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Steam boiler water level sight glass schematic (C) InspectApediaSteam Boiler Surging or Flooding
Boiler flooding & boiler surging diagnosis & repair

What causes flooding in steam boilers - why is the steam boiler water level too high?

Here we explain all of the various causes of excessive water levels in a steam heating boiler and causes of boiler priming or surging, and we suggest the remedy or repair steps for each of these problems.

Keeping the proper amount of water in a steam boiler is a critical function for both safety and to avoid damaging the boiler itself.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Steam Boiler Water Level Surging or Flooding: Causes & Remedies

Steam boiler sight glassDefinition of Steam Boiler Surging

Water level in the sight glass is seen to suddenly surge or move up and down repeatedly while the steam boiler's burner is on.

Note that it is normal for the water level in the sight glass to drop to a lower point during steam boiler operation, rising again to its proper level as condensate returns to the boiler at the end of a boiler-on cycle.

But if you see sudden or rapid up and down movement of water in the sight glass, the boiler is priming or surging.

Watch out: surging in a steam boiler, because it can include intermittent too-low water levels in the boiler, can cause the water feeder to over-feed the boiler, leading to steam boiler flooding.

[Click to enlarge any image]

Our photo shows a typical steam boiler sight glass - a key place to look when checking that the steam boiler water level is set properly. More information about using the sight glass to maintain steam boiler water levels is included in the boiler sight glass article link given just below.

Definition of steam boiler flooding

Water level inside the steam heating boiler is above the manufacturer's recommended level.

A flooded steam boiler will have trouble sending steam up into the building's heating system, resulting in poor heat or no-heat complaints.

The sketch at page top shows how to read the sight glass on a steam boiler. If water level in the sight glass is above or below the recommended level the system needs attention.

Question: Causes of high water in the sight glass or causes of flooding or surging in steam boilers?

2017/04/23 Anonymous-2 said:

What causes high water level readings in the site glass.

What causes the water level line in the sight glass to "bounce" up and down when the steam boiler is on?

This question was posted originally at SIGHT GLASS, STEAM BOILER

2017/04/18 Anonyous-2 said

I've had a boiler for seventeen years with no problem. When the water in the sight glass goes below a certain level, there's an auto shutoff mechanism that is triggered and a red light goes on. I just fill it until the light goes off and alls well.

McDonnell & Miller Probe type LWCO PS-801 (C) InspectApedia.comRecently I took the sight glass off [of my steam boiler] to clean it No problems for a few weeks, but now the safety/red light is being triggered by it being over the sight line.

Never had that problem before. Drained it and and refilled to proper level and hit reset/on off switch for the shutoff and it light went out, but this morning the water level was above the shutoff level but shutoff triggered/red light on.

The boiler low water cutoff or LWCO is a McDonnell unit. It says McDonnell & Miller ITT. Serial no at top PS-801-24

Any ideas? Thanks for any input. - 2017/04/18 by private email to editor

Causes of Flooding, Priming, & Surging in a Steam Boiler

Reply: It is not likely that simply removing, cleaning, and replacing the sight glass would itself cause abnormally high (or low) water levels in a steam boiler.

High water readings in the sight glass of a steam boiler, also referred to by the pros as a flooded steam boiler are most-likely caused by one of the following, of which surging is perhaps the most common problem.

  1. Most-Common Causes of Surging in a Steam Boiler,

    that in turn cycles the water feed valve when it shouldn't.

    Surging in a steam boiler, in turn, may be caused by over-firing (burner set too high or wrong burner capacity installed), dirt or debris in the boiler and its water, excessively high steam boiler water pH, or finally an improperly-sized or piped Hartford loop that is too small, causing a delay in condensate return to the boiler.

    Dirty water:

    Of these, the most-common cause of steam boiler surging is dirty water: high levels of sediment, rust, debris, or even oils or grease contaminants. Those form a film at the top of water in the boiler. You may see debris or even foam as well as surging water levcel in the sight glass.

