Diagnose & fix a steam or hydronic boiler low water cutoff control LWCO that's not working properly.

A LWCO that does not respond to low boiler water level is unsafe and needs repair.

In this article series 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.

Low Water Cutoff Valve not working: what to do

These questions and answers were posted originally

at LOW WATER CUTOFF VALVE - be sure to review the LWCO installation and troubleshooting advice given there.

Photo: a steam boiler sight glass showing no water on a boiler protected by Hydrolevel's CycleGard LWCO Model 450, courtesy of an InspectApedia.com reader.

Below we discuss what to do when the LWCO does not shut off the boiler in a low-water condition.

Watch out: do not allow a boiler to continue to run if it is too low or has run out of water. LWCOs are among multiple heating system safety controls that help prevent a boiler BLEVE EXPLOSION.

Bottom line: If the LWCO is not responding properly to the water level in your boiler, the most common things that a service technician would check are

1. Is the LWCO being properly maintained ?
   
   Is the LWCO being flushed at least weekly? If not there's risk that accumulated sludge and rust debris clog the float assembly and the low water cutoff valve w2ill stop working.
   
   While it may be possible to disassemble the control and use a screwdriver to loosen the sludge and then clean the control, most steam heat service technicians will simply install a new one - faster, less time with no heat, and possibly less costly when you consider the time and labor to clean the old unit.
2. What is the condition of the LWCO's sensor probe?
   
   It's common for sensor probes to become scale covered to the point that the sensor no longer responds properly to the boiler water level. The sensor probe can be cleaned or replaced.

Details and more LWCO diagnosis and repair suggestions are below.

 

Steam Boiler Water level Too Low or not visible in the sight glass / steam boiler doesn’t cut off

Our reader asked:

My steam boiler doesn’t cut off when the water level falls to a point where I can not see it. The light doesn’t come on.

My steam boiler is a manual feed unit with a Cycle Guard low water cut off middle CG450 - 2019/02/10 Stafford

Reply: What to do when the HydroLevel SafGard CG450 LWCO does not shut off the boiler at low water condition - Clean or replace the probe

Stafford,

Watch out: Thank you for asking Stafford, as you've touched on a very important safety feature designed to avoid boiler damage, destruction, or even more-dangerous conditions.

It sounds like indeed your CycleGard Low water cutoff LWCO is not working, perhaps needing sensor cleaning or, less likely, replacement of the control.

If the your SafGard LWCO used to work properly then I'm guessing that it was properly installed - the probe must be installed at or above the minimum safe water level in the boiler - so more likely there is a wiring error or the probe is scale coated or damaged and should be replaced.

Periodic maintenance is needed to keep this LWCO working properly. I quote from the company:

To insure optimum performance, remove and inspect the probe annually.

Clean any sediment or scale from the probe using a scouring pad or steel wool.

Note: Do not use caustic chemicals that could damage the probe.

Reinstall the probe and perform the Operating Test Procedure described [in the product installation sheet]..

Ask your heating service technician to perform that operation and let me know the result.

If you don't already have it, here's the manual for your

• Hydrolevel Safgard 550/650/750 SERIES 120VAC LOW WATER CUTOFF MANUAL (LWCO) [PDF] (at Hydrolevel website) 2025/12/06 - local backup copy saved as Safgard-550-650-750-LWCO-Manual.pdf

CycleGard LWCO Operation & LWCO Intermittent Level Test Feature

The CycleGard LWCO should cut off the boiler 15 seconds after the boiler water level falls to below the LWCO sensor probe (that you can't see, it's behind the blue CycleGard control box and is inserted through a tapping into the steam boiler).

Using the CG400 as an example (your 450 has similar instructions for which we provide a link below) here is how the company describes its normal operation

Burner circuit contacts open after 15 second delay in a low water condition.

Delay prevents short cycling caused by momentary fluctuations in the boiler water level.

Automatically reactivates burner circuit 30 seconds after water reaches the probe, allowing optional water feeder to raise water level above the probe.

Intermittent Level Test (ILT) feature provides maximum boiler protection by removing power from the burner circuit at set intervals.

Models ending in “1090” perform the ILT every 10 minutes for 90 seconds.

Models ending in “2060” perform the ILT every 20 minutes for 60 seconds.

- Source: Hydrolevel Safgard™ 400 Series Steam LWCO [PDF], Hydrolevel Company, 126 Bailey Road, North Haven, CT 06473 USA, Tel: (203) 776-0473 Website: www.hydrolevel.com Retrieved 2017/04/06, original source: http://hydrolevel.com/safgard-400-series-steam/

Repair for a CycleGard LWCO that Does Not Detect Low Water in the Boiler - Amber Light vs Green Light

The fact that the LWCO indicating light doesn't turn on also suggests that it's not sensing the water drop.

