Harvia Pro 26 Series Protective SheathProtective Sheath for Harvia Sauna Stoves, Pro 26 Series The Harvia WL750 protective sheath is designed and approved for wood heated Harvia wood burning heaters (Harvia Pro 26). 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,