Steam Boiler Sight Glass FAQs #4

Clean, use or fix a steam boiler sight glass
Air bubbles or dirt in the sight glass
Leaks in or at the boiler sight glass
Sudden drop in water level in sight glass

Dirt or Air Bubbles in the Sight Glass

How can I prevent the steam boiler sight glass from getting so dirty"

The sight glass gets very dirty and I can’t see the water line. How to prevent? On 2019-04-02 by Anna

by (mod) - steam boiler sight glass can be cleaned & steam boiler and its water charge flushed and cleaned + chemical treatments

Anna

A steam boiler sight glass can be cleaned by turning off the top and bottom valves at the sight glass itself, then when the boiler is cool AND IF the valves are actually shut and not leaking, the sight glass can be removed and cleaned with a small diameter bottle brush or the like.

To slow the dirt-clogging or obscuring of the steam boiler sight glass, your steam boiler may need to be cleaned - drained of sediment, and depending on water quality, use a chemical treatment.

Air bubbles in the sight glass may mean low water or boiler surging

What would cause air bubbles in the glass making it difficult to get an accurate reading of the water level. Thanks On 2016-09-20 by Colin

Reply by (mod) -

Low water or surging in the boiler can cause air bubbles in the sight glass.

If you've checked and assured that the boiler's automatic water feeder is working, or if you filled the boiler manually to the proper fill line and you're now seeing the water level in the sight glass bounce up and down or fill with bubbles, then

See STEAM BOILER FLOODING / SURGING REPAIR 

There is surging water in the steam boiler sight glass: what's normal?

Just how much movement of sight glass water is considered normal? On 2018-12-16 by Paul Reeves

Reply by (mod) - two answers: water level & water surging

Interesting question, Paul thank you.

The amount of movement of water in the steam boiler sight glass it would be considered normal or abnormal needs two separate answers.

First is the actual water level in the glass.

Water should never Disappear Completely from the sight glass and at the end of heating cycle water level should return to the mark showing normal water level as condensate returns to the boiler and the water feeder valve as water as needed.

Now with respect to surging which may have been your question,

you'll see the water level rise and bounce a bit if you feed water to the boiler manually or if the automatic water feed valve feeds water to the boiler.

Then it should settle down.

If you see water bouncing up and down like mad the boiler is surging and probably needs cleaning or some other service.

Fix Leaks at the Steam Boiler Sight Glass

Can I close off the sight glass valves if the glass is leaking?

Can I shut off the valves at the top and bottom of the sight glass and still run the system? The sight glass is leaking from the bottom. On 2016-10-11 by KEN DAVIDSON

by mod -

Yes, Ken. But you won't know the water level in the boiler.

It's fine to close the sight glass valves to stop a leak but you should go ahead and have the glass replaced.

That is, closing the sight glass upper and lower valves can stop a leak at the sight glass, but it's only a temporary measure, and

Watch out: with the sight glass closed you cannot monitor the water level in the boiler. If the boiler is run with too little water it could be damaged, ruined, or become dangerously unsafe.

Is it OK to tighten the nut of a leaking site glass if its hot?

Is it OK to tighten the nut of a leaking site glass if its hot? On 2017-06-19 by Will

Reply by mod: maybe but be careful

An experienced steam boiler service technician might indeed GENTLY tighten a loose packing nut on a steam boiler sight glass, hot or cold, but

Watch out: if you over-tighten the nut and break the sight glass you've got a bigger repair job on hand and you could be burned by hot water.

Slow leak from the top of the sight glass

Slow Drip: From top inside sight glass that moves to top or over top. On 2019-02-27 by BOB

Reply by (mod) -

Bob

I think you're telling me that there's a slow leak from the top of your sight glass.

If that's the case you can turn off the water into the sight glass at the top and bottom and then disassemble the sight glass in order to replace the seal or gasket.

Watch out: if you're not familiar with working on steam boilers it would be safer to have that job done by a trained steam boiler service technician. It's necessary to cool down the boiler and to partly-drain it to make this repair.

Sudden Drop in Steam Boiler Sight Glass Water Level at Start or End of Heat Cycle

The Sight Glass water disappears when the boiler fires

When boiler fire the water in the sight glass drops all the way down.

It's a one pipe system with a Weil McLean boiler. On 2019-02-06 by Isaac

- by (mod) - False sight glass lines on a steam boiler & other causes

Isaac,

A sudden water level drop at the start of a steam boiler heat-on cycle could be: (In our OPINION, roughly in order of probability)

• The steam boiler sight glass is being "fooled" and is temporarily not showing the true steam boiler water level. Boiling water inside the steam boiler causes the water to become mixed with bubbles of water vapor. The steam boiler is, after all, boiling water.
  
  The bubbly steam-water is lighter than the straight (cooler) water in the sight glass, so that water "falls" down a few inches into the boiler.
  
