Both of my tanks were open and one tank was full. Judging by weight of the tank. The tank that was supposed to be running the system was about half to one third full. I may have changed the auto changeover valve quickly, but I was already having problems before the switch. Another thing, the couple next to us also ran out. He had his reserve bottle off and had to turn it on. Not sure exactly how the changeover valve works, but he did have to turn it himself. He had a gauge to check his tank and the first tank was low, but not completely empty. His gauge could have been wrong and the tank empty, but I do know that mine still had at least 1/3 left maybe more.