Radiator EDR to Heat Output
The heat a cast-iron radiator delivers from its EDR (Equivalent Direct Radiation, sq ft) rating, and the gross boiler to feed it -- the counting-radiators-to-size-a-boiler direction, the complement of boiler-horsepower (which converts a boiler nameplate the other way, steam-only). Q = EDR x k, with a fixed constant per system: 240 BTU/hr per sq ft on STEAM (Hydronics Institute / I=B=R, 215 F steam in a 70 F room) or 150 on HOT WATER (170 F avg water, 70 F room) -- the hot-water basis boiler-horsepower's steam EDR omits. 320 sq ft EDR on steam = 76,800 BTU/hr; on hot water = 48,000. The boiler adds a PICKUP allowance for cold piping: gross = net x (1 + pickup), I=B=R ~0.33 steam / ~0.15 hot water, so 76,800 steam wants a boiler near 102,000 BTU/hr gross, selected by its NET rating. A sizing aid; the radiator EDR, the real piping/pickup, and the boiler's I=B=R net rating govern.
Formula and source
heat_output_btu_hr = edr_sqft x system_k (k = 240 steam, 150 hot water); gross_boiler_btu_hr = heat_output_btu_hr x (1 + pickup_factor). I=B=R pickup ~0.33 steam, ~0.15 hot water.
Equivalent Direct Radiation (EDR) heat-output conversion (Hydronics Institute / I=B=R basis), by name; the radiator EDR from the maker or a measurement, the actual piping and pickup, and the boiler's I=B=R net rating govern.
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