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Cast Iron vs Stainless Boiler

jac34 Member Posts: 63
Changing over from oil fired CI boiler with indirect DHW serving 24 rads and 2" mains. Making the switch to propane.

Getting conflicting views on CI replacement vs. stainless steel mod/con. Does one work better than the other with large CI radiators and large main pipes?

I'm hellbent on going as green as possible so I'm leaning towards high efficiency stainless mod/con, but a few installers think CI is better with this piping set up. Any opinions?


  • Jamie Hall
    Jamie Hall Member Posts: 18,935
    I presume (hope!) that your existing system is also hot water, and not steam? You don't specify. If you are switching from steam to hot water, believe me -- the material of which the boiler is made is going to be the least of your worries. And troubles...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jac34
    jac34 Member Posts: 63
    Agree with that! Yes, hot water currently.
  • Jamie Hall
    Jamie Hall Member Posts: 18,935
    I think that which will work best for you will depend entirely on how much -- if any -- condensing you might be able to get out of a mod/con. To determine that, you will need to determine your heat loss and from that what temperature your radiators will have to run at.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Steve Minnich
  • bob eck
    bob eck Member Posts: 929
    How efficient is your current oil boiler?
    If staying with oil a good three pass oil boiler like the Trio oil boiler can reduce the gallons burned in a year.
    Will an 95% AFUE LP gas boiler use a lot less fuel then a cast iron Weil Mclain GV90+ 90% AFUE cast iron boiler or even a 85% AFUE cast iron boiler?
    1 gallon of fuel oil 140,000 BTU
    1 gallon of LP gas 92,000 BTU
    You need to work with a good heating contractor that can do a heat loss on your home. You might find out that your oil fired boiler is over sized and has short run cycle times and if so then your old oil boiler could be very inefficient.
    You might be better off putting in a properly sized oil boiler than going to a 85% or 90% cast iron boiler or 95% condensing LP gas boiler.
    With the Trio oil boiler you can use the Energy Manager Plus II control and you can give the indirect water heater priority plus you can do outdoor reset and most likely run lower water temp to your cast iron radiators that will lower the gallons used per year. The Trio boiler can be run with oil burner and in the future you can install an LP gas burner if you end up wanting to go to LP gas in the future.
    Get a good heating they can help you choose to stay with oil or to go to LP gas with a 85% or 90% cast iron boiler or a 95% condensing gas boiler.
  • flat_twin
    flat_twin Member Posts: 318
    If your cast iron rads are capable of producing far more btu's than your home requires, a modcon boiler is probably a good match.
    This was the case when we replaced our fuel oil boiler with a NG fired modcon. 1850's home with 10 cast iron rads and large pipes.
    Heat loss for our house ranges from 57k to 65k btu depending on what heat loss calculator was used. Radiator output with 170 degree boiler water is right at 100k btu.
    The over sized radiation allows the modcon to heat our house with 90 to 130 degree boiler water depending on outdoor temperature. The lower the boiler water temperature, the higher the efficiency. No two homes are the same but we saved 47% on home heating costs with the modcon.

    Cast iron boilers must operate above 140 degrees to prevent damaging condensation. Modcon boilers are most efficient when they're able to operate below 140 and take advantage of condensation.

    You can use these charts to calculate your radiation output.
    http://www.columbiaheatingsupply.com/page_images/Sizing Cast Iron Radiator Heating Capacity Guide.pdf

    Slant Fin makes a handy detailed app for calculating heat loss.