Air in hot water lines

Thanks for replies. I had a little water in the holding tank, the water heater was working fine for 2 days on my trip, except for the cloudy hot water which really was just a nuisance. After reading all the comments, I filled my holding tank up. Then I ran the system on the pump. The air was gone. Went back on "city water" and also, no air. Fixed itself. I had previously checked and double checked to be sure there was water in the water heater. So unless there was no air cushion as was suggested, I cannot say exactly why it cleared up. I am amazed at all the good info that was offered. Thanks.