I realize I’m late to the party but just stumbled on this forum.
If you use a bare tank as a preheater think about condensation. Probably be a good idea to install a drip pan.
We have an electric water heater and use winter and summer preheaters.
In summer we have a simple batch heater in the greenhouse, and in winter use wood stove preheater.
Here in Southern NH is gets pretty humid so cold water condensation is a problem.