Steam boiler Sight Glass Smith Boiler Model 28A © Daniel FriedmanSteam Boiler Sight Glass FAQs
Use or fix the steam boiler sight glass

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Steam boiler sight glass: this article describes how to find & use the sight glass (or sight gauge) on a steam heating boiler to check or set steam boiler water level.

We explain where the sight glass will be found, how to read its water level, and the relationship between the sight glass reading and the procedure for adding make-up water to the boiler.

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Steam Boiler Sight Glass Use & Repair FAQs

Steam boiler sight glass explainedThe steam boiler sight glass or water level gauge allows the building owner or maintenance person to monitor the required water level in the steam boiler. A mark on the boiler body indicates the desired normal water level and a sight glass shows the current water level.

Recently-posted questions & answers about the sight glass on steam boilers

On 2017-04-23 by (mod) re: What causes high water level readings in the site glass.


Thanks for the question. Please see my very detailed reply now found at


On 2017-04-23 by Anonymous

What causes high water level readings in the site glass.

On 2017-04-22 by (mod) re: Is there water in the steam pipes when the boiler is not running


No, not if your heating system is steam heat: heat rises in the form of steam passing through the supply piping and as the steam cools and condenses back to water that condensate reuturns to the boiler.

When the boiler is OFF the condensate should have returned and

the only pipes that would contain water would be the condensate near the boiler's water level (Search InspectApedia for HARTFORD LOOP to see how that condensate line is arranged)

If you have a leak at a flange connection then you should be able to replace the gasket.

yes if your heating system is a hot water (hydronic) heating boiler and radiator or baseboard system.

On 2017-04-22 by Steve

If there is a dresser coupling that is dripping can I just replace the o ring
Is there water in the pipes when the boiler is not running

On 2017-04-06 by (mod) re: How low should the water go when the low water shutoff works

Darn good question, Tim.

I think that if 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.

Be sure to take a look at your low water cutoff control - LWCO - Or see them described at LOW WATER CUTOFF VALVE -

On 2017-04-06 by Tim Murphy

How low should the water go when the low water shutoff works
The reading on the sight glass

On 2017-03-16 by (mod) - what are the knobs for at the top and bottom of the sight glass?

Nice question, Anon.

The two knobs, one at the top and one at the bottom of the sight glass or water level gauge on a steam boiler are there to let you turn OFF the water from being able to enter the sight glass.

You'd close both of these knobs (they're actually small water shut-off valves) by turning them fully clockwise (for most valves but you may need to test yours) if, for example, your 10 year old, playing in the basement, kicks and breaks the sight glass - causing hot water to pour into the basement and making a dangerous, flooding mess.

By closing the sight glass valves you could then keep the boiler operating and keep heat while you wait for your heating service technician to show up to replace the sight glass.

And often the design of the whole assembly means that the sight glass can be replaced without having to shut down and drain the boiler - for the same reason: we can close the valves to temporarily isolate the sight glass from the boiler.

Watch out
: Normally we leave the valves open so that we can check that the steam boiler has the correct (and safe) water level.

On 2017-03-16 by Anonymous

what are the two knobs on the water level gauge for?

and On 2016-12-14 by Anonymous

Should the top and the low valves located on the sight glass stay open?

On 2017-01-02 by (mod)


Your steam boiler will almost certainly have a low water cutoff valve that is designed to be flushed frequently, usually weekly, for safety.

That will remove water. But if your boiler is over-filling and IF there is an automatic water feed valve installed, that valve may need adjustment or repair.

On 2017-01-02 by Marty

sight glass is full how do I let water out?

On 2016-12-22 by (mod) re: sight glass was overfull, LWCO shut down the system ?

I don't think that removing the sight glass is a good plan nor is it necessary. That's not the place from which to drain sludge and debris from the boiler. I would use the LWCO valve drain and flush until the water runs clean. You may need to do that several times.

I'm unclear about why the relief valve opened. That would represent a different and unsafe condition of over pressure or over-temperature in the boiler.

On 2016-12-21 by Sue

I'm sorry, I wasn't clear. The water in the glass was over-full, and a spitting stream of water was coming out of 2 of our upstairs radiators. Around the same time, the pressure valve popped on the boiler and the boiler room filled with steam.. so I turned everything off and drained the system until nothing else would come out.

Then I refilled the system to the line. True, I probably didn't wait for the water to drain back into the system from the radiators.

