that operates an electrical contact to turn the heating system off.

Watch out: 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.

Electronic LWCOs: Newer LWCO controls replace the mechanical float switch with a sensor, reducing the chances of a cutoff malfunction.

By monitoring boiler water level and turning off the oil or gas or electric heat source to the heating boiler should water level drop too low in the steam boiler, this important safety device prevents damage to the boiler should the system lose its water.


Sediment and Low Water Cutoff Valves - Flushing Needed: how to use the Steam Boiler Blowdown Valve

Double low water cutoff systemAs we explained above, mechanical-float LWCO's need to be flushed to remove sediment that could prevent the cutoff from working. In the photo at page top you can see that the owner has left a 5-gallon plastic bucket below the low water cutoff drain pipe. That's for flushing out the LWCO valve.

Watch out: if the low water cutoff valve is not flushed often enough, the residue of solid debris (minerals, rust, dirt, also referred to as "TDS" or total dissolved solids) can clog the low water cutoff valve so that it will stop working and could fail to sense an unsafe low-water condition in the heating boiler.

Low boiler water level can lead to costly boiler damage or even a BLEVE explosion (see BLEVE EXPLOSIONS). Ask your heating service technician how often your LWCO needs to be flushed.

In the photo at left you can see that the system has two low water cutoff valves installed, at two different levels on the steam boiler. Perhaps this setup was installed for an extra measure of safety?

Since steam heating systems are constantly using water, losing some of it as water vapor venting at steam radiators, and regaining water as the automatic (or on some systems manual) water feeder replaces water in the system, these systems tend to produce sediment at the boiler.

If sediment collects in the low water cutoff valve it could prevent the valve's internal float from falling as water level in the steam boiler drops, thus preventing the valve from safely shutting down the boiler should water level fall to an unsafe level. For this reason the low water cutoff valve needs to be flushed regularly, often once a week.

How to Use the Blowdown Valve - Steam Boiler Low Water Cutout Valve Flushing Procedure

Watch out: improper boiler LWCO low water cutoff valve purging or flushing can damage a boiler. Proper purging of the LWCO valve is done by slowly opening the valve's drain line, allowing the flush-water to drain into a bucket that will be dumped appropriately. As boiler experts note, purging too quickly can cause the LWCO float arm to jerk down suddenly, bending or damaging the float or its linkage. This risk is greater on high-pressure steam boilers than on low-pressure residential steam boilers.

When blowing down a control at pressure, the blow down valves should be opened slowly.The piping needs to be warmed up and stagant water in the drain piping needs to be pushed out. Suddenly opening a blow down valve causes steam to condense, which may create water hammer.

Damage to components can occur when water hammer occurs due to improper bl ow down piping. For these reasons, McDonnell & Miller recommends a dual valve blow down system for each control. Blow down the low water cut-off when the water level is at its normal level (pump/valve off) and the burner is on.  - McDonnell Miller (2015) cited below.

When we flush a steam boiler or a hydronic heating boiler low water cutoff valve we:

Safety warning: Be careful, when a steam boiler has been running, water coming out of the low water cutoff flush valve is hot and can scald a bystander.

Watch out: don't confuse water well low-water cutoff controls with heating system low water cutoff controls. The low water cutoff devices that we discuss here are intended for protecting the safe operation of building heating boilers or steam boilers. Heating low water cutoff devices (LWCO) are distinct from and have nothing to do with the well piping or building water supply cutoff safety devices discussed at WELL PIPING TAIL PIECE.

If you don't know what kind of heat your building uses, we explain how to figure out the answer at HEATING SYSTEM TYPES.

If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Question: How low should the water go when the low water shutoff works

2017/04/06 Tim Murphy said:

How low should the water go when the low water shutoff works ? The reading on the sight glass [would be where?]

This question was posted originally at SIGHT GLASS, STEAM BOILER

Reply: below the SAFE level marked on the boiler or sight glass

If the boiler or steam boiler water is anywhere BELOW the SAFE LEVEL marked at or behind the sight glass but still visible, that would be a safe LWCO level to shut off the boiler. That would be consistent with the instructions for LWCO's I've read.

The manufacturer marks a "safe" water level right on the boiler. That is in fact the point at which the LWCO valve should operate. On some LWCOs such as some McDonnell Miller models there is also a line marked right on the control body too.