    The film resists the escape of steam from the water, causing a steam pressure build-up below the film until the film surface "bursts" causing surging.

    Clean the boiler - with the boiler cooled, flush the system and consider using a steam boiler treatment or additive to improve cleaning and then to prevent surging.

    High pH:

    Too-high pH in boiler water (water that is alkaline or "basic") causes frothing and surging.

    Check the boiler water pH: the pH in a steam boiler should be between pH 7 and pH 9.

    Hartford Loop mistake:

    Diagnostic tip: if the cause of surging in your steam boiler is due to an improper Hartford Loop, the problem would have probably always been there, while if the boiler surging problem is recent, or intermittent, it's probably one of the other causes discussed here.

    Specifically, using a long nipple to connect the Hartford Loop or even with a close nipple, placing that connection too close to the boiler's water line can cause surging or steam boiler banging hammering noise.

    See HARTFORD LOOP for more detail

    Here is a RECORDING OF BANGING PIPES at a STEAM BOILER [2MB .m4a sound file recoreded 2015/11/29] provided by a reader whose comments follow.

    Play this file with iTunes or Quicktime or Windows Media player.

    Under-sized steam piping

    leaving the boiler can cause both surging and steam boiler banging or knocking sounds similar to water hammer. An under-sized steam riser at the boiler will cause steam to move upwards at such high velocity that it may pick up and bring water from the boiler itself, causing boiler water level to drop, then return - surging.

    The water feeder may then feed more water into the boiler; later as condensate and water return to the boiler the boiler will be over-filled or flooded.

    Remedy: Check the steam supply riser pipe size against the boiler's installation instructions.

    An over-sized burner or over-fired burner

    can over-heat water in the boiler too quickly, also a cause of steam boiler surging. The burner flame should not impinge any surface in the combustion chamber.

    Over-firing can in turn cause both bubbling at the point of flame contact with water-containing sections of the boiler and also sooting that reduces heat transfer and increases heating cost.

    Remedy: the service technician needs to check the boiler firing rate and the flame for impingement and to adjust the burner (gas or oil) as needed.

  2. A leak in a tankless coil

    used to make domestic hot water (not found on all steam boilers) will leak higher pressure (building water supply pressure) water into the boiler, causing boiler flooding.

    Remedy: Try turning off the water into the tankless coil and watch boiler water level over a day or so.
  3. Over-filling the steam boiler manually

    by a maintenance person - watch out, too-rapid manual filling a hot steam boiler can crack a section.

    A leaky manual water feed valve (or leaky bypass valve where an automatic water feeder is installed) on a steam boiler can also cause flooding.
  4. Delay in returning condensate to the boiler,

    due to debris clogging in the condensate return line.

    The clogged condensate return pipe slows the return of condensate into the boiler, dropping water level in the boiler, in turn causing the automatic water feeder to add water to the boiler.

    At the end of the burner-on cycle as condensate moves slowly back into the boiler, the automatic water feeder has already added water and the boiler may then be flooded.

    In addition to flooding, the delay in return of condensate to the boiler can cause condensate to back up enough to leave water in the steam main. If this occurs, steam in the main hits water in the main to cause steam system hammering or water-hammer noises, and the hammering, in turn, may cause condensate to leak out of steam vents at the end of the steam main.

    Clogging in the gravity condensate return line is common in steam systems because of the normal accumulation of sludge and debris and rust flakes that move down through the condensate return to collect at the lowest point near the boiler itself.

    Remedy: flush the steam mains and condensate return lines, or replace them.

    A check valve on the condensate return line may stick or may be partly debris-clogged, causing both delays in condensate return (doesn't open early enough).
  5. An clogged, improperly adjusted or poorly maintained automatic water feeder valve,

    or more often, an automatic water feeder valve that's leaking at a valve seat.

    Clogging from rust, debris or scale in the water feeder line between feeder and boiler can also cause boiler flooding by preventing a float-type water feeder valve from opening and closing cleanly.

