Hot Water Issue

Thanks for the replies! I forgot to mention I have been around the check valve block several times. It is working as designed. I have removed the entire water heater when I thought I would need to replace it. When there was nothing wrong found, I replaced the unit, and checked the valve and the plumbing around the heater. I understand how the check valve works for winterizing and it is a normal in-line gate valve with one red handle to move the internal parts 90 degrees. All that is okay. I understand the check valve is the first place to look, and I have done that. I have also checked the outside shower (which has never been used) and it is turned off. I'm just saying, the only thing that gets me a full tank of useful hot water is to drain part of the water out, refill with the air space at the top and reheat. I just don't know why this is so. Thanks, again, david