Yes, I have found this to be true myself. If all of the heat that was escaping up and out the 3" flue(vent) is captured and the top heavily insulated (which couldn't be done previous due to the exhaust vent), you can slowly heat water overnight to where cold must be added for showers!
I am so glad to find that others have made this discovery too!!!
You can read the article I wrote about my own experiences with this
here.
I would also encourage readers to kick back to my
home page where you can access all of my other articles on topics ranging from alternative energy, recycling, gardening, and low income community beautification.
Don't give up spreading the word on this. It is worth it!