There should not be a "head of air' in the tank. The water should push out any air. More likely the dip tube is broken. The dip tube is a plastic tube on the cold water inlet side of the tank that takes cold water to the bottom of the tank which, in turn, pushes the hot water out. When the tube breaks, the cold water immediately mixes with the hot water you have stored in the tank and causes a "cross connection" where the cold goes straight out the hot outlet instead of mixing with the hot water. In the newer tanks, they are installing back flow preventer fittings and you cannot see or access the dip tube. On the older tanks, you can see and pull out the dip tube with your finger as it just sits in the opening. A few years ago, there was a problem with the plastic being used for these tubes and they disintegrating within a couple years. Lawsuits abounded.