Boiler low water cutoff control service or repair FAQs.

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.

FAQs on Low Water Cutoff Valves on Steam Heating Boilers

These questions and answers were posted originally at LOW WATER CUTOFF VALVE - be sure to review the LWCO installation and repair advice given there.

The schematic at left, courtesy of Carson Dunlop Associates, 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 BLEVE EXPLOSIONS.

On 2018-10-29 by (mod) - the LWCO won't pull in the gas valve

Check for voltage

On 2018-10-29 by Rosanna Raziah Mohamed

on my low water cut out ,every thing is working perfect but on the controls 'B' is not pulling in the gas valve

On 2018-02-12 by (mod) - Fulton pulse PHW combustion boiler won't keep running - runs out of water

BI

It sounds as if you automatic water feeder is not feeding water into the boiler when it should. It may be debris-clogged. Have you been flushing the LWCO / Water Feeder valve?

What's the brand and model water feeder equipment installed?

Is the sensor placed at the right water level?

Is the sensor getting debris or scale blocked?

On 2018-02-09 by Bl - commercial Fulton pulse combustion boiler PHW 1000K false low-water indication

I'm working on a commercial Fulton pulse combustion boiler model phw1000k and it runs but from time to time I'd have to reset it due to low water and it's full as can be.

Probes have been replaced along with a tune up kit and relay.

Anyone have any suggestions? Thanks in advance

On 2018-01-22 by LC - sight glass filled with rust and sediment

i have a old Burnham water heater and it has been cleaned less than a month ago. I have been noticing that the water feeder has been filling water at least 7 times in 24 hours. I am not sure what is going on. I hadn't noticed this until recently.

The water level in the glass goes down extremely low until the water feeder fills again to less than 1/2?

the glass is filled with rust and sediment about 1/4 inch high and when i questioned the tech he said it was an old system.

The water heaterfeeder is UNI-MATCH Universal. Any suggestions. LC

On 2017-12-13 by (mod) - pump on our boiler takes off and runs continuously

Greg I can but guess at what's happening since I don't have a clear understanding of what heating system you are using. The presence of a sight glass means that you are talking about steam boiler. And that case I'm not sure what pump you refer to.

But sometimes a steam boiler also is connected to a loop of hot water radiator or baseboard piping for an area in a basement or low in a building. If that's what's happening the circulator May indeed be causing surging in the boiler.

On 2017-12-13 by Greg R

Sometimes the pump on our boiler takes off and runs continuously and the water rises in the glass tube. Why is this happening and how do we solve this problem

On 2017-11-08 by Mod- The low light is not working

Gerri, see the diagnosis and repair advice given

at LOW WATER CUTOFF VALVE NOT WORKING

On 2017-11-08 by Gerri - The low light is not working

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 2017-10-26 by (mod) - added water, new probe, LWCO still says "low water"

Todd

It would be helpful if we knew the brand and model of your control. But I agree if it's a sensor type low water cut off and you are absolutely certain that the water level is correct in the boiler and there is likely to be a problem with the probe or with connections to it

On 2017-10-26 by todd

my lwco says there low water i added water i cleaned the probe got a new probe still says low water i thank i my have to change the sensor but idk

On 2017-08-29 by Anonymous

How do you add water to the furnace sight gage empty

On 2017-08-17 by (mod) - instructions from the Taco LTA-2 LWCO for hot water boilers

John

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. In some installations the device can work perfectly fine mounted on a tapping on the boiler OR on the water 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. Typically that line is also marked on the boiler itself - for steam boilers, but for a hot water or hydronic boiler you'd have to review the manufacturer's instructions.

There is often an existing boiler side tapping reserved specifically for such a control.

Here are example instructions from the Taco LTA-2 LWCO for hot water boilers

Installation:

1. Install the probe above the minimum safe water level, as determined from the boiler manufacturer's literature. (See Fig. 1)

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 2017-08-17 14:08:12.510047 by John

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

On 2017-02-03 by janson

Las mant our boiler water tubes warp due to lost of water inside,cause is bec we focus/rely to wt transmitter indicates in d computer bt in fact inside the boilet is empty,now we want to install audible alarm for low water level alarm same with principle of LWCO,is it ok for us to install or put the float ball inside the steam drum?

