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.
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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.
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.
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.
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:
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.
2017/04/06 Tim Murphy said:
How low should the water go when the low water shutoff works ? The reading on the sight glass [would be where?]
This question was posted originally at SIGHT GLASS, STEAM BOILER
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.
Does it matter on a modulating condensing boiler where the low water cutoff is located? - 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]
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) [Click to enlarge any iamge]
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.
Continue reading at WATER FEEDER VALVE, STEAM or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see BOILER CONTROLS & SWITCHES - for hot water heating systems
Or see STEAM BOILER FLOODING REPAIR where we explain the causes of water too high in the sight glass or in the steam boiler.
Or see STEAM HEATING SYSTEMS & CONTROLS - home
Or see WATER FEEDER VALVE, STEAM
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(July 11, 2011) Steve said:
Hydronic hot water heat systems that have their output as baseboard heat, or as radiant heat, or are forcued air (hydronic-air) with their water-to-air heat exchanger in attics or on floor above the boiler in the basement all will have a lot of water above the level of the boiler.
That mans that a lot of water will need to leak out before these water level valves will activate. Would it not be better to sense the loss of pressure from such a leak ? You mention this "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" but do not describe pressure switches.
Good point, Steve. The LWCO has the job of protecting the boiler from damage, not protecting the building from leaks nor the piping system and baseboards from leaking out.
In a slow leak in upper building piping, the automagic water feeder keeps pushing makeup water into the system - the leak will run until someone discovers it.
In a fast leak in upper building piping people are going to see it if the building is occupied, or they'll notice that they no longer have heat - before the boiler is at risk.
In a failure of an automatic water feeder to keep water in the system or a fast leak right at a boiler, the LWCO is, as I suggested, focused on protecting the boiler from a meltdown or worse.
Hydronic (hot water) and steam heating systems, especially hot water systems, operate with pressure varying quite a bit from cold to hot - so pressure sensing coudl be trouble. Furthermore, low water in a boiler needs to be detected early to avoid damaging the boiler. Systems typically have several safety controls. The LOWCO doesn't work alone.
(Aug 31, 2011) Dave Lindsay said:
We just bought a house in upstate NY and have a Weil McLain with a low water cutoff valve very much like the one shown in the photo above. It was dripping when we moved into the house, but not very much. Then we lost power for three days with the hurricane. When the power came back on, the bucket started filling up fast -- today it filled completely in about an hour. We are complete newcomers to homeowning. Is this amount of water "normal" after torrential rains and a power outage? Should we be resetting the boiler? Any help is appreciated.
(Dec 13, 2012) John Barkosky said:
If the drain on the LWCO (McD & Miller 149700) drips a drop every 30 seconds or so, what might the problem be? A plumber gas been called, but I'm curious. A plumber HAS been called. Sorry for the typo.
the Low water cutoff (LWCO) should never drip except perhaps a few drops right after it has been flushed; dripping means (presuming you have an automagic water feeder) that you are continually adding more water to the system, wasting water and perhaps increasing the rate of debris and mineral deposit accumulation in the boiler.
I suspect that you lost power during Hurricane Irene and that resulted in a drop of water pressure in the system, perhaps leading to later dripping at the LWCO.
If you open and close the LWCO valve (over a bucket to collect water) four or five times you may be able to flush the valve seat and stop the dripping. If not, you should call a heating service tech.
Sometimes you can flush the drain valve on the LWCO several times, letting the valve "snap" back to the closed position, to stop a drip; if that doesn't work then the problem probably requires a gasket or vavle part replacement.
(Sept 1, 2011) Al Mo said:
We are about to purchase a condominium where the propane fired boiler (McLain)is on the second floor. The home inspection report recommended the installation of a low water cut off valve. The plumber feels that it is not required in residential installations. What is your take on it?
Depending on your local building codes (which your plumber probably knows better than a home inspector would) you may not be required to have a LWCO on a residential heating boiler. Required.
The home inspector is giving advice for improved safety and improved protection of the heating system. If the inspector included that as a safety recommendation it's quite sound. If s/he said it was "required" s/he may not be correct.
In at least some communities and among some hydronic boiler manufacturers a low water cutoff is being included in new installations as it's benefits are recognized: it's protecting the boiler from damage should an automatic water feeder fail to work properly or (in the case of still older systems) should the home owner forget to keep an eye on and maintain the water level in a steam boiler.
