On February 3, 2015, EPA signed a final rule that made revisions to the New Source Performance Standards (NSPS) for new residential wood heaters. This action updates the 1988 NSPS to reflect significant advancements in wood heater technologies and design, broadens the range of residential wood-heating appliances covered by the regulation, and improves and streamlines implementation procedures. This rule requires manufacturers to redesign wood heaters to be cleaner and lower emitting. In general, the design changes would also make the heaters perform better and be more efficient. The revisions will streamline the process for testing new model lines by allowing the use of International Standards Organization (ISO)-accredited laboratories and certifying bodies, which will expand the number of facilities that can be used for testing and certification of the new model lines. This action includes the following new residential wood-heating appliances: adjustable burn rate wood heaters, pellet stoves, single burn rate wood heaters, outdoor hydronic heaters (outdoor wood boilers), indoor hydronic heaters (indoor wood boilers), wood-fired forced air furnaces, and masonry heaters. These standards apply only to new residential wood heaters and not to existing residential wood-heating appliances. In this proposed action, we are proposing to allow retailers a period of time after the May 2020 compliance date to sell units that were manufactured before the May 2020 compliance date.