  Some steam heat experts call this a "false water line" in the steam boiler sight glass. It's more likely to show up in a new small steam boiler or with any steam boiler whose internal sections are becoming rust and sediment clogged.
  
  I've read that high levels of sediment, rust, scale, or even oil in the steam boiler can aggravate this condition.
  
  The "fix" is to clean scale and sediment from the boiler and perhaps to add an appropriate water conditioner.
• There's a leak somewhere in your system, in the steam boiler itself or in the steam delivery / return piping
• Your automatic water feed valve is not working properly or it has been left turned "off" or onto "manual feed"
• Your steam boiler serves multiple heating zones or one large zone and there has been a sudden surge in demand for heat
• A steam radiator vent or more than one vent is not working properly and is blowing off too much steam
• The water level was already low but you didn't notice it - perhaps because of a clogged and slow condensate return line or an automatic water feeder that's not working
• The boiler is overheating and boiling off water faster than appropriate or blowing off water at its pressure/temperature relief valve; possibly the boiler's temperature or pressure control is not working properly
• There is a hydronic heating loop and a circulator pump is sending water out of the boiler when the boiler water level is already a bit low.

Watch out: a steam boiler that runs too low on water can be damaged or even ruined and may also be unsafe. You should get some onsite help from a trained heating service technician.

Why does the low water feeder kick in at the end of a heat-on cycle?

At the end of a heat cycle, within 30 seconds after flame out, the water level suddenly drops and the LWC kicks in. Any idea what would be causing that? On 2020-11-17 by Ben

- by (mod) - possibly a slow condensate return problem

Ben

I'm just guessing from a brief 1-line e-text, but it's possible that the condensate return line to your boiler is partly debris-clogged, delaying the return of condensate;

Watch the water level in the sight glass after the water feeder has fed the boiler and then stopped.

If you see the water level continuing to creep up, say over the next hour, that suggests delayed condensate return. Some flushing and cleaning may be in order.

by Ben

Thanks, I believe you're correct. I figured it had something to do with water pressure. I drained the wet return a couple times now and it seems to be working fine.

I had a slight leak and added some boiler powder to hopefully seal it.

It worked, unfortunately, the crud it added to my system has led to a week worth of flushing and skimming.

Water suddenly disappears from the sight glass and we lose heat

I have a Crown Boiler, all Of a sudden the water keeps emptying out of the sight glass which in turn shuts of the heat to radiators, Can anyone help, I keep filling it up to the mark, when I run the heat for 30 minutes the glass empties On 2018-03-24 by Richard

by (mod) - when to check the sight glass water level

Richard:

When the boiler has been off for a time after you added water, check the sight glass again. If water level has risen above the point to which you added water, then we think there is a blockage and a delay in return of condensate to the boiler.

If that's not happening I'd look for a problem with steam vents that are not closing when the rads are hot, or for a leak in the condensate return line.

Why does the sight glass water level drop when the steam boiler starts?

Sight glass empties when furnace starts On 2016-10-25 by Anonymous

by (mod) - surging boiler problem

Anon,

When your steam boiler starts ("Furnaces" are hot air heating systems), 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.

see STEAM BOILER FLOODING / SURGING REPAIR where we explain the causes of water too high or too low in the sight glass or in the steam boiler

Why does the sight glass water level move up or down down over 3/4 the glass height?

I guess my main question is how can the water in the sight glass pulse down from over 3/4 the height down to LWCO is 2-3 minutes.

Is that much water in the boiler being turned to steam in that little time?

If not, what is happening inside the boiler? On 2018-10-12 by Bob Kos

Reply by (mod) - boiler sight glass surging

Bob

I don't want to add confusion by guessing further beyond my competence.

But that pulsing down problem acts as if there is an internal blockage in the system somewhere.

When you drained the system did you get a lot of sludge-crud? Could there be more?

Have you been flushing the LWCO as is usually required?

About the old over-fill problem: if you were able to drain the boiler and subsequently delivered steam to the building then any remaining problem would be a subtle one such as having flushed rust and debris into a horizontal section or low section of condensate return piping, causing clogging.

Beware of over-filling the boiler: if you push water out of the top of a steam boiler you can prevent it from sending steam and heat probably won't be delivered as the pressure control will sense over-pressure in the boiler and shut it off.

If there are improperly sloped condensate return pipes that could also aggravate the condensate return problem.
Such as debris clogged pipes.

Watch out: Also beware of adding a lot of cold water to a hot steam boiler at once - you can crack a section - damaging the boiler or destroying it.

Bob

If the water is actually pulsing then the boiler is surging; that can be due to contaminants in the water; there are also anti-surge additives for steam boilers.

at STEAM BOILER FLOODING / SURGING REPAIR

and

at STEAM BOILER FLOODING REPAIR FAQs

you will find a discussion of surging problems and solutions.