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Boiler Flooding

Hi - Thank goodness for this site, without this information - I might be selling our building.


We own a 4 unit commercial/apartment building that has steam heat.  The building is about 100 yrs old.    The management company replaced the boiler in 2006 with a Dunkirk PVSB-6D natural gas boiler.


We have owned the building for 11 yrs and have always had problems with the water level being too high in the boiler.  I finally have time to trace down this problem and have spent the last month working on the problem.  Based on searching the site and reading Dan's "We got steam heat", I've done the following:

- skimmed the boiler

- blow down the boiler basically every day - water is fairly clean

- replace one of the pressuretrol's (set to cut-in .5 and cut-out 1.5) 

- added a 0-3 pressure gauge

- replaced the pig tail

- replaced the main 0-30 pressure gauge

- flushed the wet return

- replace all 3 main vents with Gorton #2

- fixed a couple sagging returns

- insulated the near piping along with the main/returns

- added union after water feeder to do the broken union test - no leaks (water can only be added via the water feeder))


- from what I can tell, the system never reaches the cut-out pressure 1.5

- takes about 4 minutes to create steam

- takes about 7 minutes to vent 2 of the mains and 11 minutes to vent the other main.

- water level moves up/down about 3/4 inch

- the water level slowly moves down the sight glass until it finally hits the LWCO and the feeder adds water after a delay

- I've had no complaints from the tenants regarding heat - in fact, they had me turn down the programmable thermostat

Here is a picture of the near piping. (let me know if you need other angles)


- From the reading / research that I have done (I'm obsessed to get this working correctly),

       -  I'm thinking that the near boiler piping is not correct? 

       -   Should it have that "T" to the left and right?

       -   Is the equalizer correct?

- any other thoughts on why the boiler is over filling every night (I have put it to the proper water level every day and by the next morning it is around 5 gallons too much).

I really appreciate any help that I can get on this.


  • GordoGordo Member Posts: 532
    You Might Try

    Replacing that M&M Uni-match feeder with a Hydrolevel VXT. 
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • TeamBRAZOTeamBRAZO Member Posts: 18

    Ya, I have a vxt-24 ordered.  But the feeder is not the problem - the vxt will just give me more data and I can increase the delay to allow the boiler to re-fill.  But I think that is just a bandaid - I'd like to fix the real problem (if possible).

    Thanks for your reply.
  • nicholas bonham-carternicholas bonham-carter Member Posts: 6,540
    might have sold the building..

    congratulations on the skimming and good low pressure gauge!

    why not compare the piping you have with the mfg's installation instructions as far as pipe diameters, and height above the boiler.

    can you turn off the supply to the auto/over-fill so you can see how the waterline behaves with no feed. if you have a slow-return of condensate, a reservoir tank might be a better solution for you, as it cannot over-fill.

    although it doesn't seem so in your case, it is possible for the pigtail to be clogged, preventing the gauge, and pressuretrol from reading the pressure. runaway pressure can then force the water up into the returns, hiding it from the boiler, until the pressure drops at the end of the cycle.

    what sort of thermostat are you using?--nbc
  • SteamheadSteamhead Member Posts: 11,383
    edited January 2010
    You need to have the boiler re-piped

    using BOTH steam outlets, not just one. The water level is tilting inside the boiler, causing the low-water cutoff to trip. Also, I'd suggest using a 3-inch header.

    Like this...........................
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • I think you've got the wrong side, Steamhead

    Doesn't the Steam/ water rushing to the outlet cause the water to rise on that side? (which in this case is where the LWCO is located).   I have an PSB-6D (187,000 input) in place with just one outlet in use and it works fine with full sized 2 1/2 inch piping for the riser and header.  Both would be better, but it works and Dunkirk doesn't require two outlets even on the largest model at 300,000 input. 

    I would look to speeding up that 11 minute warm up time if possible.  This will get condensate back to the boiler faster.  This may require you to throttle back the air vents on the rads (this looks like one pipe steam)  to allow steam to favor the end of the main when the boiler first fires.  This way, the rads at the very end of the main begin generating condensate  sooner which then only has a short piece of pipe to get back to the boiler. 

