Unconfined Space : Any space whose volume is equal to or greater than 50 ft3 per 1000 Btu/hr (4. 8 m3 per kW) of the aggregate input rating of all fuel-burning appliances installed therein
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}