[jeff5may]

That's exactly why I asked the question. For a 5000 BTU batch tank, the heat transfer and mass flow rates are small enough that a single "Slinky" tube condenser doesn't have to be huge to be effective. Below about a ton of refrigeration, the "mini" systems hit the range where the system is more effective than it costs pretty quickly. Adding more efficiency past a certain point becomes exponentially more expensive. Not so true with the 2 tons of capacity. I'm not trying to cram 50 meters of copper tubing through a little bung and spiral it all inside a residential water heater tank.

The 2 ton system I will be tinkering with falls above the "mini" size class. It's a super common capacity for apartment complex and condo communities as well as smaller single family homes. In the commercial sector, this size condensing unit powers beer caves and glass door beverage coolers around the world. To most small business owner and upper middle class individuals, as well as most home owners, the outdoor units are priced low enough to justify replacement without a lot of consideration. As such, working units can be found second hand very easily and are highly affordable.

Call me a scavenger, tell me about how you HAVE TO replace your primary vehicle every 7 years and your HVAC system every 10. Tell me about the risk of having to do maintenance or being stuck on the side of the road, or burning up in the summer, or freezing in winter... I'll just nod my head and let you justify your monthly payments for all that stuff. Me, I pay rent and live my life like a nomad. Rather than go on vacation for a minute somewhere, I pack up and go: living there is so much more satisfying. This is America: even when there are no jobs, there are ways to earn money and live well. Even when the housing market is heavenly and tied up, there are still affordable places to live. Believe it or not, I have been there and done that. My retirement will be spent mentoring and consulting, paying it forward to the kids, watching them walk that beam way up in the air where I used to. Watching TV and playing bingo just doesn't do it for me. But I digress...

I am not dead set against using a BPHE to feed the water heater. For my purposes, it would for sure be more directly measurable, compact, and replaceable than a tube slinky. But for a DIY project, it would need a lot more brazing and pipe fitting than the slinky rig. A continuous run of copper tubing with a single u-bend at the end has only one point of failure and a super low chance of ever fouling, whereas the BPHE has all kinds of fittings that can leak, plus the relative certainty of needing cleaning in the future.

To refine my question, can someone relate to me the equivalent relative effectiveness of the two types of exchange methods? For an entirely wrong example, an answer would be something like "2 runs of 1/4 inch tubing 10 meters long would have about the same exchange rate as a 10 plate bphe 5x12 inch flowing 5 gallons per minute at blah blah dT". I don't need a dissertation, just some ballpark figures to relate the cost vs. effectiveness factors involved. Needless to say, an immersion element would rely completely on convection and direct radiation. A bphe would rely on a small circulation pump, which for this discussion is another element of complexity (and point of possible failure/maintenance/expense/control) to consider.