I could warm the sand in a small amount of hot water and then it would expel the heat off of it instead of absoribg it.
How does that save water, then? You're running hot water to heat up sand so that you don't have to run more hot water....
Now, the heating the sand in other methods has possibilities, but the logistics of moving all of this hot sand/hot rocks around has me puzzled.
Wouldn't you be better off just getting a hot tub/jacuzzi that heats the water in place?
I said small amount of water.
Anyway, it seems that the best insulating material is wool or felt scraps.
I could fill watertight bottles (I've got plenty of those) with the above, with just enough heavy material in the bottom to make it sink. The wool and felt will keep the water hot for about an hour, and the bottle would displace a lot of water.
Now, what heavy material can I use?
hot tubs and jacuzzi's are expensive, and I don't have the area available for either. Plus, they take a lot of water to fill. Plus, a hot bath is nice, but it's not something I do every day. I don't stay in the tub for an hour, nowhere near that.
I'm musing about ways to save water, and keep nice and warm lying still in a relaxing bath with my eyes closed for 20 minuetes or 1/2 hr. Who care what it looks like, it's not like there's a lot of traffic going through my bathroom when I'm taking a bath.