Miss. Code § 75-51-1
All water heaters installed in any dwelling or in any other building within the state shall be provided with an approved self-closing (levered) water pressure relief valve and temperature relief valve or combination thereof.
No individual, firm, corporation or business shall install any new automatic hot water tank or heater of one hundred twenty (120) gallon capacity or less without installing thereon a pressure-temperature relief valve so labeled by the manufacturer's identification stamp or cast upon the valve or upon a plate secured to it.
No individual, firm, corporation or business shall install, re-install or re-locate any existing automatic hot water tank or heater of one hundred twenty (120) gallon capacity or less without installing on such tank or heater an approved type water pressure relief valve and temperature relief valve or combination thereof.
No individual, firm, corporation or business shall install, sell or offer for sale any relief valve, whether it be pressure type, temperature type or pressure-temperature type, which does not carry the stamp of approval of the American Society of Mechanical Engineers or the American Gas Association.
Miss. Code § 75-51-1