Heating System Low Water Cutoff Safety Switches LWCOs

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 Associates, explains how the low water cutoff valve works.

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: (shown above) 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.

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

How to Use the Blowdown Valve

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:

• 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).


• Usually the total volume of water flushed i
  
  s 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.

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

Our reader asaked:

How low should the water go when the low water shutoff works ? The reading on the sight glass [would be where?] 2017/04/06 Tim Murphy

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.

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

boiler BLEVE EXPLOSIONS.

For boilers using an automatic water feeder valve, the most-common cause of low boiler water level is failure to properly clean and maintain the water feeder valve, and the most-common cause of failure of the LWCO safety control to function is the same: failure to clean and flush the valve as specified by the manufacturer.

At LOW WATER CUTOFF VALVE NOT WORKING we describe the steps recommended by Hydrolevel if that company's CycleGard LWCO is not responding properly to low water levels in the boiler.

Where should the LWCO be mounted?

Another InspectApedia reader asked:

Does it matter on a modulating condensing boiler where the low water cutoff is located? -John 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

John

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:

Low Water Cut-off Installation on a Hot Water Boiler (Level Control)

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.

Source: PEERLESS Series 63/64 Gas Boilers INSTALLATION & OPERATION MANUAL [PDF] retrieved 2017/08/17, original source: www.peerlessboilers.com

Should I Install a Low Water Cutoff / Water Feeder on my Hydronic (Hot Water) Heat Boiler?

Summarizing: example of LWCO & Water Feeder on a Hydronic (Hot Water) Boiler

For other readers: the pressure/reducer/water-feeder used on residential boilers does not really regulate the boiler pressure except insofar as the feeder will add water to the boiler if the boiler pressure falls below 12 psi (or other settings if the device has been adjusted or modified).

At the other end of boiler pressure, on a residential boiler the temperature/pressure relief valve will open and spill water (and pressure) if boiler pressure exceeds 30 psi.

So together those two devices manage the high end and low end pressures of the boiler.

The combination of starting water pressure in the cold boiler and the temperature to which the boiler rises are the main determinants of the actual operating pressure in the system - normally ranging between about 12 psi and under 30 psi. It's the aquastat that controls boiler temperature.

To be clear, in the U.S. ASME does not require that a LWCO (low water cutoff) float type control be installed on residential hydronic (water not steam) boilers; the International Mechanical Code (IMC) does want to see a LWCO on all boilers, both steam and hydronic. You can find LWCOs and water feed devices from various manufacturers including McDonnell & Miller.

The illustration below is adapted from

• McDonnell & Miller's HEATING BOILER BASIC OPERATION [PDF] McDonnell & Miller, retrieved 2019/11/06 original source: controlcomponents.com.au/Open-File/557/Boiler%20control%20basics.pdf
  
  can also be downloaded free from InspectApedia.com
• WATER FEEDER VALVE, STEAM illustrates the float controlled valve on a STEAM boiler - less common on residential boilers.

In North America and many other countries a residential hydronic boiler's water supply is controlled simply by an automagic boiler feed valve , and where required an additional backflow preventer to protect the water supply system piping - such as this Watts boiler feed valve (below)

Around our location in Mexico the same solar water heaters as your Costa Rica unit are very widely used.

Watch out: a water pressure-reducer feeder valve is NOT a heating boiler safety device and does not protect against loss of water from the boiler nor from improper boiler pressure conditions caused by other factors.

The ONLY job of an automagic water feeder is to set the initial "cold" pressure in the boiler. That's why some codes and some inspectors want a separate LWCO even on residential hydronic boilers, not just on steam boilers.

Is it correct to keep our hydronic boiler pressure up to 15-18 psi to get heat upstairs?

Our reader Star said and asked:

Hey, excellent resource - thank you for all your hard work and long hours compiling it for our benefit!

We recently purchased a former commercial building that we are going to renovate and occupy as our home.

It comes with a 1970's hot-water heating system with a boiler (see attached photo). The makeup water valve has always been kept shut off, and the pressure in the 'closed' system drops by about 1lb every couple of days.

The residential zone is two storeys (2 zones to this system) and I've been keeping the pressure between 15psi and 18psi to push against the 12' of head pressure to feed the upstairs radiators.

My questions: does that sound correct so far?

And, regarding the pressure regulator just after the makeup water valve - I was told that it's "faulty" and won't control the temperature [sic - she meant "pressure"] properly (hence the makeup water cannot be left turned on, I have to bring it up to 18psi and then shut it off again).

