Steam Boiler Sight Glass FAQs

Boiler sight glass control valve
Low water in boiler sight glass, boiler turns off
Unexplained increase in water level in sight glass

Steam Boiler Sight Glass Control Valves

What are the knobs for at the top and bottom of the sight glass?

What are the red valves on top and bottom of the sight glass? Should they be closed or opened? Thank you! On 2015-10-26 by Anonymous and On 2017-03-16 by Anonymous

Should the shut off valves at the top and bottom of my steam boiler sight glass be kept open or closed ? On 2016-02-24 by Joan

by (mod) - sight glass control knobs let water in/air out & help with sight glass cleaning or replacement

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

If the glass / sight gauge is leaking or broken you'd close both valves to stop the leak and to permit replacement of the glass.

Watch out: if your boiler has an automatic water feeder you may be OK while waiting for the sight glass to be replaced, but if you are feeding water to your boiler manually you need that sight glass to know how much water the boiler needs.

Watch out: If you fail to keep water in the boiler it may be unsafe or it may be damaged.

If you need to close the boiler sight glass valves:

In the sight glass photo just above on this page you will see two red control valves that are used for this purpose, but on older steam boilers those control knobs may be a bit smaller in diameter and often are black Bakelite plastic.

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.

Are the sight glass valves normally open or closed?

Should the top and the low valves located on the sight glass stay open? On 2016-12-14 by Anonymous

by (mod) - normally open unless there's a leak

Most service techs and building owners keep the top and bottom sight glass valves open so that the water level in the boiler is visible.

But if the sight glass were broken or leaky, in an emergency we may keep heat on by closing those two valves.

Check the instructions for your specific steam boiler as your manufacturer may have different advice.

Watch out: if the steam boiler sight glass valves are closed and you cannot monitor the boiler water level you are depending on an automatic water feeder to keep the boiler safe and operational.

If the steam boiler water is added just manually, you cannot know how much water you've added without first opening those valves on the sight glass.

Should the both the top and bottom inlet valves be fully open to operate properly

Should the both the top and bottom inlet valves be fully open to operate properly? 2017-11-06 Paul

Reply by (mod) -

Nice question.

As long as the valves are open at all the boiler water level gauge or sight glass, whose purpose is only to show boiler water level, will work fine.

It's possible that people close the valves to reduce surging in the sight glass;

Watch out: It's also possible that a partly closed sight glass valve may be more prone to clogging.

I suspect some people might stop down the valves to reduce surging of the water column in the sight glass. In my view that's not the best "repair" even though we discuss it in the article above on this page.

If the boiler is surging that might cause other trouble. More on surging is

at STEAM BOILER FLOODING REPAIR where we point out that surging can foul the water feeder.

Also at CHEMICAL TREATMENTS, BOILER we discuss boiler chemicals added to control surging.

And if the valve is closed too much, a nearly-closed inlet valve crud in the boiler could more easily block water entry into the sight glass, rendering it inoperative.

I think the main use of those little valves is to permit change out of a broken sight glass or to keep the boiler running (by closing the valves) while waiting for a broken sight glass to be repaired.

Do I use the knobs on the sight glass to re-fill the tube?

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? v

by (mod) - not exactly - adding water to the boiler should make water appear in the sight glass

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.

Provided both the top and bottom sight glass valves are open, water can flow into the sight glass from the steam boiler's interior, and he sight glass then indicates the level of water in the boiler itself.

If water is low in the sight glass (below the indicator mark or line) 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
or

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.

Is it safe to run the boiler with a broken sight glass by closing its two valves?

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 2016-12-10 by Rick

Reply by (mod) -

A steam boiler can run with no sight glass, but

Watch out: first you have to turn off the upper and lower valves and then confirm that there is no leakage. That's a reasonable emergency measure to keep heat on while waiting for the repair person to replace the sight glass.

Watch out: with no sight glass you cannot visually monitor the boiler to assure that the water level is proper and safe. Some steam boilers using automatic water feeders may still work just fine.

