Harvia WL800 36/duo Protective Sheath (side)Harvia WL800 36 duo Protective Sheath (side) The Harvia WL800 protective sheath is designed and approved for wood heated Harvia wood burning heaters (Harvia Pro 36 36 Duo Steel). The protective sheath considerably reduces the safety distance to combustible materials in practice, this makes the wood burning heater fit in a smaller space. The protective sheath is attached to the wood burning heater and conforms to the shape of the wood burning heater,