    The VXT can be really helpful.  What I have found is that compact boilers tend to overfill right at the end or especially after the end of the heating cycle.  This is especially the case with the electronic probe on that boiler because it is placed so high.  During firing, water splashing on the probe tells it there is enough water in the boiler.  However, once the firing ends, the splashing stops quickly, making the probe think it needs water.  In addition, the water level drops because all the steam bubbles that were filling the water and causing the level to rise collapse.  The feeder will kick in until the water level is once again about halfway up the gage glass.  Then all the condensate in the system returns and the boiler overfills.   

    I suspect the boiler does not run out of water during firing, but after the cycle ends.  I have set my delay as long as ten minutes, so condensate has plenty of time the return to the boiler to completely refill it before the next heating cycle.

    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • RAFRAF Member Posts: 65
    Somthing simple

    Have you made sure all the your radiator valves are open all the way and that none of the washer have fallen off the stems this would allow water to collect in the radiators when the system is running and flood the boiler when it shuts down.

    May want to check it out
  • nicholas bonham-carternicholas bonham-carter Member Posts: 6,540
    good point

    well said, however the return of condensate from radiators with closed/faulty valves usually takes much longer, and is accompanied by much hammering.--nbc
  • TeamBRAZOTeamBRAZO Member Posts: 18

    Thank You for all the replies - appreciate your time.


    Redo piping - yes, I don't think the near boiler piping was done correctly and I plan on redoing it after the heating season.

    VXT Feeder - I have one on order and after reading your suggestions - I think that may be what keeps the system in balance.  I don't like the idea of the "feeder being the solution" - but it might just be the case with the "compact boiler".

    Radiators - I have not touched or looked at any of the radiators so far. I know that this may be part of the problem, but I wanted to first knock out all the items that I can get done before giving notice to go into the apartments. 


    Yesterday I throttled back the water valve before the feeder to just a trickle coming thru the feeder.  I then went back over to the building this morning and the water level was correct.  I then watched a heating cycle and timed the mains - took 7 minutes to get steam to all 3 main return ends.  This is compared to 21 minutes before I insulated, added new vents, flush wet returns, cleaned boiler, etc.

    Even though this is a HUGE improvement, I still thick it is too slow.  Also, now that it is much faster - it also sucks the water out of the boiler very quickly.  I would say that it took about 30 minutes from start (with water at correct level) to LWCO.  Then the feeder kicked in (just adding a small amount of water) a few times and I timed how long the sight glass returned above LWCO from the returns - which took about 3 minutes.

    So, I'm still confused on why so much water is leaving the boiler??  But that being said, the VXT should at least solve the flooding problem while I attempt to figure out the wet steam issue.  Also, the pressure on the 0-3psi gauge never goes above .2.

  • Jamie HallJamie Hall Member Posts: 6,708
    The timing

    isn't that far off.  Nothing wrong with 7 minutes at all, if that is from boiler startup.  Remember that you have to get the thing to boil first (ever timed a tea kettle?) and then get the steam out to the ends of the mains.

    I'm going to venture to say that a large part of your water departing the boiler is in that single steam connection.  That'll suck water out of a boiler faster than you can possibly imagine, and it takes time for that water which is sprayed all over everywhere in the piping to come back.  If a boiler was built with two steam outlets, there was a reason for it.  Until that is fixed, I don't think there's much point in looking around for other causes of slow return -- although they may exist.

    Pressure is fine as is.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • TeamBRAZOTeamBRAZO Member Posts: 18

    Thanks for the reply.  Ya, the 7 minutes is after the header reaches around 200 degrees (measured with a "laser temp" tool).  So, total time from start of burners to all 3 loops being 200 degrees at the return ends is around 10 minutes.

    I know very little about this whole steam stuff, but because of the searching on this site and my many hours standing in the boiler room watching and my many "try this...yep that is IT...nope that didn't fix it" hours - I feel like I'm close.

    I'm going to start planning and finding the piping to redo the near boiler piping. I'm so disappointed that the HVAC firm that put in the boiler 3 years ago did not follow the directions - they just hooked it into the existing piping.  I was not managing the building and it was done in a "no heat" situation, but they had an open checkbook to do it right and they still didn't.

    Thankfully, there is a site like this - I'm very handy and can do just about anything with the right information.

