This month is when I've switched to a hot bath instead of a shower because of the cold air, must be near 55 gallons of 130F. If somebody can save a couple bucks using near-free junk to capture some of this...

I've always read that crossflow heat-exchangers transfer more total heat than a similar sized uni-directional flow (don't know why...) so here's my idea.

Raise the water heater, set 55-gal drum on floor next to it after wrapping with insulation. Get a 30-gal drum, flip it over, wrap with insulation, bolt bottom of the 30-gal drum to the inside of the 55-gal lid.

The hot drain water flows down into the center section, hits bottom and makes a U-turn and flows up around the circumference where it exits the lid near the edge. The outlet turns, goes through an "S" trap then flows to drain.

To make it cross-flow, cold water that is headed to the water heater is detoured to go into the lid near the edge and coils down the outer chamber and then makes a "U-turn". Then it continues on, coiling up the inner chamber where it exits near the center of the lid, and the short insulated exit pipe immediately feeds the water heater.

The outer chamber is only warm because it has been cooled slightly from the incoming cold tube, and the then-warmed incoming tube-water is further heated by the hotter inner chamber. The warm outer chamber also acts as an insulating jacket around the inner hotter chamber. This would further be helped by the heat in the inner hotter chamber wanting to rise instead of mix with the outer chamber.

My shop gets grease in 30-gal drums (18-wheelers) and we throw them away all the time, although I would prefer the outer 55-gal drum to be one of those one-piece plastic types I get from the horse ranchers.