It was free, but would not drain out of the bottom spigot. We twisted that off and white crystal clumps and whitish water came out.

It is still heavier than it should be. Should I let a gallon of muriatic acid soak inside for a couple of days to dissolve the deposits? Or just toss it?

I cleaned some calcium out of a samovar a while ago. It was a pain.

Vinegar wouldn't hardly touch it.

I did end up using some Muratic Acid (HCl). It still wasn't the easiest stuff to dissolve.

I'm sitting here wondering what Calcium Deposits (weak base) would do to your reaction, or what low levels of Ca++ would do to your fuel.

Perhaps not the best idea to get Ca++ deposits in the engine (if that would happen) :P

I got all the fittings off and took out the heating elements. I rinsed out a pile of rock-salt-looking crud. Every fitting had a brown-white paste coating the inside. The sacrificial anode was fully sacrificed, down to a tooth-pick size wire.

I reinstalled a newish PRV and gauge. I plugged everything up, put in about 5 gallons of water, and added 2 gallons of muriatic acid. It has been generating a very slow release of gas (hydrogen?) that has not registered as any measureable pressure. It is standing upright, and I am going to let it sit for a few days.

I let the WH sit upright for a few days, and then on its side. After a week I have now filled it with water with the muriatic still in it. I have been sloshing it around a couple of times a day. So the acid has had opportunity to work on all parts of the interior.
i have checked the pressure every day or two. It got up to 10 psi once (upright). I vent it each time. When I put it on its side it went up a little for about a day. Now, full of water, it hisses only slightly when I vent it. I think it is about done.