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Btu's in hand, how to size radiator?

mikemac52 Member Posts: 47
I have my coolcalc manual J in hand and the Btu requirements per room. How close to come to the btu numbers required with the capacity of the radiators? Should I leave any headroom? The cast iron radiators were quite large and will be downsizing some of them for space saving reasons. (I mean, the wife told me to...)


  • JohnNY
    JohnNY Member Posts: 3,231
    Size them to the heat loss of the room within 5%.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
    And, since this is tagged under hydronic, not steam, be sure to account for the temperature you plan for your circulating water.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,572
    If you are only downsizing some of the cast iron radiators, be careful of your zoning. If you have existing radiators which are 200% of design with new that are 105% of design, you would be wise to zone them separately or add TRV's
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mikemac52
    mikemac52 Member Posts: 47
    Thanks for the quick replys! Hoping to keep the water at 140 F. Would have been 130 F but for a bad interior design Choice...