The A200 Outdoor Wood Boiler Treatment and Rust Inhibitor by Alternative Heating & Supplies has been specifically designed for outdoor and indoor wood boilers with mild or stainless steel construction. The A200 may also be used as a full spectrum water conditioning agent for other hydronic and solar heating applications. The A200 is the only environmentally friendly outdoor wood boiler treatment and rust inhibitor. One quart treats 200 gallons. Buying 2 quarts at a time saves on shipping... long shelf life so you're ready for next year!