During operation of the system, wood pellets are burned in a stove and the flue gas produced is drained to the environment through the inner tube (𝑖. 𝑡.) of a stainless steel TTHE-HL, arranged vertically. Ventilation air flows in the inner annular section (𝑖. 𝑎.) and is heated by the flue gas, as well as by the combustion air that flows in the outer annular section (𝑜. 𝑎.). The flue gas flows counter-currently to the ventilation air and combustion air. The parameters used in the Analytical Solution (AS) and Numerical Solution (NS) of the TTHE-HL models are listed in Table .