LARGER VIEW of a steam boiler low water cutoff controlGuide to Heating System Low Water Cutoff Safety Switches - 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

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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 2015, 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

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

  • Place a 5-gallon bucket under the end of the flush valve drain pipe, being careful not to bang into piping and maybe cause a leak

  • Open and shut the flush valve several times, opening it briefly - just a few seconds, then closing it each time. This avoids a large surge of cold water entering (and possibly damaging) a hot steam boiler, and it also seems to help stir up and remove sediment from within the low water cutoff valve.

  • Watch for clear water: After flushing the LWCO valve several times, when we see that clear water is coming out of the boiler, the job has been done. But notice: often if you hold the flush valve open longer than just a few seconds, you may see clear water coming from the boiler.

  • Check for clear flush-water once more: But if you close the flush valve and open it again you may see new brown sludge water coming out of the valve again! So when we think we've cleaned out the low water cutoff valve by this flush procedure, we close the valve, wait about 15 or 20 seconds, and then try it once more. If we see that the first water coming out of the valve on this last try is clear too, then we figure we've flushed out the valve successfully.

  • If an automatic water feed valve is installed: you'll hear new water flowing into the boiler just after each time you open and flush the LWCO valve. That's another reason to do this flush procedure gradually - so we don't damage the boiler by filling it with lots of cold water when the boiler is itself very hot. We discuss automatic water feeder valves for steam boilers

  • If only a manual water makeup feed valve is installed: you'll need to add water to the steam boiler, filling it back to the "full" line marked on or behind the sight glass. We discuss manual water feeder valves for steam boilers
    at WATER FEEDER VALVE, STEAM. It's best to add water to the steam boiler slowly to reduce the chances of cracking (more of a risk with cast iron boilers).

  • Usually the total volume of water flushed is 2-4 gallons.

  • Dump the flushed boiler water down a building drain or into another approved outdoor area where some rust and sludge won't contaminate anything, but to not dump hot water into a cold sink or toilet - it may crack.

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.


Continue reading at WATER FEEDER VALVE, STEAM or select a topic from the More Reading links shown below.

Suggested citation for this web page

LOW WATER CUTOFF VALVE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about diagnosing & repairing LWCOs low water cutoff switches & controls on heating boilers

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References