I have found several good systems for heat recovery of drain water from a shower, that I can not use.
They all work on the same basic idea of a big water pipe with one or many smaller pipes wrapped around it. The drain water from the shower heats the big copper pipe, which in turn heats the smaller pipes where the incoming cold water to the shower flows. There is a heat transfer from the drain water to the incoming cold water. When the cold water heats up, we can use less hot water and still achieve a nice warm shower as before.
Here are some links to different units of the vertical type:
The problem is that all of them have this vertical design and can not be fitted underneath our shower cabinet that is placed on the floor. They are great if you are doing work on your house, or for installation when you build your house. We have a shower cabinet that is fully enclosed and abut 4 inches above the floor. I need to fit a heat recovery in this tight space. I was thinking that I could just get a long 32mm hose and the fit an equally long 12mm copper pipes inside of it, forming a simple heat recovery system. I even went to the library to borrow some books on thermodynamics to learn more. Then a month ago I was so happy to find someone selling a heat recovery unit that will fit underneath a shower cabinet. I decided to buy one.
Continue reading here: 2.The small bathroom heat exchanger - miljødusj