An air gap in a drainage system is the unobstructed vertical distance of free space between the outlet of the waste pipe and the flood-level rim of the receptacle into which the waste pipe is discharging. As an example in a commercial restaurant, the air gap would be between the drainpipe of a salad bar and the floor sink or tub drain.

There are deep wells (more than 25 feet deep) and shallow wells (25 feet or less). A typical well pump can pull up about 25 feet of water. If the well is deeper than 25 feet, the pump will have to be installed at the bottom of the well to push the water up. Pumps are better at pushing than pulling.

Inspection of private water sources, such as a well, is beyond the scope of a home inspection.

Check for physical damage to the tank, particularly rust and corrosion on the bottom of the tank. Check for water marks on the floor of the tank. Check for a leaking TPR valve and discharge pipe is installed. The covers at the electric heating elements should not be disturbed. Corrosion can be found where the water pipes are connected to the top of the tank. The electricity or the fuel may be shut off. There could be scorching at the burner cover area. The heat rollout shield should be in place. The draft hood and vent connector are often loosely attached. All tanks should be accessible with at least 24 inches of working space around them.