Whenever I turn on the hot water, the water spits out for a few seconds, expelling air. After this initial spitting, the hot water comes out fine. Where is this air coming from? It occurs at all hot water faucets in a two-story house. The cold water does not have this problem. Thanks.
Cold incoming water is about 5% air. As water heats, air entrapped in the water expands to rising bubbles. The bubbles settle in high places. So long as they are tolerable, they do no harm. Piping the water heater to trap air at the top loop while using an automatic air vent can reduce or eliminate the annoyance.