    McDonnell & Miller describe a BROKEN UNION TEST [PDF] at the water feeder that can detect this problem. [

    Click to enlarge any image]
Broken union test by McDonnell & Miler at InspectApedia.com
  1. Motorized steam valves

    (depending on other boiler condensate return piping system components & design) can cause boiler flooding. The steam valve closes, cutting off steam pressure that would have pushed more condensate into the boiler through the condensate return piping;

    Next the automagic water feeder puts makeup water into the steam boiler.

    Finally, when on a next heating cycle the motorized steam valve opens the pressure returns and more condensate is pushed into the boiler, flooding it.

    For readers unfamiliar with motorizes steam valves we quote this example from Heat-Timer's information on motorized steam valves:

    The MPC Platinum control Series is designed to operate a steam boiler or a steam valve to provide steam outdoor reset. Utilizing the steam cycle concept and the outdoor temperature, the control varies the duration of the steam supply to the building in a cycle based on the outdoor temperature.

    Each of these options has a set of pre-configured, but adjustable settings to meet the specific needs of each building. - retrieved 2017/04/23 (my dad's birthday), original source: www.heat-timer.com/motorized-controls/ Heat-Timer Corporation 20 New Dutch Lane, Fairfield, NJ 07004 USA, Phone: 973 575 4004
  2. Steam push in narrow boiler sections:

    thanks to Dr. Steam, Dan Holohan (in various texts) for the following more technical example of a boiler design that can cause boiler flooding:

    A boiler with narrow sections, when heated, may suffer what I [DF] call "steam push" - rising bubbles in the boiling water "push" water level in the boiler higher than the water level shown in the sight glass (that doesn't contain an equalizing steam pressure).

    At the end of a heat-on cycle when pressure in the boiler drops and bubbles are no longer pushing water up in the narrow boiler sections, water in the sight glass falls back into the boiler, raising its water level. In my less-expert opinion this doesn't seem likely to introduce a big volume of water but over many cycles it may indeed have an effect.

    This is probably an extension of steam boiler flooding due to over-firing.
  3. A damaged or failing Low Water Cutoff or LWCO control or valve

    a damaged or clogged LWCO can cause over-filling of the steam boiler if it is not properly-sensing the boiler water-level.

    In discussing abnormal water levels in the steam boiler sight glass in the question by Anonymous-2 above, the homeowner was describing a McDonnell-Miller PS801 / PS-802 type Low Water Cutoff Valve. This is an electrionic water level sensor that uses a probe inserted into the boiler rather than a float-type mechanical water level sensor.

    The fact that a LWCO is a probe-type that doesn't have a moving float assembly should be a tipoff to an electrical or electronic component or probe failure if the steam boiler is flooding.
    Reader followup from Anonymous-2 said:

    I thought I'd let you know what it turned out to be: a relay to the low water cut off probe. Replaced the whole LWCO box & its working like a champ.

Below: an image of some of the interior electrical components of the McDonnell & Miller 801 series LWCO control provided by the reader.

 

Steam Boiler Water Feeder & LWCO Manuals & Steam Boiler Flooding References & Research

 


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: what causes flooding in steam boilers?

Interior view of McDonnell & Miller LWCO 801 series probe type cutoff control (C) InspectApedia.com JB@anon asked:

STEAM HEATING SYSTEMS & CONTROLS does not say what causes high level of water in the steam boiler.

Why your boiler in the few days became over flooded.

Reply: List of causes of flooded steam heating boilers

On the steam heating system page that you cite above, you will find this statement:

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

But to help out, here's a bit more detail:

An inspection by a trained heating tech will surely be more reliable than an online guess sight-unseen, but here are some common causes of steam boiler flooding:

- Failure to adequately flush the LWCO can leave sludge in the system, or even just dirty boiler water can gum up operation of the automatic water feeder so that it is not closing completely when it should.