Do you think if I adjust the pressure regulator that I might get it to do what it is supposed to?

There's obviously a small leak somewhere - and the system has been gone over thoroughly before we purchased looking for leaks (found none that they would admit to ;-) ). Any input would be appreciated, thanks.

We'd like to be able to go away for a couple of months in the winter time but would have to hire someone to come in and tweak the pressure once a week or so as the system stands now. - On 2019-11-06 by Star

As for: "won't regulate the temperature properly" - I meant to type 'pressure' not 'temperature' and missed it on the proof-read! Got the gist.

I repaired a leak in my radiator system on my Jeep Grand Cherokee (1998 5.9 Limited) with White Pepper - made a poultice and spooned it in - lasted for 60,000 miles+ until I sold it!

inspectapedia.com is such an excellent resource - 5 outta 5 on Trip Advisor ok?! Lol, too bad.

So a float-controlled water feed valve is in my future - I have one of those on the Chinese Solar Water Heater that I installed on my roof in Costa Rica :-) Thanks so much, really enjoying this!

Reply by (mod) -

Star

Well as you know there are "leak stop" products that can be added to a boiler to flow to and "stop" small leaks - though some experts worry that those products can foul other controls on the hydronic system.

(My friend Paul Galow (Poughkeepsie NY) poured oatmeal into his car radiator to stop a hot water leak there).

RE: does that sound correct so far?-

I'm not sure which "that" sounds correct but in general losing water isn't generally OK for the reasons we've both cited: convenience and safety and reliability of the system;

If you refer to leaving the makeup water valve OFF - you can do that on a system that's being inspected and manually-fed often enough to keep it safe - but that's not acceptable for most residential heating systems as most homeowners simply won't do it or don't want to or don't know how.

For tall buildings you might need to boost the cold or starting boiler water pressure

a bit above 12 - which will be OK only if the pressure when the boiler is hot remains low enough so as not to dump the TPR valve. (That would also be unsafe).

If you mean "do you think I can get the pressure regulator to do what it's supposed to" - I answered that: yes by servicing it or if that doesn't work, replacing it.

Finally: for a typical two story residential hot water heating system you leave the controls as factory set at 12 psi cold.

Are your pressure calculations for pressure necessary to get hot water heat to the second floor of your building taking into account your elevation above sea level?

In general 1 psi will lift water 2.31 feet. The author's of B&G's nice book

• HOW HYDRONIC [HEATING] SYSTEM COMPONENTS REALLY WORK [PDF] point out on p. 8 that for a boiler located in the basement of a 2 story building you need to lift water from the boiler up a total of about 19 feet.

(19ft. total lift height) / 2.31 lift per ft = 8.23 psi. So 12 psi or even your 10 psi of cold boiler pressure would be fine.

Watch out: There are additional safety concerns with setting the boiler pressure too low including a risk, in a particularly hot boiler, of hot water flashing to steam at the top of the heating system - a dangerous condition as well as one that can make for strange noises in the heating system. So go with the factory settings.

You can get into trouble fooling around trying to fine-tune the starting pressure setting by adjusting the regulator (yours doesn't work anyhow) because when you observe or measure the system pressure you're thrown off by the actual temperature in the system - unless you let the system cool down completely.

Finally: "float operated" water feeders are used on steam boilers and can be installed on residential hydronic boilers too but are not common. Most people find that the pressure/reducer/regulator factory set at 12 psi will keep water in the boiler just fine for decades until that valve clogs up.

Followup by Star - how much pressure to get heating water to the second floor

As a retired JM Boilermaker/JM Boilermaker Welder (36 years) I'm still learning!

This is small stuff compared to what I'm used to in my trade (ie 10" water feed piping, 72" gate valves, etc) but it's all working under the same principles of course. Back to my own personal house boiler.

Regarding the leak - don't believe I'll find it so I'm going to carry on as is.

The tradesmen searching for the leak previously removed nearly all the insulation from the piping which is very unfortunate - and there is evidence of scale and deposits buildup at multiple locations, but nothing wet. The town we live in switched to a full R/O system about 10 years ago so the scale issue is done at least (particularly for build-up inside the pressure regulator on the screen).

From the calculations in my original comment (below, originally posted at PRESSURE REDUCING VALVE WATER FEEDER INSTALLATION) (multiply feet of rise by 0.43 and add 3#)

I have determined that I should only need about 10# pressure to feed the 2nd floor radiators. I'm going to let the pressure drop now until the radiators are cold on the 2nd floor and I'll know where the "sweet" spot is for certain.

