HEATSOURCE 1 stainless steel water chamber surrounds the firebox, producing the maximum hot water available from your fire.  The water chamber is built of heavy 10 gauge stainless steel 409 grade.  Grade 409 stainless steel is especially formulated to accept the expansion and contraction of a heating system without cracking.  This type of stainless steel to be used on a wood fired furnace there was clearly only one choice - 409.  While some furnace manufacturers boast 304 stainless steel, this type of has great surface appearance for restaurants and hospitals, and is much more expensive, but it is not the best product for a wood furnace as it will crack; it cannot take the expansion from the heat.