Why Boiler Sight Glass Full Of Water [5 Easy Solutions]

A few issues can cause the boiler sight glass full of water such as dirt in the boiler, priming or foaming, the flame issue, narrow sections, and water feed trouble. 

The steam boiler sight glass works for the house owner or maintenance person to monitor the required water level, in such case the boiler water level may get too high.

If it gets full of water constantly it will certainly be damaged. It could lead to a dangerous explosion or fire. Therefore it needs urgent repair or replacement. 

Why Boiler Sight Glass Full Of Water [5 Easy Solutions]

The operation of a sight glass requires the presence of water. Fortunately, rather than being replaced, many sight glasses may be cleaned and troubleshot.

You will find five simple problem solutions in this article, and they are not difficult to implement. 

Note: You can also read the Utica boiler troubleshooting guide.

1. Dirt In The Boiler

Steam bubbles rise to the top and break loose with ease when the water is clean. However, steam has a harder time escaping unclean water.

The volume of the steam bubbles grows as it rises to the surface. As they attempt to breach the surface of the unclean water, these bigger bubbles cause the water line to bounce, which may cause boiler sight glass full of water.

Once you know how to drain water from sight glass it will be easier to solve the issue. 


To begin, double-check that your top and bottom sensor valves are both working and capable of shutting completely. Without a proper seal, cleaning is pointless and sometimes injurious.

Then, under your sight glass, establish a temporary connection to the drain valve. This line will safely remove extra water from your sight glass while also injecting ammonia into it.

Carefully but completely shut the differential pressure valve under your sight glass, then close the indicator valve above it. Slowly open your sight glass’s drain valve, then completely open it to release all of the water.

To be sure most of the water has been blown out of the sight glass, open the top indicator valve slightly and let the steam pressure blow out any remaining water. All that is left in your sight glass right now is mist.

2. Priming Or Foaming

The strong splashing of the water is known as priming. When bubbles develop on the water’s surface, foaming occurs. Priming is followed by foaming, which is far worse than priming.

Foaming forces water into the system, resulting in water hammers, uneven heating, and water level issues in the boiler.

You can tell whether the boiler water is foaming by taking a sample and boiling it on the stove. When doing this test, use a thin container to lower the water level in the boiler. 


Examine the ph of water if there is foaming. The pH level should preferably be either seven or nine.

Foaming is caused by a high pH (high alkaline) and is one of the most prevalent reasons for boiler troubles.

If the boiler is just storming, the water is most likely filthy. Clean the boiler according to the manufacturer’s instructions.

3. Overheating Issue

This is a typical issue with oil and natural gas burners. The flame must have the appropriate size and form for the space.

If it isn’t, it will overheat a section of the boiler, causing steam to develop more rapidly, which may cause the full of water in the sight glass. 


The water on the opposite side of the boiler will decrease to compensate for the steam bubbles racing up one side of the boiler. The sight glass will show this rising movement.

Check the fire rate and flame pattern, and make any required adjustments. To solve this issue, you may need to add chamber material.

4. Narrow Sections Of The Boiler

With home steam boilers, this is an issue that arises from time to time. The increasing steam bubbles will raise the boiler water level over the level in the indicator glass if the portions are too small.

This is due to the lack of steam in the gauge glass’s water. Until the burner turns off on the pressure troll’s cut-out setting, the two columns of water sit at separate levels.


The water in the boiler drops below the water in the sight glass as the steam bubbles dry.

Water dropping into the boiler from the sight glass compensates for this. The low-water cutoff or automated water feeder is normally activated by this.

In this case, you end up with a boiler that has constant water level problems.

This issue is inherent in boiler design, and dirt exacerbates it significantly. Underfiring the boiler might sometimes help to fix it.

As a result, there are fewer steam bubbles, giving each one more space to breathe. However, do not boil the water to the point of simply simmering it full of water in the sight glass.

5. Water Feed Valve

While a faulty control may need replacement, most steam boiler LWCO and water feed issues may be traced down to a clogged strainer or screen in the device, or filth that has blocked the float. This might be the cause of your water full issue. 


Check the maintenance directions in the water feed valve’s instructions. It is likely that it is blocked with dust.

The two 67 switches are supplied by neutral from the burner’s opposite side, while terminal 2 is fed by hot power regulated by the thermostat.

ITT UNI- MATCH water feeder terminal 2 neutral straight from the transformer, terminal 3 hot power from terminal 2 ON LWCO, do it yourself or locate a specialist who may help you in case of difficulties.

You may be changing the sight glass on the boiler to have an alternative to this sight glass. A surge column looks just like a sight glass, except it’s made of pipe, not glass.

You build it from two tees, a few nipples, and a short length of steel pipe, which you’ll place between the bulls of the two tees. Hook up your low-water cut-off.


How much water should be in the sight glass on a boiler?

Using the sight glass, check the water level. Within one inch of the middle, the water level should be. Pull the lever on the water supply valve and watch the water pour through the sight glass. You may shut the feed valve by pushing the handle up once the meter has reached the desired level.

What is the ideal fill level for a steam boiler sight glass?

The water level in a boiler sight glass should be at least half full, but not more than three-quarters full. On certain boilers, the drain valve on the boiler must be opened every two weeks to enable the silt on the bottom of the water tank to drain.

Why does the water in the gauge glass rise and fall so dramatically?

Surging occurs when filth and oil collect on the surface of the boiler water. The water is lifted as the steam seeks to break out, causing the surge. This is seen in the gauge glass. Because steam systems are exposed to the elements, they must be cleaned on a regular basis.

Is it possible to operate a boiler without a sight glass?

Yes, Dennis, the steam boiler may operate without a sight glass if the top and lower valves are closed and there is no leakage.

What was the right volume of water in the boiler when you filled it up?

The NOWL (normal operational water level) should be in the center of the gauge glass. The water level must be determined in two ways in all steam boilers. The gauge glass is the first and most straightforward method.


You will need protective gear to keep your face, arms, hands, and chest safe from excessive heat and any accidental mist when cleaning or solving issues in your sight glass.

If you have any further questions, leave a comment. We will recommend the easiest option.

About William

William is the founder of Fireplacehubs.com. He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

2 thoughts on “Why Boiler Sight Glass Full Of Water [5 Easy Solutions]”

  1. Hello. The water level in the sight glass rise during the summer months when the heat is off. I changed all the fill valves so I know they don’t leak. But the water rises slowly in the sight glass. What could cause this?

    • Hi Mark

      Generally, the boiler sight glass full of water due to one or more reasons like dirt in the boiler, priming or foaming, the flame issue, narrow sections, and water feed trouble.

      Well, as you have changed all the fill valves, now check these other issues & try to find out the main culprit behind the problem. Then, fix it following our solution guide in the article.

      Hopefully, you will get the solution.

      Wish you all the best.


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