Within how many inches of a gas appliance metallic vent should PEX not be installed?

The correct answer is 6 inches, as PEX (cross-linked polyethylene) tubing should not be installed within this distance from a gas appliance's metallic vent. The reason for this requirement relates to the thermal properties and installation safety considerations of PEX.

PEX is susceptible to heat and can deform or degrade if exposed to high temperatures. Metallic vents associated with gas appliances can become quite hot during operation, which could lead to the PEX tubing failing if it is installed too close. Keeping a minimum distance of 6 inches ensures that the PEX remains safe from heat exposure, reduces the likelihood of potential damage or failure in the plumbing system, and maintains the integrity of the gas appliance’s venting system.

This safety guideline is an essential part of maintaining proper clearances for different types of materials used in plumbing and heating systems to ensure efficiency and safety in installations.