Originally Posted by Vicr
There can't be a pressure change when the water lines are empty. There wasn't a tornado or any other atmospheric change to cause the water heater to sense a change in pressure. You comments are not relevant or useful and I did read the manual thank you. If you can't answer my question in my original post please don't post conjecture, hyperbole and infer I can't read an owners manual, thank you.
Wow, what a hostile reply. I didn't suggest there was a change
in air pressure, just that the air pressure would be below the threshold pressure that would trigger the WH to come on. The first sensor in the WH water inlet is a pressure sensor. It most likely uses a pitot tube that senses water flow by a difference in water pressure that drops as it passes by the tube compared to a static pressure chamber in the sensor. Airplanes and boats use pitot tubes to measure speed by the pressure changes in the fluid in which they are traveling.
Fact is, when water system is drained, the water heater should be shut off or have the fuse pulled to prevent accidental turn on in an empty system.
I in no way inferred you hadn't read the manual, I was just providing a link to the manual that might have provided you the answer to your own question. I'll not argue with you, I've always believed Mark Twain's saying, "Never argue with a fool, onlookers may not be able to tell the difference."