Guide 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.
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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.
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.
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
(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).
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
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?
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
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
at WATER FEEDER VALVE, 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 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).
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.
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The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
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 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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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