The calculation standard: steady-state heat gain/loss and surface temperatures of insulated flat, cylindrical and spherical systems. Every figure on this site and in the free calculator follows its simplified still-air form (combined h≈10 W/m²·K); the full standard iterates surface coefficients with wind, emissivity and orientation. ISO 12241 is the international twin. When a spec says 'losses calculated to C680', this is what it means — and why two vendors' numbers should agree.
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