Any danger in this? Is a vacuum created when the pressure drops too low?

You said: "While the manufacturers are careful to say that the pressure/reducer/regulator should not be used as an automatic water feeder, that's what almost everyone relies on in residential hot water heating systems."

This page states: "a properly-installed float-operated water feed valve should all be installed." I take it to mean this is the proper alternative. I have been discussing the installation of such a device with my local plumber.

Thanks again for you time and trouble - please take a moment to answer my two questions above.

by (mod) - If you can't find leak in hot water heating boiler ... now what?

Star

I have added some parts labels on your original photo.

I see a gas fired hydronic (hot water) heating boiler with at least 2 circulator pumps, and with what looks like asbestos lagging coating the boiler top and sides (if so, hazardous, do not disturb it). I think that earlier in its life this boiler may have been oil or even coal fired.

You are right that a hydronic heating system should not be consuming water; there is a leak somewhere; often a small boiler leak seeps out on a hot component of the boiler, piping, or radiator/baseboard where the water evaporates; therefore you may not find a puddle or obvious leak point.

However there will in such cases always be a build-up of crud and corrosion and deposits of salts left behind as the water evaporates, so a careful and thorough inspection can find such leaks unless the leak is where piping or components are in an inaccessible area such as a wall or ceiling cavity.

While the manufacturers are careful to say that the pressure/reducer/regulator should not be used as an automatic water feeder, that's what almost everyone relies on in residential hot water heating systems.

Sometimes the regulator can be cleaned - there's a screen that clogs - to get it working again; most techs don't bother, they just replace it. Your system is, in a subtle way, unsafe if its water feed system isn't working since when no one is around the system loses water and can thus be damaged or could overheat.

Your comments are on a page for BUILDING water pressure regulators.

To read about the most-common type of pressure-reducer / regulator used on hot water heating systems please see

PRESSURE REDUCING VALVE WATER FEEDER INSTALLATION

Choose the Correct LWCO to work with a McDonnell & Miller water feeder

Our InspectApedia reader asked:

I have a McDonnell LWCO and its a gas fired unit (Burnham), what auto water feeder should I get?

Should I get the McDonnell to match the LWCO?

I installed the McDowell & Miller water feeder, and the unit is getting power, but won't automatically feed the water when the LWCO is active.

I know there is supposed to be a small delay, but even after 5 minutes, the unit is not feeding the water. The water feeder does have power, and can be manually fed if I push the button.

Seems like the LWCO is not giving the signal. I have tried all the different settings on the water feeder, without luck. Any ideas? - Evan 11/28/12

Is there a support number for the mcdonnell & miller water feeders? - Evan 12/15/12

Reply:

Sure, Evan. You probably had a little trouble finding the McDonnell & Miller website because of a series of name changes, additions, and modifications.

The McDonnell & Miller (now a Xylem brand) WFE Uni-Match water feeder controls varies depending on the low water cutoff LWCO Model that is wired to work with the Water Feeder valve

In the instruction manual for these water feeders you will find a table of wiring diagrams for each WFE - LWCO combination so you'll see that the water feeder will work with a variety of LWCO controls.

The water LWCO manuals that I've reviewed usually have wiring details for just about every major brand of water feed control.

STEAM BOILER WATER FEEDER VALVE MANUALS lists a range of brands and models and gives instructions as well as wiring and compatability with various LWCOS in each IO manual.

McDonnell & Miller, ITT McDonnell & Miller, or McDonnell & Miller a Xylem Brand can be contacted at the company's website page dedicated to McDonnell & Miller heating system controls:

http://unitedstates.xylemappliedwater.com/brands/mcdonnell-miller/ - general products

http://unitedstates.xylemappliedwater.com/brands/mcdonnell-miller/low-water-cut-offs/ - low water cutoff products

At the web link we give above you can find product literature about these and other McDonnell Miller controls & products.

But good luck finding a technical support telephone number directly at McDonnell Miller's new website. Instead the company is going to direct you to a local sales and service provider - that may be your best bet.

If you get somewhere better calling the company at the numbers we give below let us know as that result will assist other readers too.

The McDonnell Miller corporate headquarters is now at XYLEM Global Headquarters, 1133 Westchester Avenue, White Plains, NY 10604, Tel: +1-914-323-5700, Fax: +1-914-323-5800

LWCO Low Water Cutoff Valve Manuals & Instructions

Moved to STEAM BOILER CONTROL MANUALS