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Boiler Sizing Question!

Jamie_6 Member Posts: 710
In the last few years I have been using Slant Fins Load Calc. to determine the amount of heat necessary to heat homes with hydronic heat! I have never had a problem with this calc. but I also never ran it on a home like I did today. The house was a three story stone home built in the 30’s. It has huge radiators and enormous mains to go with them! The load calc. came up with a boiler about 84,000 Btu’s. I really don’t know what to do in this situation. This seems like the boiler would be way too small. Maybe it would be the right size with smaller radiation? It just seems like there would be way to much water in the system.

What do you guys think? Have any of you run into a similar situation?


Pompetti Heating & Air Conditioning, Inc.


  • Mike Kraft
    Mike Kraft Member Posts: 406
    Go with the load!


    Always size a home for the heat loss with hot water heat.Only size a steam boiler to the sq' of radiation.I recently converted a home with iron radiators......the radiator in the master bedroom had an EDR of 70K.The home needed a boiler @ 90K.You have a perfect scenerio for resetting the water temperature in this home.Don't be noivous:)

  • Be careful though...

    Although the heat loss calcs are correct, if there is a lot of high mass radiation in the system, make sure that if you're using a smaller boiler that you provide some means of avoiding long term condensation production.

    If it were my job, I'd consider using a condensing appliance and REALLY knock the snot out of his fuel bills (50% reduction common..)

    If you decide to not go with the condensing technology, make sure you provide some means of protection for the boiler, otherwise, the boiler will die an early death from the side effects of condenstion production, or thermal shock and stress to the boiler section.

    As for protection, it can be as simple as a thermostatic device installed in the return line of the boiler, or a 4 way, or VSI. Your experience will rule.


  • Steamhead
    Steamhead Member Posts: 14,840
    Jamie, ME and Cheese are absolutely right

    You don't need a boiler any bigger than your heat-loss.

    However, I think you have an old gravity system here. It's way too easy to over-pump such a system, which will reduce efficiency. Size your circulator (system circ in a primary-secondary application) to the total EDR rating of the radiation, and you'll come close to mimicing the original design flow with gravity circulation. The system will actually heat faster this way- ask me how I know that!

    Go here-

    for a handy chart and a more detailed description of how this works.

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  • Jamie_6
    Jamie_6 Member Posts: 710

    Thanks SteamHead! I'm going to try and bid the job that way!

This discussion has been closed.