Which of the following fittings is not allowed to be concealed according to gas fitting regulations?

Concealed gas fittings are prohibited from being hidden within walls or other structures due to safety concerns. The regulations are designed to ensure that such fittings remain accessible for inspection, maintenance, and emergency disconnection. In the event of a gas leak, it’s crucial to have immediate access to all components of the gas system for quick action. Concealed fittings could impede this access and increase the risk of dangerous situations going unnoticed.

Other fittings like unions, compression fittings, and right and left couplings can typically be installed in a way that they can remain accessible. These fittings have specific uses and can often be located in areas that allow for easy maintenance without compromising safety. Therefore, the clear preference in the regulations is to keep gas fittings that require regular inspection and maintenance above ground and readily accessible.