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

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 / FURNACE DIAGNOSIS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

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

What 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. In short, LWCOs are among multiple heating system safety controls that help prevent a boiler explosion or BLEVE (see BLEVE EXPLOSIONS).

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 valves are installed on all steam boilers, most commercial heating boilers, and some home heating boilers (hydronic or hot water heating systems).

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

Original LWCO valve designs 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.

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

As 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.

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 at Water Feeder Valves, Steam
  
  
• 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 Valves, 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.

Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: AQUASTAT CONTROL Functions and CAD CELL RELAY SWITCH (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.

Watch out: 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.

Questions & Answers regarding this article

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

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• InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
• InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
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• Additional technical contributors & reference sources for this article are listed below.
• Daniel Friedman - InspectAPedia^® Website Author/Editor
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

STEAM HEATING SYSTEMS
  AQUASTAT CONTROL Functions
  BACKDRAFTING HEATING EQUIPMENT
  BANGING HEATING PIPE NOISES
  CAD CELL RELAY SWITCH
  Check Valves Flow Control Valves Flo Control
  Chemical treatments, Boiler
  Circulator Pumps & Relays
  COMBUSTION GASES & PARTICLE HAZARDS
  CONDENSATE RETURN PIPES, PUMPS, STEAM
  DIAGNOSE & FIX BOILER
  DRAFT HOODS - gas fired
  DRAFT MEASUREMENT, CHIMNEYS & FLUES
  DRAFT REGULATORS, DAMPERS, BOOSTERS
  ELECTRIC MOTOR OVERLOAD RESET SWITCH
  ELECTRICAL POWER SWITCH FOR HEAT
  Expansion Tanks
  FIRE SAFETY CONTROLS
  Gauges on Heating Equipment
  Limit Switches, Boilers
  Low Water Cutoff Valves, Boilers
  MIXING / ANTI-SCALD VALVES
  Pressure & Temperature Settings, Controls
  Pressure Switch, Steam Boiler
  RELIEF VALVES - STEAM TP VALVES
  Sight Glass, Steam Boiler
  Spill Switches
  Stack Relay Switch
  Steam Pressure Gauge
  Steam Vents
  THERMOSTATS, HEATING / COOLING
  LOW WATER CUTOFF CONTROLS
  Water Feeder Valves, Steam

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
• Links to our list of additional information on heating system inspection, repair, maintenance
• ...