A boiler for generating steam at gage pressures in excess of 15 psi (gage pressure of 103 kPa), or for heating water to a temperature in excess of 250°F (121°C) or at a gage pressure in excess of 160 psi (gage pressure of 1103 kPa)
NFPA 31 Standard for the Installation of Oil-Burning Equipment, 2001 ed. https://www.nfpa.org/Assets/files/AboutTheCodes/31 /LPI-AAA_ROPagenda_06-09.pdf, (Std: Install of Oil-Burn Equip - Amer) {195 NFC}