On 2017-01-21 by (mod) - is it normal for the water level to go below the line in the sight glass?

Sue:

"Normal" is not quite the word I'd choose for seeing the water level disappear from the steam boiler sight glass every few days.

It is normal for a steam boiler to consume some water - as water is lost in the form of steam at the radiator steam vents at the start of each heat-on cycle.

But most steam boilers include an automatic water feeder valve that feeds make-up water into the boiler as needed. If your water-feeder valve is a manual one - not automatic - then you have to do that yourself every few days.

OR if it's an automatic valve the valve on your particular boiler may need cleaning or repair as it is not admitting water when it should.

Call your heating company to help get the water feeder working properly - otherwise you risk loss of heat, damage to the steam boiler, or in extreme cases (where several things all go wrong) an unsafe boiler.

On 2017-01-21 13:30:40.840590 by Sue

Is it normal for the manual water feeder on a gas furnace to go below the line every few days? my furnace level seems to go below the line and shut off and I have to fill it about every 3 days

On 2017-01-21 by (mod) - water overfills sight glass and doesn't stop

Gllenn

Your water feed valve is malfunctioning and needs to be serviced, repaired or replaced.

On 2016-11-06 by Gllenn

Water fills glass tude and doesn't shut off. Why?

On 2015-11-29 by Jan

The LWCO was installed new at least 5 years ago and we had no problems as part of the total boiler replacement. This LWCO is cylindrical and is labeled as Self-Cleaning. It is electrically connected to the damper.

The location hasn't changed. The service tech replaced the thermostat. Yesterday, for the first time I noticed the heat was not maintaining at thermostat setting overnight. When I investigated the boiler, I noticed the red light on the LWCO.

I increased the thermostat setting another 5 degrees and the boiler kicked on, the red light went off for about a minute. Then the red light came back on and the boiler kicked off.

Just a side question on the thermostat... there are two switches inside that can be turned on or off depending on the system. If mine is a hot water boiler run off natural gas using radiators throughout the house, should the thermostat settings apply to "hot water (3CPH)" OR "Gas (5 CPH)" ? The tech had it set at gas.

On 2015-11-29 by (mod) - clogged screen in LWCO or inadequate cleaning

Jan often problems with a LWCO are traced to a clogged screen in the device or inadequate cleaning;

If, however, the problem you report has been present since the control was installed "new" I suspect a level adjustment or location problem. Has your heating service tech taken a look at the control?

On 2015-11-29 by Jan

I have a self-cleaning LWCO on a Smith hot water boiler. LWCO red light shows often and cuts off heat even with thermostat calls for more. At least 5 years after new install and without prior problems, replaced Honeywell round thermostat 2x and still have heat cut out with LWCO showing red. Auto water in. Any ideas?

Comment: use pressure to detect loss of boiler water?

(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.

Reply:

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.

It's not "water pressure" but rather the water level in the boiler that determines when the LWCO operates to shut down the boiler.

Please see details at LOW WATER CUTOFF VALVE, BOILER

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.

Question: the LWCO is dripping - how do I stop the leak?

(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.

Reply:

Dave,

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.

John,

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.

Question: should we install a low water cutoff valve?

(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?

Reply:

Al Mo

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.

Question: what should the steam boiler pressure be

(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?

Reply: about 0.5 psi

Andy on residential low pressure steam systems the pressure is very very low, usually around 0.5 psi. See details

at STEAM BOILER PRESSURE 

Question: Mcdonnell & Miller Uni-match water feeder and series 67 LWCO diagnosis

(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.

Reply:

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.

Question: no water came out of the LWCO when I tried to flush it

(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.

Reply:

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.

Question: LWCO keeps cycling through a 90 second self-check and shuts off the burner

(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.

Reply:

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.

Comment: Found the problem with the McDonnell #67 LWCO

(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.

Question: Safgard model 170, LWCO - boiler overflowed

(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.

Reply:

The valve needs maintenance, repair or replacement.

Question: INsurance company requires me to install 2 LWCO's on my commecial steam boiler

(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?

Reply:

John

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.

Question:

2015/10/26 deborah said:
low watter box buzzing then stopped

Reply:

Deborah: did you try flushing the valve per the homeowner routine?

...

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

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