That could've accounted for it being overfull. But that doesn't explain why an hour later the LWCO valve turned on and shut down the system. I couldn't even see the level in the sight glass, so it was definitely too high! So I drained it all again, and the water was just as sludgey this time as the last.

I think there's a ton of sediment in the system (upon refilling, globs of brown water swished up into the glass). Is there a way to remove the drain/ spigot thingee so I can clear it out?

I've removed the glass before but it's higher than where the sludge has probably settled in the boiler. I'm terrified of breaking it off (and then being without heat in the 20 degree weather we've been having here.) Is there some kind of glue on the threads? It looks gunky and white. If there's another article I could read that could help, that would be great.

On 2016-12-21 by (mod) re: draining water at the boiler won't fix spitting steam vents


I'm not clear how draining a boiler would fix a "spitting valve" in an upstairs floor. More likely you were seeing steam or water leaking out of a steam or hot water radiator at a valve or vent.

Your description sounds as if no one has been flushing the low water cutoff LWCO valve. Take a look at the maintenance instructions for yours. Typically it's a weekly-task to prevent valve clogging - that can be unsafe.

Also, IF your steam heating system's condensate return piping is clogged or partly clogged, or steam traps at radiators are clogged with rust and crud, then the return of condensate to the boiler could be slowed. In that case, filling the boiler while there's condensate still in upstairs piping means that later as that condensate returns the boiler is over filled.

And you are also right to question the automagic water feeder. If it's not working properly it too could be the culprit.

On 2016-12-21 by Sue

So I just drained our system because it was acting funny, spitting water out of the valves, creating a mess in our dining room. Then I manually refilled the system (we don't have an auto-refiller) until the water level in the glass was perfect.

When I went down to check it again after an hour, the glass was over full (couldn't see the line at all). So I drained some water out of the system (it was dark brown and gross).. only to have the low water shutoff light go on. How can that be? I still can't see the water line, even though I do see a tiny bit of sediment floating in the glass, so isn't it OVER full?

On 2016-12-11 by (mod) - is it safe to run the boiler with a broken sight glass

You can run your boiler safety, Rick, provided you make sure that the water level in the boiler is close to what is required.

Finding that out can be difficult without the sight glass. In the worst case, if water comes out of the sight glass valve when the Lower Side Glass valve is opened and we know we have the bare minimum volume of water in the boiler.

At the same time if water comes out from the top of the sight glass fitting when the upper site glass valve is open then you've overfilled the boiler.

If you can't keep water in the boiler without it leaking because of the broken sight glass then you can safely run the boiler.

Watch out: Take care of that if you run the boiler with too little water it could be damaged, destroyed, or dangerous.

On 2016-12-10 by Rick

I just went to clean the sight glass on my steam boiler and the tip of it broke.

I tried to pick one up at Home depot, Loews and sears with no luck.

My system does have an automatic water feeder, is it safe to run the boiler until I"m able to get a replacement?

On 2016-12-08 by Anonymous


The problem could be clogged steam traps but the most-common problem, assuming the valves to each radiator are fully open, is non-functioning steam vents.
Search InspectApedia for steam vents to read details.

Every radiator has a steam vent - see

Steam traps are a separate device - see

On 2016-12-08 by Bruce Hevner

I have just bought a large old house (80 maybe?) that has (two pipe) steam heat. None of the radiators seem to be heating completely.

Only about 1/3 of the radiator gets hot. I am "assuming" that the steam traps could be dirty/faulty?

I don't see any traps that I can identify located on the piping coming out of the radiators. Do ALL radiators have traps? Could a trap be located IN the radiator?

On 2016-11-06 by (mod) - sight glass shows too much water in boiler

Glenn, your automatic water feeder needs to be cleaned and perhaps repaired to avoid over-filling the boiler.

Temporarily in an emergency to keep heat working one can on most steam boilers, turn off the water supply into the automatic water feeder and manually feed water into the boiler using a bypass piping loop that passes around the automatic water feeder - IF your boiler has that feature.

Take care to avoid shocking and cracking the boiler by sending in too much too cold water too fast.

Watch out: For safety it'd be best to have a trained professional repair or replace the water feeder asap.


On 2016-11-06 8 by Glenn

Glenn (no email)

COMMENT:Glass sight tube,fills with water from automatic feeder.what do i do to stop feeder from putting to much water into boiler?

