BTW I've been assuming that in the event of the apocalypse, and the water supply being cut off, we'd still have a hot water tank full of water. But it's an older house, and we hadn't got around to installing a backflow valve (it was on our list), and having a break in the water supply meant the contents of the tank was siphoned out of the house and onto our driveway.
So if you're prepping for the apocalypse make sure you have a suitable valve.