Yep and the answer was because the new code says double wall only. Basically the inspectors way of saying I'm not thinking about this.
I think I may have a solution that may be close in costs.
The old exchanger was 300' of 3/4" pex would be about $200
here's my new thought off the top of my head and 10 minutes of research
basically an approved double wall flat plate exchanger a second pump and some form of flow sensor telling me when to activate that pump (only when hotwater is demanded basically).
pumps are $65, ebay suggests a 40 plate exchanger is $120 but I need to do the math to see if it's large enough. flow sensors I've yet to research but I can't imagine they are too much. The bucket is a bucket and a leak detector should be cheap cheap available off the shelf anywhere.
This only works because I have an arduino for my temperature controller and a change like this is 1 extra relay and some code.
I need to do the math on the flat plate exchangers to figure out exactly how many sqft of surface area I need...
My wife's response was not fit for public ears and she said just do it.