On 2016-10-25 by (mod) - why does the sight glass water level drop when the steam boiler starts?


When your steam boiler starts ("Furnaces" are hot air heating machine) if water disappears from the sight glass I suspect the boiler is surging. It is not normal and may not be safe to see no water in the sight glass.

I'd ask for an evaluation from a trained steam heat service technician as running a steam boiler with too little water can damage the boiler and can be unsafe. Also ask if there is a low water cutoff valve installed and be sure that safety device is working.

On 2016-10-25 by Anonymous

Sight glass empties when furnace starts

On 2016-10-11 by Thomas

When the boiler was running for about an hour I noticed the water gauge was about empty. Is this because all the water was in the form of steam? Is this dangerous?

On 2016-09-28 by (mod) - air bubbles in the sight glass may mean low water or boiler surging

Low water or surging in the boiler;

On 2016-09-20 by Colin

What would cause air bubbles in the glass making it difficult to get an accurate reading of the water level. Thanks

On 2016-08-01 by (mod) - how do I regulate the water level in the sight glass?


If your boiler has an automatic water feeder it or its control needs cleaning and adjustment;
If your boiler is fed manually, someone over fed it, or perhaps it was fed water before condensate from floors above had returned to the boiler.

If the water level is still visible in the sight glass and only slightly above the indicator line on the boiler, say an inch or less, cleaning and water feeder may still need adjustment but it doesn't sound like the water level is dangerously high;

If the top of the water column is higher or is out of sight entirely then you should call for service and repair; that will involve both draining some water from the boiler and fixing its water feeder.

On 2016-08-01 by David

Sorry. Let me rephrase. The water level in the sight glass is above the index marker. Is there a way to regulate the water level so the water level can go back down. I am getting more water pushing through the system then before.

The sight glass on the boiler is above the water index line. Is there a way to regulate this. I believe my boiler is over filling

On 2016-02-10 by (mod) - how do I stop the sight glass from filling from the steam side

Dave I don't understand the question.

Normally the sight gauge shows the water level in the boiler. Its bottom is open to boiler water and its top open to the air or steam above the water level in the boiler. Water enters at the bottom sight gauge port.

Watch out: If your gauge is filling from the top then the boiler is over-filled and possibly also the sight gauge bottom port is manually closed or is clogged. Or the automatic water feed valve is not mounted properly or is not working properly.

On 2016-02-10 by Dave

How do I stop the gauge glass from filling into steam side

On 2015-12-21 by (mod) - how do I flush out the sight glass and valves?


Use the brand and model of your low water cutoff valve to get the proper disassembly and cleaning instructions for your valve.

It may be something a homeowner could do but there are risks including scalding burns, having to drain down the boiler if the valve can't be isolated by shutoffs, and on old equipment there's a risk that you take the valve apart but can't reassemble it because you need replacement gaskets or o-rings.

So consult the manual and consider asking your heating company to do the job.

On 2015-12-21 by will wallace

on a weil-mclain 68 steam boiler about 20 years old, i have always periodically "flushed" a few gallons of dirty water out of the system using blowdown valve (i think that is what it is called -- you can see it attached to black structure (yellow handle) to right of sight glass in picture above.

now the water barely comes out, so it must be clogged. is this something I can clean out myself?

On 2015-11-21 by (mod) - why does water level in the sight glass keep going up?

Check for a failed water feed valve or if your boiler includes a tankless coil, it could be leaking into the boiler;

CHeck also for a problem with delayed return of condensate to the boiler.

On 2015-11-21 by Derrick

Water level keeps rising when not running

On 2015-10-26 by Anonymous

What are the red valves on top and bottom of the sight glass? Should they be closed or opened? Thank you!

On 2015-10-19 by gil

My water continues to drain daily and I must fill it about 3 times a day... and it makes a noise. What is wrong with my boiler, I have a utica team boiler

On 2015-10-07 by (mod)

Most-likely, yes - look for the index mark on the boiler that indicates the proper fill level.

On 2015-10-03 by If the glass tube is filled to the top, is boiler over-filled?

if the glass tube appears to be filled to top, is boiler over-full?

On 2015-09-01 by (mod) - do I use the knobs on the sight glass to re-fill the tube?

Mae: the little handles or valves on the top or bottom of the sight glass itself are not there to "re-fill" the tube. They are there to allow you to keep boiler water out of the sight glass (tube) in the event that the tube is cracked or broken, thus avoiding draining the boiler.