In sum, you must 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 and if your boilers low water cutoff doesn't work to either add water or turn the boiler OFF, then the boiler could be damaged, destroyed, or dangerous.

Will the steam boiler still work OK if I close the sight glass valves?

Will the steam boiler work if the sight glass valves are kept shut?

I have two LWCO valves on the boiler but have a small leak and need to fix. On 2019-03-31 by Bruno

Reply by (mod) -

Yes, if the valves at the top and bottom of the steam boiler sight glass are closed no water enters the sight glass - the steam boiler will still operate, but you cannot see the water level in the boiler.

Watch out: if your steam boiler does NOT have an automatic water feed valve then you could lose heat by failing to notice that the boiler needs water.

How do I stop the sight glass from filling from top valve, the steam side

How do I stop the gauge glass from filling into steam side On 2016-02-10 by Dave

Reply by (mod) -

Normally the sight gauge shows the water level in the boiler because 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 I suspect that 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.

When adjusting the water level with the sight glass, what should the sight glass valve settings be?

When adjusting the water level with the sight glass, what should the valve settings be?

Open on bottom/ open on top, I noticed that the water level doesn't fluctuate when the top is closed. On 2019-02-20 by Chris

Reply by (mod) -

Except for the case when the sight glass is leaking and the valves have to be closed the normal position is open at the top and bottom so that the water level in the boiler can be continuously monitored.

Sometimes the technician may slightly close the bottom and top valves as an effort to deal with surging however that's not the best repair for surging problems in the boiler.

Unexplained Increase in Water in the Boiler Sight Glass

Steam boiler flooding after installing new controls

I just had a new Safeguard 450 LWCO installed, and a VXT auto water feed installed on my steam boiler. The old mechanical LWCO valve failed.

The water level in the sight tube is now all the way up, and there seems to be no steam reaching the radiators.

The pressure gauge reads zero.

The basement is very warm though. Is the boiler over filled with water? Should I call the service company back to look at this? On 2018-11-11 by Mike F. -

by (mod) - Steam boiler is over-filled: find where the water is coming from

Mike that sounds like an installation or adjustment error that may have flooded your steam boiler.

I'd ask the company to return, drain the excess water from the boiler, and check the adjustment of the automatic water feeder and the position and operation of its water level sensor.

Other possibilities are a tankless coil leaking into the boiler OR a bad control or valve or water feeder leaking into the boiler.

Let me know what you're told, and post a photo of the full sight glass, of the boiler, the LWCO, and the VXT water feeder (one image per comment) if you can; that may permit us to comment further.

by Mike F. - I just drained this boiler

I drained about 14 buckets of water out of the boiler earlier this morning, until I could see a true water level in the glass tube ( half way up ).

I fired up the boiler and watched it for about 20 -30 minutes. The water was slowly rising again. I shut it down, and drained two more buckets out of the boiler.

After a few hours of rest I took a look and the tube was all the way up again.

Drained 18 buckets out and found some bad flooding by the radiator in the basement. Second time I had flooding this week.

First time was after they installed the new LWCO and Auto feeder earlier this week. Steam and water was leaking out of the top of the viewing tube, and a new joint that they installed by the water feed was also leaking. This morning the repair guy arrived and after some testing he discovered that the new VXT 120 auto feeder is defective.

It was allowing the water to slowly enter the boiled, but showed no gallons being used. He has bypassed it for now until they can come back and install a replacement later this week.
I not feeling to good about these guys.

I hope they can really fix this thing for good. I already sent them a check for the full amount of the installation of the new equipment that they installed ( $ 942.00 ).

Do you think I have a case to ask them for a credit on my account for the all of the problems ( flooding, no heat or hot water )? If so, what would a fair amount be?
Thank You, Mike F.

by Mike F. - The boiler is filling up again - why?

I can't believe this, but I checked the boiler and it's still slowly filling up with water !

