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Calculate btus for OIl and Gas steam

Carl1680 Member Posts: 2
Hi I have a 200 year old house and have done some work to update the insulation in the walls and windows and doors. I have a oil fired steam boiler from like around 1950's I havnt decided to stay with oil or go to a new gas steam boiler just quite yet. I went to Utica site and found a calculator to determined the amount sq ft radiation I need. I need 725 sq ft of radiation, but before I call in a steam boiler specialist I have 2 questions hopefully can be answered. 1 how many btu's will I need so I know the person I hire gives me the right amount of btu's and 2 If I decide to convert my radiators to hot water and use a high efficient gas boiler like Viessmann how do I figure out that amount of Btu's as well. Also there is no water tank being installed that is completely temperate.


  • Joe V_2
    Joe V_2 Member Posts: 234
    You will find this link very helpful

    read throughit.  your answer is around the second or third page.

  • Carl1680
    Carl1680 Member Posts: 2
    Converting steam to hot water

    HI Joe, How would I go about calculating the radiators from being used as steam to being used as hot water?
  • Jamie Hall
    Jamie Hall Member Posts: 23,302
    A few more thoughts...

    First off, the size of the steam boiler required is determined by the installed radiation, not -- repeat, NOT -- by the heat loss of the structure.

    Now if you change the amount of radiation, basing it on a conservative estimate of the heat loss in each room, then you can add up the installed radiation area (termed EDR) and determine the boiler size based on that.

    On the other hand, if you keep the installed radiation -- which I would recommend, unless you are doing a pretty drastic remodel -- just add up the EDR of the existing radiation, and there is your boiler size.

    Really simple.

    But to repeat: the boiler size for steam has nothing whatsoever to do with the heat loss of the house!  The installed radiation may -- you can size that to heat loss -- but the boiler must be sized to the installed radiation.

    It doesn't matter whether the fuel is oil or gas.  A BTU is a BTU, and an EDR rating is an EDR rating.  Which fuel you use is a matter of preference and cost and availability.

    Now converting to hot water... we generally do not recommend doing so; there are just too many ways to get into varying amounts of trouble ranging from relatively mild pains in the pocketbook to truly catastrophic.  There is no good reason to do so; a good steam system, properly adjusted, will be within a few percentage points of an equally well adjusted high efficiency hydronic system, and it is unlikely -- at best -- if you would ever recover the additional cost (which could be very large indeed) in fuel savings..

    However, if you were determined to do so, a BTU is still a BTU.  However, your installed radiation will not deliver the same BTUs on hot water as it will on steam.  On steam, the figure is 240 BTU/hr/square foot EDR rating.  On a high efficiency hot water system (trying to condense to get the efficiency), the figure will be about half that -- say 130 to 140 BTU/hr/square foot EDR rating.  Therefore you will need to evaluate the ability of your installed radiation to meet the heat load of the structure; then you could size the BTU rating of the boiler to suit.

    For budgeting for the conversion, you would need to have the existing system evaluated pretty carefully, but for starters, assuming that the existing radiation is usable and doesn't leak at the higher pressures, you will need pretty much all new piping.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • KC_Jones
    KC_Jones Member Posts: 5,737
    Just a homeowner...

    Another thing to mention on a HW conversion is if the radiators can even be converted.  My house has steam only radiators that can not be converted (not that I want to).  You also don't mention what type of steam you have?  Do you have one pipe, two pipe (with or without air vents).  This information would help get you even more advice.  Also as mentioned hot water conversions can be risky.  Don't get caught up in a modern high efficiency boiler either (hot water).  Those depend on low temperature water to work correctly (140 return temp max I think).  The pros know all this better than I do, just mentioning some of the things I have read on this topic.  This seems to pop up every few weeks on this board.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15