Harvia Protective Sheath Pro 20One Side, Protective Sheath for Harvia Sauna Stoves, 20 Series The Harvia WL500 protective sheath is designed and approved for wood heated Harvia wood burning heaters such as the Harvia 20 Pro SL Duo, 20 Boiler 20 SL Boiler, and 20 RS LS Pro. The protective sheath considerably reduces the safety distance to combustible materials. The WL500 is comprised of steel material coated in black paint. The Harvia WL500 protective sheath is designed and approved