- Surges in the water line can cause the automatic water feeder to also surge and insert extra water into the boiler; dirty or oily water in the boiler can cause surging. If this is the case the boiler needs cleaning.

- High pH in your water supply can cause foaming and water feeder troubles; but if your boiler flooding problem is an new trouble on an old system I tend not to suspect this cause - unless your water system source has also recently changed; If you see foamy water in the sight glass this could be the problem.

- Tankless coil leaks - some steam boilers and many hydronic boilers make domestic hot water with a tankless coil. If the coil itself is leaking, because house water pressure (inside the coil) is higher than boiler water pressure, the leak will flood the boiler.

If you have a tankless coil on your steam boiler you might diagnose this by turning off water into the tankless coil and observing if the boiler flooding stops.

- A gravity condensate return with motorized heating zone valves: this explanation is a bit technical, but basically, the operation of the zone valve can interrupt condensate return causing the automatic water feeder to send more water into the boiler.

- Steam boiler running too hot: for example if the oil burner nozzle was changed to a higher gph rate the boiler may be over fired, causing steam to leave the boiler too rapidly for the condensate return rate - I see this as a different type of surge that causes the water feeder to send water into the boiler; then as the condensate later returns the boiler is flooded

- B&G CounterPoint from September 1996. B&G is a producer of heating system control

- Automatic water feeder bypass valve leak: if your automatic water feeder has a manual bypass loop and valve that valve might be open or leaky.

- B&G and some other manufacturers point out that on occasion someone else is adding water to the boiler - improperly - causing boiler flooding

- Steam boiler piping not properly sized or installed around the boiler - but if this is the cause of boiler flooding you'd see this problem probably from day one of the installation.

- Automatic water feeder not properly installed - e.g. too high on the boiler - but if this is the problem you'd get flooding from day one of the installation.

Question: how to track down cause of steam boiler flooding problem

2021/02/03 Reader M.K. wrote:

Hi there, I have a residential steam boiler that keeps flooding and has to be drained for 40 minutes twice a week.

The weird thing is the water is turned off to it and it keeps flooding. Any thoughts? Plugged pigtail? Bad pressure switch. I have seen 15 psi once or twice before draining it. Thank you for your time -

Matt Krewer - mkrewer@towncountryservices.com
Service manager
Town and Country Services
Tonica,Il 61370 USA - offices in Princeton IL and Tonica IL.
Tel: 815.220.5107
Website: https://www.towncountryservices.com/contact/

Moderator reply:

Typically that is a delayed condensate return problem

Reader follow-up:

Any ideas on what I can check to confirm that?

Moderator reply:

I think we're maybe people would do if you could hire a heating professional who's familiar with Steam Heat and he or she would try flushing the piping in condensate system

Reader follow-up:

I am a heating professional, familiar with steam heat just a weird situation. Thanks anyway

Moderator reply:

Oops. Got it. I still think about this some more and I'll see what else I can come up with. At least you should be gratified that I was telling a stranger to call a heating professional.

Reader follow-up:

Definitely! I went over today, cleaned out the pigtail, the pressure switch, replaced the sight glass and cleaned the ports. The boiler is set for 2 psi, the sight glass was 2/3 full. I watched it cycle on pressure but from some reason it keeps flooding with the water turned off to it.

Moderator reply:

Two things. If this is a residential system is typically should be operating around point five PSI so if someone has said it to 2 PSI that's telling us that they had a problem pushing steam somewhere.

Sometimes, in turn, that's a clue that there is a rust or debris clogged somewhere or that there's improper routing of piping.

Second, if I understand you correctly, you got water turned off at the water feeder.

There should be no other potential water source except for condensate. So if the boiler is flooding that's telling me that there's a condensate blockage and very slow return to the boiler such that while the boiler is operating the water feeder is feeding more water in.

I don't remember if we test on this before but is this a 1 pipe or a two pipe steam system?