    Next up will be the VXT-24 Feeder (temporary get by) and then re-piping the near boiler piping.
  • BillBill Member Posts: 52
    Waterfeeder overfill

    BoilerPro's analysis of low water content boiler overfilling is right on. Your description is exactly what happens to my system (a Lennox, which is the same as Dunkirk, boiler). The waterfeeder only ever feeds a couple minutes after the burners shut off(and when it does come on, the waterlevel is still well above the minimum watermark the mfg has on the boiler, which makes me think as you say that the probe is really up too high).  When the condensate finally gets back the waterlevel is too high. As a result I just turned off the feed valve to the waterfeeder.  I add water lost because of evaporation from wet steam myself when needed.  My waterfeeder is a McConnell & Miller Uni-Match feeder. I opened it up and its already set to the slowest feed rate and maximal time delay, which I think is only 90 seconds, and this still results in boiler overfilling. How do you go about adding in your ten minute delay?  That would be about perfect for my system? The situation I worry about at present is that if i am absent during a period of cold weather and leave the waterfeeder off, if for some reason water loss accelerates, although the probe may shut off the burners, with no water replacement the boiler wont come on and the house temp could drop below freezing. If I do leave the feeder on, than after a period of days the boiler waterlevel may get so high that the burner will stay on indefinitely to try to satisfy the thermostat as very little heat reaches the radiators from the flooded boiler.
  • GordoGordo Member Posts: 532
    edited January 2010
    Replace The M&M Uni-Match Feeder

    With a Hydrolevel VXT feeder.  That feeder has a nice set-able delay feature.  It also has a feature that if it has to feed more than twice without satisfying the LWCO, it will lock out and not flood the boiler.

    The Uni-Match is obsolete.  They all should be replaced.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • TeamBRAZOTeamBRAZO Member Posts: 18
    Short Cycling???

    I've now installed the VXT water feeder and set the delay to 8 minutes (I timed the water return after shut-down at 6 minutes) and the water feed to LWCO.

    The concern that I now have is will the boiler short cycle and is this bad?

    Here are some timings:

    -- Cold start boiler will run for approx. 35 minutes before water goes below LWCO

    -- Takes about 6 minutes for the water to return to the boiler and the boiler starts up again.

    -- Boiler then runs for approx. 11 minutes before the water goes below the LWCO.

    -- The pressure (0-3) gauge only shows .2 - so no cut-in or cut-out

    Is this ok?  Will the boiler still heat the building correctly?  Are the cycle times with-in "normal" ranges?

    Thanks again.
  • Jamie HallJamie Hall Member Posts: 6,708
    No, not really

    that is, no, this sort of behaviour really isn't normal.  I take it that what happens is that the water level slowly drops for the 35 minute run; then the LWCO cuts out and the burner stops, and the water (all of it? probably not...) comes back in 6 minute and the LWCO cuts in, then 11 minutes of firing and the LWCO cuts out again, and so on?

    It won't hurt anything that way, but you are losing efficiency for sure, and you may have trouble getting enough heat on very long runs, such as coming back out of a setback.

    What this tells me is that somewhere in your system water is stacking up.  Granted, the pressure never gets very great -- on the order of ounces -- but somewhere out there that's enough pressure to keep the water from coming back in.  Without knowing the whole system (and you will have to look everywhere!) I wouldn't want to say what it is.  However, among the things I would look for would be an F&T trap which was set high up near a return (they take some head -- a foot or so -- to open, so if they are less than a foot below a horizontal pipe such as a return, they could store a lot of water) or a check valve (particularly a spring check, or one with a weight on the swing) in a similar sort of location -- that is, where water can back up behind it without building up much head or... maybe a return pipe which is very close to the water line, but slightly above it...

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • RAFRAF Member Posts: 65

    1  What is your definition of flooding?  We talking about a water gauge that is filled to the top or water coming out of the vents and hammering in the mains? I may have missed this in you past posts.

    2  Have you checked your radiator valves ? I have seen radiators with bad valves some will hammer and others will just fill up and girgle. You stated your tenants ask you to turn down the T-Stat makes me think they may have shut a valve or two.