It's not just theory. Unlike a hydronic (hot water) heating system, a steam boiler consumes some water every time the burner runs and boiler cycles. The rate of water consumption varies among systems and buildings, but it's never zero.
In conditions at which there could be a loss of water supply to the building, I'd consider the LWCO an absolute necessity to protect the boiler.
(Oct 4, 2011) Andy said:
I have recently purchased a home with steam heat. Previous homes had Forced Air Gas heat systems. When the boiler turns on, the water pressure gauge registers 0. What should the pressure be?
Andy on residential low pressure steam systems the pressure is very very low, usually around 0.5 psi. On this page at Continue reading we provide an INDEX to RELATED ARTICLES that includes a live link to - STEAM HEATING SYSTEMS titled
Pressure Switch, Steam Boiler
and perhaps also
Pressure & Temperature Settings, Controls
(Oct 23, 2011) Charles said:
My boiler have Mcdonnell & Miller Uni-match water feeder and series 67 LWCO. This morning the water fail to stop feeding the boiler. Now, I have to cut off the power to the Uni-match feeder. Otherwise, it will flood the boiler. Is the problem with LWCO? Please advise. Thanks.
Charles, the low water cutoff (LWCO) is designed to be flushed, typically once a week, by the homeowner. If your LWCO was not being flushed, sediment can collect in the system leading to controls jamming.
If you are not familiar with the flushout procedure for the controls, I'd immediately call a heating service company and ask them to send a steam heat repair tech to your home. S/he will clean or repair the controls as needed and at the same time s/he can explain to you what you need to be doing to prevent the problem from happening again.
(Oct 29, 2011) Steve said:
I went to turn on heat for first time this year but noticed my LWCO was rusty, and no water came out when I went to draw. The water level looks to be at proper level in the viewer but am still concerned. I turned the heat on and have been closely monitoring the boiler and saw that the water level in the viewer went all the way to top. I will be having the LWCO fixed by a professional but in the meantime I do not know what warning signs to look for if there is a serious impending problem.
(Nov 1, 2011) margaret Hager said:
Good article. Question: if the automatic refill is broken will the low-water shut-off still turn burner off if water runs too low? Thanks.
The boiler will runif the LWCO is clogged and draining, or it might. BUt
Watch out: this is a dangerous condition as the safety control intended to sense loss of water and then shut off the boiler is no longer working. I would shut off the system and call for repair.
(Nov 11, 2012) Darren said:
I have a steam bolier with an auto feed and LWCO. Every few minutes the LWCO performs a self check that lasts for 90 seconds. During the self check, the the LWCO shuts the burner down. Is it normal for it to check itself this often? It's been this way since it was installed in 2006, but lately I've been thinking that something might be wrong. Thanks.
This sounds like a control defect to me: it's time for a service call.
You could first try going through the homeowner's normal LWCO flush-out routine to see if that works.
Also if your unit produces error codes check the code in your specific LWCO installation and maintenance manual.
(Dec 20, 2012) Bill said:
The self check is probably checking for any more return water...to prevent flooding (typical problem with newer smaller steam boilers). You may want to see if you can extend this "check" time or even eliminate it, and then keep eye on site glass for too high of a water line.
(Dec 27, 2012) Emily said:
I think I have identified my problem on my LWCO but want to make sure. I removed the mechanical head 6667 on my Mcdonnell #67 and it sounds as if there is metal in the copper rod. Is there supposed to be a sound? I don't want to order new if that is a normal sound and might not be the problem. Thanks.
(Dec 28, 2012) james said:
My Safgard model 170, LWCO is installed between the boiler (above the boiler) and the non bladder expansion tank hanging from the basement ceiling. Recently, the boiler relief valve overflowed. I drained the expansion tank twice.
Overflow situation was not corrected.
With the second draining, no water came out from the tank. I measure the temperature with a multi meter on the piping before and after the LWCO. There is a 6 degree Celsius difference.
What from the LWCO is blocking the water flow? Could it be reset without replaced?
Thanks. Happy New Year to all.
The valve needs maintenance, repair or replacement.
(Apr 20, 2015) john said:
an insurance company told me I need 2 low water cut offs on my commercial steam boiler? are there extra tappings on my weil mc clain boiler?
I don't know - as we don't know what boiler you've got installed. Take a look at the boiler itself and also at the installation and operation manual - or give us the model and brand.
2015/10/26 deborah said:
low watter box buzzing then stopped
Deborah: did you try flushing the valve per the homeowner routine?
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