RAP developed a model rule for use by U.S. state and local air quality regulators to reduce nitrogen oxide (NOx) emissions from water heaters. This technical support document was published to assist regulators and staff in understanding and making use of the model rule. It describes why water heaters are a significant source of air pollution, why NOx emissions standards are an excellent tool for reducing the environmental impact of water heaters and how those standards can promote electrification and market transformation. It also explains in detail the design and structure of the model rule.