The sight glass indicates the level of water in the boiler itself.

If water is low in the sight glass one must either open a manual boiler water feed valve (find the water pipe bringing water to your boiler and follow it to the valve0
one must repair an automatic water feeder valve that is not working.

WATCH OUT: do not leave a heating boiler turned on or running if it is low on water. The boiler could be damaged and the system is UNSAFE.

On 2015-09-01 by Mae

I have tried to refill the glass tube with water, but have had no luck. I turn the red faucet handle and nothing happens. Am I doing it the right way?

On 2015-02-22 by (mod) - sight glass was empty, we added water, now there's too much water in the steam boiler

Ask your heating company about adding an automatic water feeder valve and a low water cutoff valve if you don't already have working versions of those on your steam boiler.

>Renee the boiler should have a drain valve. Let the boiler cool to avoid scalding burns and don't force an old stuck drain or you may get into new trouble.

Sometimes crud clogs the sight glass ports and can confuse water levels.

The boiler drain can be used to drain excess water from a boiler but better is to use the flush valve on the low-water cutoff control.

On 2015-02-22 by Renee

Our sight glass was empty, so we manually added water (per previous instructions from our HVAC company)only to the correct fill mark. Now the sight glass is overfull, all the way to the top, so the boiler must have added too much water somehow.

How do we drain some water from it, so it gets back to the correct level? What can happen from the sight glass being overfull?

On 2014-02-24 by Anonymous

While you are flushing the boiler LWCO the water level in the sight glass will drop; when you stop flushing the boiler the automatic water feeder will restore the boiler to the proper level and the level in the sight glass should return to normal.

On a manual feed boiler the user restores water to the level marked at the sight glass.

On 2014-02-24 by Opening the Drain line only

Should this empty the sight glass or will there still be a hold up of liquid in the sight glass ( Water and steam valve still open)

Question: water is rising in boiler sight glass but I'm not adding any

sight glass on steam heat furnace with indirect heater water rising without adding it I have a manuel feed I took off union checked
manuel feed not leaking any ideas indirect heater 4 yrs old - Harold Nobile 3/3/12


Harold, if your system does NOT have an automatic water feeder and you see water level apparently rising in the boiler anyway some explanations could be:

Question: boiler sight glass problems?

I had a fill sight glass replaced by a service technician because it crack. Before that repair, my boiler was automatically feeding water. After the repair, it seems that I manually have to open the valve. There are three valves that are close together and seem to be connected to the same pipe.

Now I have to turn my boiler on much higher than before to get it warm in the house. - Anon 10/23/12

steam boiler's sight glass is rusty and every time the boiler comes on the automatic water feeder,feeds water to the boiler filling the sight glass above the required level..

i have been reading the other problems on this site there a required height for the auto feeder to be installed in relation to the sought glass ? - Phil J 11/2/12


Anon: you want to call your heating service tech to have him/her show you the water feed valve setup and to check the condition of the automatic water feeder. Perhaps the sight glass repair stirred debris that has entered a valve or control. Try flushing things out.


There should be a tag, mark or line on the steam boiler face behind or next to the sight glass giving the proper fill level. Filling above the glass would be unusual. Perhaps your automatic water feeder meds cleaning, service, or replacement.

Steam Boiler Sight Glass Leaks, Breaks, Repair

Please see STEAM BOILER SIGHT GLASS REPLACEMENT for steps in repairing or replacing a leaky or broken steam boiler sight gauge or sight glass.

These heating boiler leak diagnostic articles may also help troubleshoot steam boiler sight glass crud or leaks:


Continue reading at SIGHT GLASS, STEAM BOILER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BOILER LEAKS CORROSION STAINS - how to diagnose & evaluate leaks on or in heating boilers

Or see BOILER LEAKS, HOW TO LOCATE - how to determine that a boiler or hot water system is leaking and how to find hidden leaks


Or see LEAKS at BASEBOARD, CONVECTOR, RADIATOR - hot water heat: how to find leaks at baseboards, radiators

Or see STEAM BOILER FLOODING REPAIR - causes of high water in the steam boiler

Or see SIGHT GLASS REPLACEMENT- how to repair or replace a broken sight glass on a steam boiler

Or see STEAM BOILERS GENERATORS CONTROLS, PRATT for some interesting history of steam generators & steam turbines.


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