In a rare idle moment, I was contemplating the torrents of hot water that disappear down the plughole whilst having a shower, and thinking what a huge waste that is. A quick google reveals that heat recovery systems are possible, but I didn't stumble across any off-the-shelf systems.   One of these days I fancy building a system myself. Plenty of space under the suspended floor. My initial thoughts are to swap out a length of the 40mm plastic waste pipe for a compatible metal pipe, and