[LazyPup]

It is definitely not coming from the water heater. Here again, the internal pressure of a water heater is equal to the applied pressure from the water source. Given that the internal pressure is 2 to 3 times greater than the air pressure surrounding the tank any opening would result in water leaking out, not air going in.

Theoretically if a water heater were heated too hot it could produce steam which could leave the illusion of air in a line, but even if there was steam in a water heater that steam would immediate condense back to water when it goes through the water lines and begins cooling. In addition, the boiling temperature of a liquid is proportional to the pressure. While water boils at 212degF at standard atmospheric pressure at 35psi the boiling point of water is approximately 240degF and at the code maximum of 85psi the boiling temp of water is over 300degF

For that reason all water heaters are required to have a TPRV (Temperature/pressure relief valve) which is preset to disharge at 210degF or 150psi.