Am I correct that when you add water you do see water enter the sight glass? So we think the sight glass is accurately showing boiler water level and isn't blocked itself?

OK so what to do: Here are verbatim instructions from the CG400 Instruction Sheet.

1. Check to see if a foaming condition in the boiler is preventing the control from accurately detecting the water level.

With power to the system on, and water above the probe level, set the thermostat so there is no call for heat and allow the burner to shut down. Then, slowly lower the water in the boiler to a point below the probe.

The amber LED lamp should turn on and the relay will open after a 15 second delay.

Once this has occurred, reestablish a call for heat and the boiler should not fire.

Now raise the water level to an operating level.

The amber light should turn off and the control should reenergize the burner circuit after a 30 second delay.

If the control functions normally during this test, but allows water to drop below the level of the probe when the test is conducted with the burner on (as described in the Operating Test Procedure above), the boiler is foaming.

The CycleGard provides protection for foaming boilers [see "CycleGard LWCO Operation & LWCO Intermittent Level Test Feature " above on this page].

However, to provide optimal steaming performance, it is still recommended that you clean the boiler in accordance with the manufacturer's instructions.

The foaming condition is caused by machining oils, elevated mineral content from make-up water, grease and other contaminants in the boiler water.

Cleaning the boiler will eliminate foaming conditions in the boiler and provide for optimal steaming performance.

See also BOILER CLEANER CHEMICALS / TREATMENTS

2. Check terminal block wiring to insure that all connections are correct.

3. Check the probe installation to insure that there is 1/4" clearance from any surface within the boiler or pipe. (Refer to Step 3 on page 1 of this instruction sheet.

If the Hydrolevel CycleGard 400 Amber Light is On

The amber LED lamp indicates that the water is below the probe.

If the gauge glass shows that the water is at the correct operating level and the amber LED is lit check the following:

1. Check for plugged gauge glass.

2. Make sure probe lead wire is properly secured to the terminal.

3. Check for proper ground between probe and boiler shell. Excessive use of Teflon tape or sealing compound may isolate the probe from the boiler shell.

4. Remove probe and examine for oily residue. Clean probe with steel wool and skim boiler.

If the Hydrolevel CycleGard 400 GREEN LED Light is on

The green LED lamp indicates that the control is conducting an Intermittent Level Test.

The burner does not fire during the test period. See Intermittent Level Test Feature on this page for more details.

See the complete instructions for your HydroGard™ CG450 at

Hydrolevel Safgard™ 450 Series Steam LWCO [PDF], retrieved 2017/04/06, original source: http://hydrolevel.com/safgard-400-series-steam/

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

What does the green light mean on my low water cutoff?

Green light is on - tell me please what that means - On 2023-02-08 by anthony zambrana

I drained water from Burnham steam boiler to replace leaking glass tube. After replacing glass tube with new gasket, now green light on mcdonnell low water shut of does not stay on as before repair, please hepl. tks mike b

Answer by InspectApedia Publisher - GREEN LED lamp indicates that the control is conducting an Intermittent Level Test OR system is operating normally. RED = low water

@anthony zambrana,

In General on LWCO controls, The green LED lamp indicates that the control is conducting an Intermittent Level Test OR that the LWCO is in normal operation.

For controls using an intermittent level test, the burner will not fire during the test period. See Intermittent Level Test Feature on this page for more details.

If the LWCO light never goes off, youu may need to flush the low water cutoff to remove debris.

Here is a specific example: from the Xylem McDonnel & Miller LWCO manual cited below:

The probe is a manual reset low water cut-off control. The control unit has a red LED light to alert personnel to a low water condition and a green LED to indicate normal operation.

When a low water condition occurs, the burner turns off and the red LED begins to blink.

When the water level is not restored to a level above the probe within 60 seconds, the control locks out.

Press the reset button when the water level is restored to a level above the probe.
- source: Xylem McDonnell & Miller PSE-800-M Low Water Cut-off INSTRUCTION MANUAL [PDF] (2019) at Xylem, local backup copy saved as Xylem-McDonnell-Miller-PSE-800-Manual.pdf

The low light is not working on my low water control

The low light is not working, I flushed out the furnace and the low light never came back on after the levels were low enough to sense it.

I filled the furnace again to the fill line. The furnace is not coming on. Yes, I checked the thermostat, yes they have new batteries, yes the pilot light is on. But the furnace is not coming on. Could it be the low light switch or sensor that is effecting the whole system? Please advise. On 2017-11-08 by Gerri -

by Mod- The low light is not working

Gerri, see the diagnosis and repair advice given above on this page

at LOW WATER CUTOFF VALVE NOT WORKING

Leaky Watts SAN89D low water cutoff: will parts from a newer control fit to do a repair?