Question: where should the LWCO be mounted?

Low water cutoff control locations on a hot water boiler (C) InspectApedia adapted from Hydrolevel Saftard 1150 instructions cited in this articleJohn said:

Does it matter on a modulating condensing boiler where the low water cutoff is located? - 2017/08/17

Illustration above showing acceptable locations for the low water cutoff on a hot water heating boiler, adapted from Hydrolevel's Safgard 1150 LWCO instructions cited below. [Click to enlarge any image]

Reply: the height or elevation or position of the low water cutoff sensor has to be correct


Watch out: Absolutely, yes, the height or elevation or position of the low water cutoff sensor has to be correct, but the sensor and control does not always have to be mounted right on the boiler itself.

While float type low water cutoffs must be installed on the boiler at an appropriate tapping or sometimes in conjunction with a gauge glass, probe-type LWCOs can be installed either into a side tapping on the boiler or for some equipment, the electronic sensor probe type LWCO can be mounted in a tee on the supply or return piping from and to the boiler.

Basically the LWCO sensor must be at or above the minimum safe water level for the boiler as specified by the boiler manufacturer.

Typically that line is also marked on the boiler itself - for steam boilers, but for a hot water or hydronic boiler you'd you may have to review the manufacturer's instructions. There is often an existing boiler side tapping reserved specifically for such a control.

Appropriate tee fittings can be installed on the supply or return piping for LWCOs to be mounted in those locations.

In some installations the device can work perfectly fine mounted on a tapping on the boiler OR on the water supply line or on its return line (at the right height).

The low water cutoff (LWCO) has a sensor - varying by the LWCO design - that has to be put into the boiler at a location that allows the sensor to detect the fall of boiler water below a "safe" level in the boiler.

On hot wter boilers using a Taco LTA-2 LWCO the control is installed as follows:

1. Install the probe above the minimum safe water level, as determined from the boiler manufacturer's literature. ... NOTE: This may be in a tapping on the boiler or in the boiler supply or return piping.

2. Install the probe to extend into the boiler cavity or piping to make contact with the water.

3. Install the probe so that the exposed portion of the stainless steel is a minimum of 1/4" from any grounding surface inside the boiler (to prevent the probe from shorting).

On hot water boilers using a Hydrolevel company's Safguard 1150 LWCO for hot water boilers are installed thus:

The Safgard 1150 must be installed at or above the minimum safe water level established by the boiler manufacturer. The 1150 can be installed directly into the boiler if a suitable tapping is available . The 1150 can also be installed in the boiler piping using a standard 3/4" tee.

On a Peerless hot water boiler such as the hydronic version of the Peerless Series 63/64™ Gas Boiler, the LWCO is installed as follows:

LWCO on a Peerless hot water heating boiler adapted from Peerless Series 63/64 TM Gas Boiler I&O Manual (C)

Low Water Cut-off Installation on a Hot Water Boiler (Level Control) [Click to enlarge any iamge]

a. A hot water boiler installed above radiator level or as required by the authority having jurisdiction must be provided with a low water cut-off device either as part of the boiler or at the time of installation.

b. If a probe-type low water cut-off is used it should be piped into the boiler supply piping as shown in Figure 6.8.

Peerless steam boilers require that the LWCO be installed as follows:

Low Water Cut-off (Level Control):

a. Float Type: Install the float type low water cutoff (if provided) in conjunction with the gauge glass as shown in Figure 6.3.

b. Probe Type: Install the optional probe type low water cut-off into the 3/4" tapping on the right side of the boiler. Refer to Figure 6.2 for tapping location. The supplied control incorporates a 60 second off cycle after a 15 minute on cycle. This
feature allows the water level in the boiler to settle so that the probe can sense a true water level.

LWCO Low Water Cutoff Valve Manuals & Instructions

McDonnell & Miller Probe type LWCO  PS-801 (C) InspectApedia.comIllustration: A McDonnell & Miller LWCO PS-801-802 851 series probe type low water cutoff control. [Click to enlarge any image]

Taco LFM and LTA LWCO controls as shown in the company's catalog & at Home Depot at


Continue reading at WATER FEEDER VALVE, STEAM or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BOILER CONTROLS & SWITCHES - for hot water heating systems

Or see STEAM BOILER FLOODING REPAIR where we explain the causes of water too high in the sight glass or in the steam boiler.




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