Many, but not all, flexible gas appliance connectors are rated for exterior installation. You should check the manufacturer’s tag on the connector to verify. The yellow and gray appliance connectors manufactured by Dormont are exterior-rated, for example, but not the blue ones. 

    The appliance connector shown above, connected to a tankless gas water heater at an exterior wall, is definitely rated for the location. But it is 1/2” diameter although the minimum service required by the manufacturer is 3/4”, and  protrudes from the wall about 14”, which makes it subject to damage. So that particular installation is not acceptable.

    To learn more, go to What are the requirements for installing a gas appliance connector?

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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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Here’s links to a few of our other articles about gas piping:

• How can I tell the difference between Corrugated Stainless Steel Tubing (CSST) and a Flexible Appliance Connector (FAC)? 

• Is black iron gas pipe code approved for exterior (outdoor) installation?

• Can gas piping be installed in a supply or return duct or plenum? 

• Is it normal to smell gas near a natural gas meter?