re plumbing the gray water recovery unit into a cold water intake tank.
Yes - that is something I would absolutely do differently today. At the time, I literally had the cold water line to shower running in basement right next to the shower drain.
On the positive side, it is very tangible on how much energy I am saving as I have to back off the amount of hot water by lowering mixing valve.
Today, I would put a similar device on the dishwasher drain (not the clothes washer as we use cold water) and each shower. And yes, all prewarmed water into water holding tank before demand hot water heater.
The temperature of the holding tank water may not go up much, but those saved BTUs remain.
The sand in the soft copper was a pain, but lots of shaking and tapping got it all out. First, I tried to blow it out with water pressure - not a good idea as it just packed in the sand. To dry it out I had to put the unit in the oven at 200 F for about a week
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I have seen other hot water recovery designs and one I like is a copper based counter flow "tube within a tube". I will try to find that and post it. The problem with that design is the lower amount of contact area for conductive heat flow. But a LOT simpler and quicker to make (no filling soft copper with sand and coiling it).