I have a Watts 89d low water cut off that is leaking inside the switch from tapping on it

Are there any other LWCO's that will fit? these old 89d lwco's are hard to find and very expensive Thanks, Mike On 2022-04-19 by Mike

Answer by Inspectapedia Com Moderator : no

@Mike,

I'm so sorry but I don't know of an alternate-brand part that will fit inside of a Watts 89D LWCO for sure, but you might check with Watts about their San89 float and switch assembly to see if that can work.

Quoting:

Series SAN89 Float and Switch Assemblies for Servicing Low Water Cut-Offs feature a one piece unit that facilitates installation and ensures the most up-to-date construction.

Model SAN89D is a complete float and dual switch assembly.

Model SAN89S is a complete float and single switch assembly. Maximum Working Pressure: 15psi (103 kPa).

WATTS SAN89 LWCO INSTALLATION INSTRUCTIONS [PDF] (2008) Watts USA: 815 Chestnut St., No. Andover, MA 01845-6098; www.watts.com
or Watts Canada: 5435 North Service Rd., Burlington, ONT. L7L 5H7; www.wattscanada.ca

My Burnham Steam boiler is flooding because the LWCO won't stop: how do I fix it?

I have a Burnham Boiler that by looking at the sight glass all the way full and excessive water feeder run time leads me to believe my boiler is flooded/flooding?

My McDonnell PS-892-24 LWCO does the cut off and triggers the United match WFE-24 water feeder. But the water feeder seems to run all the way till the sight glass fills up and then some.

There’s no click shut off sound just as if it comes to a slow running water stop sound. Could the lwco not be satisfied?

The probe is set at approx 2/5 of the sight glass. Once it passes that 2/5 mark the lwco red light goes away but the feeder keeps going. Could the probe need cleaning? On 2021-10-19 by Robert

by inspectapedia.com.moderator

@Robert,

It's just a guess but it sounds to me as if you are water feeder control or float is not working properly. Open that's corrected by cleaning and Flushing the controls otherwise it's time for control replacement.

by Robert

@inspectapedia.com.moderator, I thought if I have a probe I don’t have a float in the tank. This also seems to be what happened last year. As I went to flush it for this winter coming the sight glass was full. It’s not good to have it that much full and pressurized right?

I also can’t seem to find my boiler safe fill line mark. So if cleaning the probe doesn’t fix it then it’s most likely the water feeder not working?

by inspectapedia.com.moderator - Could the probe need cleaning?

@Robert

sorry you're quite right; I should have focused first on cleaning or descaling;

Still, the valve does have to have moving parts to open and close a water supply - electrically - in response to the probe's finding the absence of water

The probe is itself an electrical sensor; if de-scaling it doesn't fix it, we ought to be able to first test (electrically, using a VOM or DMM) the water feeder (at the probe contacts) to distinguish between a probe that is itself defective vs. a water feeder that's not responding properly to the probe's signal.

 

When the boiler calls for water I lose all my steam pressure

Just hooked up a Utica steam boiler for steaming crabs when the boiler calls for water I lose all my steam pressure is there any way to keep my pressure or get get it back quickly. On 2020-03-24 by Terry

by danjoefriedman (mod) -

Pretty interesting use, Terry - I'd need to know more about what you've done to your boiler.

IN general a residential steam boiler operates at very low PSI - around 0.2 to 0.5 psi.

At those low pressures, if you are trying to jet your crabs to steam those fellows (I imagine they hate it), it makes sense that you'll lose all steam pressure in the boiler very quickly.

But I do NOT suggest fooling with the boiler's pressure settings as doing so can make the heating system unsafe.

Watch out about adding frequent shots of very cold water to a hot steam boiler: you can crack the boiler.

 

Thank you to our readers for their generous comments

Good through answer I commend you my friend - On 2019-10-30 by Anonymous

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Continue reading at LOW WATER CUTOFF VALVE, BOILER or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articlesoilers

• BOILER OPERATING PROBLEMS
• BOILER WATER FEEDER MANUALS
• LOW WATER CUTOFF VALVE, BOILER
• LOW WATER CUTOFF VALVE NOT WORKING
• SIGHT GLASS, STEAM BOILER
• STEAM BOILER FLOODING / SURGING REPAIR
• STEAM BOILER LOW WATER CUTOFF LWCO MANUALS
• STEAM HEATING SYSTEMS & CONTROLS - home
• STEAM BOILER WATER FEEDER MANUALS
• WATER FEEDER VALVE, STEAM

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