Placed another call for the service guy to come back and try again to figure out what's happening. The glass tube was filled all the way. I drained out about 4 buckets of water. This seems to be an unending nightmare.

by (mod) - Signs of a leak into the steam boiler

Mike what are the diagnostic things I forgot to ask you is whether or not this boiler includes a tankless coil for making domestic hot water.

If it does and if that coil is leaking that would be adding water to the boiler independent of a problem with the automatic water feeder.

Other problems that we look for are a leaky water feed valve or delayed condensate return to the boiler because of the kind of said return blockage or clogged,

And other steam boiler flooding problems that you will see in our article on flooded steam boiler repair

STEAM BOILER FLOODING / SURGING REPAIR

by Mike F. - Bad ball valve flooded my steam boiler

So.. It was discovered that one of the new on/off ball valves was defective, allowing the water to by-bass the VXT auto feeder.

The water feeder is not bad, as first thought. I'll check the water level in an hour or so to make sure that it's not rising again. Hopefully this will be the end of this nightmare. I'll say a prayer.

 

Why is water rising in boiler sight glass when I'm not adding any

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

Reply:

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:

• Condensate is returning to the system from steam radiators and condensate return piping
• A manual water feed valve is leaking into the boiler
• A combination boiler that includes a tankless coil for making domestic hot water is suffering from a leak out of the tankless coil into the boiler
• The sight glass water level was artificially low during the heating cycle because yours is a smaller modern steam boiler with lower volume than traditional boilers; bubbles in boiling water lead the sight glass water to fall during the heating cycle.
  
  Details are at SUDDEN DROP in SIGHT GLASS WATER LEVEL - at start or end of a heating cycle - false water line?

Water level in sight glass goes too high after I used a boiler cleaner

Before firing up the boiler this fall, I drained the system, flushed and refilled to 2/3 up on the sight glass.

I have a manual feed. I added boiler cleaner (Fornax) through the relief valve pipe, then put the relief valve back properly.

When I turned on the system, the boiler started up OK, but the then the water in the sight glass eventually went down to the cutoff level, shutting off the boiler.

Then it returned to a level allowing the boiler to come back on and kept recycling like this.

I had to adjust the water level in the sight glass to higher than 3/4 the way up to keep the level from going down to below the cutoff when the boiler is operating.

What happened? I could keep the sight glass level at 1/2 to 2/3 up last year without it going to the cutoff level.
Thanks,
BK On 2018-10-09 by Bob Kosiba

by (mod) - how to use Fernox™ boiler cleaner

I suspect that scale and crud has clogged your LWCO or automatic water feeder and that those devices need to be flushed.

The Fernox boiler cleaner (not Fornax) instruction sheet is at BOILER CLEANER CHEMICALS / TREATMENTS

If this is NOT the product you used, let me know; Fornax seems to be something else.

Water keeps creeping up in my steam boiler sight glass

I am SO disappointed. After removing so many parts on my boiler, the water is still creeping up in the sight glass, so that means it is my tankless hot water heater. which will now need to be removed. Is that a complicated job?

Expensive? I have all new valves and piping and now am faced with this project. How involved s it. Do you have an article on your website I can read?

I'm discouraged. On 2017-05-18 by Phil

by (mod) - repair for steam boiler flooding

Phil,

This is a pretty common problem, so you're not alone. See STEAM BOILER FLOODING REPAIR and let me know how well that works for you.

by Philomena

It's a cast iron boiler but it a case of replacing very rusted pipes, flanges,maybe the tankless water heater IF the water continues to rise after these repairs.

I'm thinking I will get it all fixed up and there will not be anyone to repair it and any prospective buyer will be too scared to buy it.

IF after all this, if it is the tankless heater, I think I need to replace the entire system.

It's the lack of qualified HvAC people that is the major problem now and people are afraid of these boilers. Maybe I need to begin getting estimates but Ed reassures me it's going to be fine.

Thanks, Daniel, for being there every time I become a nervous Nellie.

by (mod) - when to replace a steam boiler

I agree that it makes sense to compare total repair cost to boiler replacement cost.