    3 Read page 5 of your owner manual before repiping near boiler piping. I think paragraph 4 or 5 should be of interest.

    4 I have worked in this business from age 14 I am 56 now and learn somthing new every day. The one thing I do know for sure is I have seen more boiler piped wrong than right and for some reason they work go figure.

    5 Again What is your definition of flooding?
  • TeamBRAZOTeamBRAZO Member Posts: 18
    definition of flooding

    Thanks for the reply.

    Here is the flooding data:

    - overnight the boiler will go from perfect water level to 6 gallons too much

    I've only had the VXT installed for 1 evening, but it kept the water level correct by having a 8 minute delay on the feed.  The VXT meter was the same as last night.  It seems that it just sucks the water out of the boiler - the timing to fill all the mains is around 11 minutes from cold start to return hot.

    The VXT at least temporarly "fixes" the over fill (until I can re-do the piping), but I'm concerned with the cycling.  I'm not sure if 35 minutes from cold, then 6 minutes off, then 11 minutes on, etc -- will be a problem keeping the building warm.
  • RAFRAF Member Posts: 65
    6 Gallons

    Ok 6 gallons if you leave it go for two nights are we still a 6 gallons?
  • TeamBRAZOTeamBRAZO Member Posts: 18
    edited January 2010
    Day two

    Pretty close - it might add just a bit but it doesn't keep on adding 6 a day.  We had someone else managing the building earlier this year and they had not been to the boiler in a month.  When we drained the water, we had about 8 gallons of water to much. 

  • RAFRAF Member Posts: 65
    short cycle

    From your post you state your cycle is short do to low water in the boiler

    Do I think this would be give you a problem heating a four family house yes I do but that is something you will have to find out.

    I would be concerned with the amount of make up water being add this will destroy a boiler faster than anything. I would run this unit for a night or two and not drain out the water to see if you get more than your 6 gallon overfill.

    Again the stacking up of condensate was brought up .  I am going to  assume all piping is clear on your returns and all radiator valves are open every thing is good. Then you may need to think about a receiver pipe on this system.  You may want to do the math to see just who much condensate you could expect from this system. I am also assuming who ever installed the boiler took the time to size it correctly to your system.

    New boilers are great but they do not hold a lot of water in them
  • RAFRAF Member Posts: 65

    OK on the eight gallons now if all radiator valves are open then I would look into piping a receiver pipe onto this system. Again assuming none of your returns are pluged allowing condensate to stack up in them.
  • TeamBRAZOTeamBRAZO Member Posts: 18
    receiver pipe

    Thanks RAF - appreciate your help.

    I'll look into a receiver pipe - I'm not familiar with that at all. I'm hoping that the building stays warm - it has for the last 5 days, but it was in the low 30's.  This week it will be in the teens, so we'll see. 
  • RAFRAF Member Posts: 65
    Take a look

    Go to

    Weil Mclean Boilers

    Product Literature

    Current Boilers

    EG Boilers

    Installation Manuals

    Page 16

    Receiver Pipes I am sure there is other locations for this information also.

    Good luck would like to know how you make out with this. Rich
  • TeamBRAZOTeamBRAZO Member Posts: 18
    Update - partial success

    Just a quick update to our boiler flooding problems.

    I added a VXT water feeder, set the feeder to 8 minute delay and the water feed rate to 1 gallon per feed.  I made those decisions after "watching" a few cycles to see how long the water took to return to the boiler.  My concern then switched to "will it heat the building" with the shorter cycles caused by the LWCO/VXT delay.

    I was very cold last week and the system ran without problems.  I had no complaints from the tenants and the water level stayed correct.  The only question now will be the makeup water rate - over 6 days it has used 3 gallons of water.  I still need to get into the apartment and see if anyone has the vent off a radiator or if there are any leaks - I'm guessing this will be the case.

    So, I now feel IN CONTROL - took a months time of major research and trail and error to get here but so far so good.

    Now, I have an order in for the Fittings and Nipples required to re-pipe the near boiler piping - which is certainly wrong and causing wet steam and the boiler to cycle on LWCO.  I plan on doing that project at the end of the heating season when I can turn it off and work over a number of days/weeks to get it done.

    The VXT is awesome!  Not necessarly the fix, but makes it somewhat stable while I tweak the rest.
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