Direct-Fired Appliance : A fuel burning appliance in which the products of combustion (flue gases) are mixed with the medium (e. g. , air) being heated
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}