"Ken Hall" wrote in message
Vacuum. Put a straw in a glass of water. Put your finger over the end
and
lift it out. Same thing with the heater. If air is not allowed in, the
water cannot get out.

Why doesn't opening hot water faucets prevent/releive the vacuum?

Ken

The hot water faucets are the bottom of the straw, You need to open the
top. You can open ten faucets, but they are all fed from the same line
coming out of the tank and no way to get air back into it.