Wood-Burning Sauna Stove w/ Firebox Extension, 9.8kW The HUUM HIVE Flow LS Sauna Stove with a Firebox Extension has an innovative design, ensuring that 75% of the heat from the firewood directly heats your sauna rather than escaping through the chimney. It stands out as one of the most efficient and environmentally sustainable heaters for sauna rooms from 282 to 635 cubic feet (CF). Required sauna stones (10 boxes) sold separately. Innovative patent pending furnace construction BImSchV Certificate for HUUM Hive Flow compliance Designed, engineered, and made in Estonia For sauna rooms from 282 to 635 cu. ft.