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Suggestions for dealing with oversized steam boiler. Plugging up burner?

ChrisF Member Posts: 46
Since moving into my current home, 2 years ago, I've always had some problems with convector radiator noise (water hammer) and spitting. The steam boiler heats well, but I was told by a heating technician last year, that the boiler is oversized for the house, and heating up too fast. The boiler is a Dunkirk model PSB-5D and only about 7 years old. It is a 160,000 btu boiler, and my heating space is under 1800 square feet. After trying many numerous things, I couldn't get rid of the water hammer. Other than replacing the boiler, the only other option was plugging up one of the four burners, and that is what the technician did. It seems to have resolved about 75% of the problem, but I am still getting noise from the convector radiator farthest from the boiler. The technician did mention he could plug up one more burner if needed. My question is, are there any other solutions to getting around an oversized steam boiler? Not sure what it would cost to replace the boiler with a smaller one, but I'm assuming it's between 5 and 7 grand, so I'm looking for other options. Thank you.

current Dunkirk boiler:

Style of convector radiators throughout house:


  • Jamie Hall
    Jamie Hall Member Posts: 23,322
    There are a number of ways to deal with an oversized boiler, and down firing it -- if the manufacturer of the boiler and the burners lists that as an option -- is one. If they don't, it isn't. Further, you can't just "plug a burner". Consideration must be made to maintain reasonably even heating of the boiler, and the combustion has to be verified and adjusted with the necessary instruments.

    Now. That said. The oversized boiler is not the cause of either water hammer problems or spitting. The spitting -- and possibly the water hammer -- is more likely from two sources: excess pressure at the boiler and poor drainage of condensate. The excess pressure is easily remedied: the pressuretrol should shut the boiler off at no more than 1.8 psi, and it should cut back in at a bit of 0.5 psi. This is easily adjusted by the screw on the pressuretrol, and is the first thing to check.

    Convectors on one pipe steam systems are notorious for spitting and water hammer, though. I notice on that the convector in your photo has the vent at the same end as the inlet. This doesn't help, but may not be easy to fix.

    So... first, are there main vents on the steam mains? If not, you need them. Second, try slowing the radiator vents down -- smaller vents. Third, make sure that the radiator inlet valve is fully open. Fourth, go through the whole system -- all the piping -- and make sure that every foot of every pipe is properly sloped to drain.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ChrisF
    ChrisF Member Posts: 46
    @Jamie Hall Thank you for that information. I understand, but I'm not a heating expert, and was pretty much at wits end, so I went along with the plugged burner. It seems to have resolved much of the prior issues, but I am not sure

    I know that the previous homeowner had oil heat, and up until about 7 years ago, it was converted to gas and the new boiler was installed.

    The pressure troll was actually one of the first things that was checked and is currently adjusted as low as it can go.

    Would it be better to change the convectors with cast iron baseboard radiators?

    As for the main vents, I have two big mouth main vents that were put in last year. All of the radiator vents were replaced with smaller ones, as well. The slopes of all the pipes were checked on three different occasions, and everything is right with that.
    Also, I don't have any adjustment for the radiator inlet valve.

    Quite honestly, I'd prefer to just change out the whole system to make it right, because it doesn't seem like any technicians who actually have come to diagnose the problems have resolved it.

    big mouth main vents:

  • ethicalpaul
    ethicalpaul Member Posts: 5,704
    edited December 2020
    You situation is a shame to me because I really like the way your boiler is piped. You've got great height, and a beautiful looking header.

    I think you are certain to have to live with short cycling, which isn't really horrible for your boiler, I feel, compared to, for example, a hot air furnace, or a mod-con boiler.

    You could try a pressure-triggered delay. It's pretty easy to set up with a low pressure switch (by Dwyer or other) and a simple delay relay. It may not be as useful to you since you don't have cast-iron radiators that would hold heat for many minutes. The only way to know if it would help would probably be to implement it. It's under $200 in parts. Even less if you get the switch from ebay.

    It would take the short cycling off your pressuretrol (which is not really meant to handle this job), and it would let you experiment with the delay time.

    Here's an example relay: https://www.amazon.com/gp/product/B08P53JFD1/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • mattmia2
    mattmia2 Member Posts: 9,662
    How stable is the water level in the boiler while it is steaming? is it stable or does it bounce around?
  • ChrisF
    ChrisF Member Posts: 46
    @mattmia2 It bounces a little, but nothing drastic.
  • ChrisF
    ChrisF Member Posts: 46
    @ethicalpaul Thank you! I will look into that option.
    EBEBRATT-Ed Member Posts: 15,540
    Yeah, somebody did a decent job piping the boiler so replacing shouldn't involve a terrible amount of piping.

    I would continue to work on vents and getting the system to operate properly with the boiler you have for now

    If you getting water hammer and spitting vents you have to fix that before the new boiler is installed.

    An oversize boiler will not cause water hammer and spitting if you keep the pressure low

    Where are you located? Try "find a contractor " on this site
  • WMno57
    WMno57 Member Posts: 1,269
    I've been following your other thread about Replacing One Pipe Steam. Your current 7 year old system looks very nice to my untrained eyes. I think you will be able resolve the issues when you find the right steam Pro.

    If the only solution is a smaller boiler, you could get some money back by selling your current 7 year old boiler. It appears to be in good shape, and someone out there needs your bigger boiler.

    I question if the gas lines meet your local codes. If nothing else, they should be secured better. If you do find the right steam guy, you may want to have him re-pipe those in steel.
    I DIY.