If a boiler is an old steel unit that's rust damaged, I lean towards replacement.

For a cast iron boiler in good condition and that needs a control repaired or replaced, often that's a sensible approach.

by Phil - plumber coming to do steam boiler repairs

Well, my plumber returned from vacation. He came to check out the boiler; he is coming tomorrow to remove the 4" diameter pipe on the return line; there is a very small leak right there. It looks like a tough piece to remove. He said it would be too difficult for him to unscrew so he is going to cut it out with his Sawzall.

He is also going to replace the steam vent that hisses and shoots steam up to the ceiling. Then he is going to replace the water feed valve with a newer style. IF the water continues to creep up in the sight glass, he said it would have to be the tankless hot water heater, which would need to be replaced.

Oh boy, I hope I am making the right decision to keep this old boiler going. I dread the thought of removing it; I have no hatchway. I think Ed is one of the last repair men who is willing to work on a boiler this age in this area. I'm hoping that after all of these repairs ($1,500 so far), I am doing the right thing.

I had the boiler off for most of last week when our temperatures dropped all week long. I blocked off my kitchen, hanging blanket on two doorways and using my gas stove for heat. If this had been the middle of winter, I don't know what I would have done. I'm in Massachusetts.

I'll let you know if it turns out to be my tankless water heater. Poor boiler. Poor me. I live alone in this old house.

by (mod) - water still creeps up in the steam boiler sight glass

Phil

there are several causes of steam boiler water creep up or overfilling ranging from a leak in a tankless coil leaking into the boiler to a water feed valve problem.,

See STEAM BOILER FLOODING / SURGING REPAIR at https://inspectapedia.com/heat/Steam-Boiler-Flooding.php for details.

by Phil - I continue to have problems with sight glass, creeping up.

Sorry Daniel, to be back again. Still waiting for HVAC man to call me. I have been co concerned about what is going on with my old National boiler, and you have been very helpful. (I cannot locate our earlier exchanges). I continue to have problems with sight glass, creeping up.

I have noticed a new small drop of water coming from the pipe at the bottom of my boiler which I THINK is called an out take. It is in the area where the plumber removed a pipe a few weeks ago and had a lot of trouble with the connections.

When the water in the sight glass creeps up, I release some of it to get the level down. It seems to me that the boiler has been going on for just a short while. My thermostat is set at 62 in the house; the current temperature outdoors is about 50.

I decided to remove water and the furnace turned off! I was told by HVAC Plumber #1 that I should always remove water when the boiler is running and listen for the furnace to go off. He said that proves that the relief valve is working. Others say you shoulod reduce the water when the furnace is not working.

But now the furnace is completely off.

I am worried because I had the problem on Saturday of my National Radiator original pressure going from 1 to 4 and a vent emitting a LOT of steam.

I have left two messages for the plumber. I have even left a message for Plumber #1. I called the Gas Company this morning, but they are useless. They just advised me to call a plumber. Not easy to do with a vintage boiler.

I am sorry to bother you once again.

I have no one to help me; no one understands this system. And I am elderly and live alone.
Do you have any ideas - once again.
Thank you so much!!!!

by Phil

Once again, Dan, thanks for your continued help. I will certainly read all of the information you provided.

And good to hear you confirm that even though my old "scary" boiler is ancient, it's "good enough" to keep around. I feel that way, too. It has been dependable for the 50 years I have lived in this house. And it was old even then.

Do you answer questions every day of the week?

You do such a thorough job. I don't know how you keep up with all the questions people ask you.

You offer such a great service and I'm very grateful.
Thanks once again.
Have a good day.

Reply by (mod) -

Phil,
I answer as many questions as I can, sometimes getting behind, and on occasion taking time off for a wedding.

Your questions help us see where we need to add information, clarify text, or do more research.
Daniel

by (mod) - Everhot Tankless Water Heaters

Phil,

I've made good use of old cast-iron coal-fired boilers converted to gas or oil, properly fired and tuned and cleaned and running at about 79% efficiency - good enough to defer moving to a newer high-efficiency heating boiler in many cases.

