Harvia Protective Sheath MProtective Sheath for Harvia Sauna Stoves, M Series The Harvia WL400 protective sheath is designed and approved for wood heated Harvia wood burning heaters (Harvia M1, M2, M3 and M3 SL). The protective sheath considerably reduces the safety distance to combustible materials which allows for more peace of mind in smaller environments. The WL400 attaches to the wood burning heater and conforms to the shape. With its slim look and steel black material,