I have been helping a small church with a problem and need some advice. We suspected a leak in a copper hot water line in the slab. I put a gauge on the hose bib at the mop sink , opened the valve and closed the input valve to the hot water heater. The pressure drops (about 4psi over two hours) but comes back up later (quite a bit, almost double). I expect this to be caused by cooling and heating in the tank. Should I turn off the heater for a while and run the test and if so how long?