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underfiring a steam boiler

Adam_13 Member Posts: 49
Hello all!  We're looking at an oil to gas conversion in small house with one pipe steam. Th current boiler is firing at an input of 140,000 btuh with a doe output of 115,000. The steam system load is 87,000 (including a missing radiator and the piping pickup) and the boiler has a tankless coil for DHW. We're going to abandon the tankless in favor of a stand alone gas water heater, and so the boiler will only be used for heating. The boiler is about 10 years old. Assuming the boiler to be 80%, the gas conversion burner should only need to fire at 107,000. The near boiler piping is copper and done poorly, so it will need to be repiped. We're wondering

1. Can we fire the boiler at 107, 000, or will it be stuck on "simmer" instead of steam?

2. It's a newer boiler but needs reping and it is oversized. Should we just replace the boiler with an atmospheric gas burner model instead of going with the conversion burner?

We want to keep the steam and are wondering what would be the most cost effective route. Any and all thoughts/comments would be greatly appreciated. Thanks for your time, -Adam


  • JStar
    JStar Member Posts: 2,752

    What make and model is your boiler?
  • Adam_13
    Adam_13 Member Posts: 49
    the boiler is a

    Utica SFH3100STLB
  • JStar
    JStar Member Posts: 2,752

    According to Utica's literature, their 140K and 90K boilers are both 3 sections and have the same water content (10.5 gallons). Dropping inbetween to 107K should be fine.

    10 years is sort of the beginning of the "what if" stage. It should, realistically, have another decade of useful life, but may develop those pesky control and mechanical failures in the near future. You can get great benefits from converting the existing boiler to gas. Just be aware that you will spend a decent amount of money to do it. Add to that, the other piping repairs, and you may be close to the price of a new boiler. If it was just the conversion burner, I would lean towards yes. With all repairs...maybe...
  • Steamhead
    Steamhead Member Posts: 16,820
    But you might be able

    to use the conversion burner in another boiler, if the present one dies.

    Regardless, the conversion will run more efficiently than an atmospheric- I'd say go for it!

    Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Adam_13
    Adam_13 Member Posts: 49
    I agree that 10 years

    is at that "what if" point, however the conversion burner and repipe would probably be the least expensive route now. Since the boiler can be down fired, and since the bsmt has been finished, we're leaning towards the conversion burner.

    Thanks to both JStar and Steamhead for your help and for checking into the boiler info

    Are the gas conversion burners noticeably more efficient than atmospheric? I tend to prefer the atmospheric burners because they are silent and usually can be converted to millivolt aftermarket, which works great with steam when there's a power outage.
  • Ifollowinstructions
    Ifollowinstructions Member Posts: 27
    Conversion burners

    If your looking to install a gas conversion burner  in a oil boiler good luck!  You may as well put a coal fire in there.  All the calculations in the world will not save you on this one.  You will get heat eventually!  Spend the extra few bucks on a new boiler. 
  • Ifollowinstructions
    Ifollowinstructions Member Posts: 27
    Great idea

    Great idea!  buy another oil boiler and install the gas conversion back in it. Maybe keep a few gallons of oil available..if gas runs out  install the oil burner back in.
  • JStar
    JStar Member Posts: 2,752

    This statement is neither fair nor accurate.

    Gas power burners in oil, wet-based, boilers work very well. There's quite a bit of history on this forum to back that up, too.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    What would your choice be for a replacement boiler

    Since you have reservations about replacing the oil burner with a gas burner, what would be your ideal replacement boiler?--NBC
  • Paul48
    Paul48 Member Posts: 4,469

    Smith series 8 with an oil burner or a G-8 with Carlin ez-gas, same exact boiler supplied by the manufacturer either way. That's all I have to say.
This discussion has been closed.