Btu's in hand, how to size radiator?

mikemac52

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

Size them to the heat loss of the room within 5%.

Jamie Hall

And, since this is tagged under hydronic, not steam, be sure to account for the temperature you plan for your circulating water.

Zman

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

mikemac52

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...