Well, it's not working, because it's not hooked up.
Last week, I took the "Intro To Solar Hot Water" workshop through the
Midwest Renewable Energy Association.
The instructor told me that external tube-in-a-tube heat exchangers work well in that they are simple and don't clog with sediment the way some other heat exchangers can, but they are pretty in-efficient.
Also, after taking the workshop, I don't think I want any plastic involved ANYWHERE in my solar heating system. I used PVC connections to make a coupler to attach the heat exchanger to the hot water tank, but it was only because it was cheap and available. I couldn't find the right brass/bronze parts anywhere.
I have just been pretty busy lately, and we have hit our cloudy season about a month early around here.
I don't think I will have a "for real" system hooked up until the spring. Until then, I will keep learning all that I can about solar water heating, maybe take another workshop and learn more about solar sighting.
In the workshop I took, there was a photo of a tube-in-tube heat exchanger, similar to the one that I made, only it used 2" copper tube on the outside and FOUR 1/2" tubes running through the inside. That makes a heat exchanger with four times the surface area of the one that I built!
Building a heat exchanger of that style may be an upcoming project.