Your gas water heater should be 18 inches off of the ground, as per the National Fuel Gas Code. This is the standard for water heater installation requirements. This is a requirement for gas heaters that are kept in garages, but it’s still a good idea to follow this code wherever your water heater is located, if possible. Basement water heater installation doesn’t require an 18-inch elevation, but it certainly can’t hurt.
This rule is in place for the safety of you and your family, to help prevent fires and even explosions in the event that a flammable substance is spilled near your gas heater.
These rules are part of the Uniform Plumbing Code, and one of the reasons this code exists is for the sake of safe installation and maintenance for water heaters. It should be noted that per the Uniform Plumbing Code, the distance of 18 inches is not from the bottom of the heater to the ground, but from the ground to the pilot light or other lighting mechanism.
When installing your heater 18 inches off of the ground, it’s important to remember that the stand between your heater and the ground should be strong enough to hold up a full tank, not just an empty one.
While the 18-inch rule is part of the Uniform Plumbing Code, regulations can change from city to city. Make sure to check with your local offices prior to installation.