Reader follow-up:

It is a one pipe system and he also mentioned a couple radiators had been removed. I’ve only been working on it a couple years but he said the last few years it has flooded. All the way up to the second floor I appreciate your time

Moderator reply:

Let me say before going further that I don't want to pose as a Superworld a class steam expert. I have worked on in debug some steam systems and of course I'm interested in them and have written about them. The expert of course is Dan Holohan .

What do you think about this? For the system to flood up to the second floor suggests that there may be multiple blockages in condensate return. It seems to me that if the blockage were only on a lower floor that steam entering those radiators would flood those radiators and then stopped entering the radiator so it couldn't produce enough condensate to push water up to higher floors.

Also although we don't know where the problem is yet we know there's been a history of Trouble getting steamed to the various Heated areas otherwise you wouldn't have seen the pressure pushed up to four times normal PSI.

It's just an opinion but I also think that in a one pipe system it's easier to get a clogged and malfunction at a radiator base because you have to have available that flows steam in one way and condensate the other.

It's a bit more complex and it's possible the openings are smaller. In an old building there's plenty of opportunity for us to fall down in the radiator and clogged the condensate return.

What do you think about going to one of the radiators and pulling off he steam entry and condensate return at the radiator bottom to see what you find. If you choose one that's been reported to have been cold frequently are continuously we're more likely to see a lot of rust and crud in the radiator bottom. It may be that the client is facing an extensive cleaning As well as some parts replacement .

You probably know better than I that most of these systems also have other traps In the steam piping besides at radiators so this Stepping in the bugging, following the piping and finding One or more of those .

Watch out: Also double check that the water feed shutoff is fully, truly off.

Reader follow-up:

Awesome thank you for your time. I really appreciate it. I’ll look into this

Moderator reply:

When I should have been sleeping last night the shutoff valve question turned its squeaky handle until I had to get up and sneak into the living room to send a text about it.

I've seen stop valves that leaked due to a groove in valve seat or a damaged washer, and gate valves that you think are closed but are not fully down (sometimes the stem breaks). A ball valve is often more reliable.

Anyhow as you know, even a slow leak at a stop valve, just like a leaking water feeder, an fill up piping to the top of a building.

It's similar to the problem of a leaky diaphragm on a pressure-reducer water-feeder valve used on some boilers.

I'd check that first before tearing open any piping.

Reader follow-up:

There are actually 2 ball valves between the meter and the valve I’m seriously doubting it would go through 2 and the meter wasn’t turning at all while I was there.

Moderator reply:

So we have slightly simplified the task;
Think about what else we can eliminate;

In sum it sounds as if enough steam is making it upstairs to then condense and fill piping all the way back down; That is, I'm doubtful that condensate from downstairs would somehow get pushed upstairs
- unless there is a condensate collection container and return pump somewhere in the building - not common on residential buildings with their own steam boiler.

On 2014-10-15 by Ori - too much water in my boiler

I have a 20 year old Peerless gas fired steam boiler. Lately I find my self removing water from the boiler rather than adding water.

The boiler has a manual feed water valve. Every few days theremseems to be more water than before.

Any suggestion?

On 2014-10-15 by (mod) - signs of steam boiler flooding

Yes, usually when the steam boiler water level is too high that means that there is a delay in condensate returning to the boiler followed by an automatic water feeder valve adding makeup water. Then as the system cools off and condensate finally returns, there will be too much water in the boiler

Your heating service tech will look for a clogged condensate return line or clogged steam trap.

 

Question: steam boiler keeps flooding - high water in the sight glass

(Apr 1, 2013) Luis said:
I have a gas bolier which continues to overflow. The sight glass floods even some pipes that run though the floor ang o up the wall with a steam trap a gauges at the to of the pipes. I called a company but they have not been able to find the problem.

Reply:

Luis if the steam boiler keeps overflowing, or flooding, most likely the water feeder is out of order.

See details at STEAM BOILER FLOODING / SURGING REPAIR


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