Everhot tankless water heaters are still sold, as Everhot, now in Waterville MA, is a division of Therma-Flow - s

ee EVERHOT WATER HEATERS

Also see this article

INDIRECT FIRED WATER HEATERS https://inspectapedia.com/plumbing/Indirect-Water-Heaters.php

Low Water in Sight Glass, Boiler Turns Off

Boiler sight glass level drops below the low water cutoff valve & shuts off the boiler.

Used Fernox cleaner, left it in 4 days (cold weather so it got circulated). Drained the the boiler, refilled with Fernox Protector added.

Water level in sight glass set just above 3/4. When boiler is fired up, it pulses down to below LWCU and shuts off fire until the water pulses up
again.

Does this mean the boiler is too small for the house?

Before firing up the boiler this fall, I drained the system, flushed and refilled to 2/3 up on the sight glass.

I have a manual feed.

I added boiler cleaner (Fornax) through the relief valve pipe, then put the relief valve back properly.

When I turned on the system, the boiler started up OK, but the then the water in the sight glass eventually went down to the cutoff level, shutting off the boiler.

Then it returned to a level allowing the boiler to come back on and kept recycling like this.

I had to adjust the water level in the sight glass to higher than 3/4 the way up to keep the level from going down to below the cutoff when the boiler is operating. What happened? I could keep the sight glass level at 1/2 to 2/3 up last year without it going to the cutoff level.
Thanks, - 2018-10-11 Bob Kosiba

by (mod) - water feeder is not working properly

Bob

I would not assume the boiler is under-sized;

First I'd find the water fill mark indicator on the boiler or sight glass assembly. I'm not sure what 3/4 means in your question: 3/4 an inch above the sight glass bottom? 3/4 of the sight glass height?

There should be a mark on the boiler and also an indicator in its instructions giving the proper sight glass and boiler water levels.

Next: I'd want to check out the automatic water feeder and LWCO (low water cutoff valve) to be sure that they are working properly.

Next I'd check out the condensate return: you could have a blockage or delay in condensate return

Only if this problem you describe has been the case at every heating cycle since the boiler was first installed new would I be quick to suspect its size as a possible problem.

You will want to review our boiler size discussion at RADIATOR VOLUME & BTUs

We describe using Fernox at BOILER CLEANER CHEMICALS / TREATMENTS

Followup by Bob Kosiba - no automatic water feeder in use

No automatic feeder.

When I say 3/4 I mean 3/4 of the Height of the sight glass (almost to the top.

I just now filled it to within a 1/2 inch of the top with the boiler off.

When turned on, it took maybe 5 minutes to pulse down to where it cut off.

A minute later it came up above LWCO, proceeded up to 1/2 height, then started pulsing down again to LWCO, and around we go.

I don't believe it was always acting like this.

Some years ago I accidentally left the fill water valve on and filled the system on all three floors

After I drained it and cleaned up the mess of water on the floors, it seemed to be working OK.

Could this have any bearing on my problem?

by (mod) - clean the boiler LWCO control & look for delayed condensate return

Bob

Filling the steam pipes up three floors and then draining them out would not itself cause surging in the boiler or variation in water level in the boiler years later

except for the chance that that "flush-out" of higher building steam pipes might have moved a big dose of rust and sediment down to the boiler and its controls.

So it would be reasonable to ask your steam heat service technician to inspect and clean the boiler and its low water cutoff control.

Also there's a sensor probe in the water feeder that, if scale coated, can cause delay or improper sensing of boiler water level - so it could need cleaning or replacement.

If the problem is delayed condensate return to the boiler that could explain what you're seeing.

IF you were using an automatic water feeder (you say you are not) there are programmable water feeders that can use a timed delay between sensing low water and when water is added to the system - to avoid over-filling the boiler due to condensate return delays.