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over sized steam boiler

working with an older house with a replacement boiler that was installed about 5 years ago, since then the house has gone through some changes and the owner has removed over half of the radiators.

he is currently having issues with the water feed valve over feeding the boiler and the relief valve opening up

the areas where the radiators have been removed have been converted to circulating hot water. the circulating water loops are coming off of the bottom of the steam boiler with a plate fin heat exchange to separate the boiler from the hot water loop

the boiler is rated at 400k btus with a gross output of 310k btus. natural gas

the connected edr to the boiler as of now is around 400 edr

the hot water circulating loops are around 100,000 btus

the boiler is rated at 970 edr

is there any way to de-rate the boiler to shed some capacity to get closer to the connected radiation

i am thinking that even if he has added the circulating loops to the bottom of the boiler this still has no affect on the undersized connected piping, because when the thermostat calls for steam to the connected radiation the boiler will be at rated capacity and delivering the edr that the boiler is capable of delivering

the original set up was a parrell flow/upfeed system, where the piping was split to where the supply main went to the back of the house and then split at the back of the house where each main wrapped around the perimeter of the house and back to the boiler

the piping is still set up as a parrell flow /upfeed feed system but without all of the piping it had at one time

the owner disconnected 1/2 of the house and left the other half connected

has anyone out there run into this issue before?

what is the best approach to handle this and keep the owner from having to replace the boiler


  • VA_Bear
    VA_Bear Member Posts: 50
    What is causing the overfill situation?

    Is the beast a gun burner or an atmospheric and what is it's fuel du jour?
    PORKCHOP Member Posts: 21
    oversized boiler


    i think the overfilling is coming from the following: water that is in the boiler is leaving due to the velocities in the undersized piping, the piping was correct at one time until the homeowner disconnected over 1/2 of the piping system and now the boiler is still operating at the same capacity and generating the same rated EDR that the boiler was rated at when installed a few years ago so when the boiler comes on the water is leaving the boiler and the feed valve opens to maintain water level then after several fill cycles the boiler, connected piping and radiators are all full and the 15 psi relief valve is relieving off of water pressure instead of pressure created from the steam

    i have sent a note to the boiler manufacture to see if there is a way to derate the boiler to reduce the capacity, hope to hear something back tomorrow.

    but was hoping to hear some input from some other heating contractors that may have seen something similar.

    PORKCHOP Member Posts: 21
    oversized boiler

    fuel type is natural gas
  • JStar
    JStar Member Posts: 2,752

    In order to create 15 psi, the entire system would need to be filled with water, right up to the highest radiator. Seems unlikely. Check the pigtail. You may have two separate problems.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,296
    Here's an article that might help.


    I think you're on the cause of the problem. Not sure if you can derate this one enough. It's liable to turn into a condensing boiler.
    Retired and loving it.
  • VA_Bear
    VA_Bear Member Posts: 50
    edited November 2013

    Atmospheric burner systems don't like modifications. Blocking a port to a burner will create hot and cold zones and can cause material stress which results in rapid failures of the heat exchanger. Reducing the gas pressure can and often does lower the flame propagation rate enough to allow the flame to move into the air mixing venturi and the flame being out of place for the design can cause some very nasty fires and explosions. Neither leave pretty results. Sounds like time to run a heat loss calculation and get the right size boiler. Sorry. Sounds like a good time to correct the piping and sizing problems as well.
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