Applies to low-pressure solar gas water heaters, with or without a backup water heater using liquefied petroleum gas (LPG), natural gas, or electricity as an energy source, operating at working pressures below 3.0 kgf/cm², intended for domestic or commercial use, of the thermosiphon and self-contained types, fitted with a thermal storage tank having a capacity not exceeding 0.5 m³, that are imported into, or manufactured or marketed within the United Mexican States.

Proyecto de Norma Oficial Mexicana PROY-NOM-039-ENER/SE-2025, Rendimiento térmico, ahorro de gas y seguridad de los calentadores de agua solares de baja presión y de los calentadores de agua solares de baja presión con respaldo de un calentador de agua que utiliza como combustible gas L.P. o gas natural. Especificaciones, métodos de prueba y etiquetado (Mexican Official Standard PROY-NOM-039-ENER/SE-2025: Thermal performance, gas savings and safety for low-pressure solar water heaters and low-pressure solar water heaters which use a liquefied petroleum gas or natural gas water heater as backup. Specifications, test methods and labelling)

The notified draft Standard establishes minimum values for thermal performance, gas savings, test methods, labelling, and safety specifications.