There are three ways of installing a WWHR system, depending on how the preheated cold water is used.
The preheated water can be plumbed into both the mixing valve of the shower and into the cold water supply of the water heater. In this case the flow of the drain water is equal to the flow of the cold water through the WWHR system. This is called a balanced-flow setup and gives the best performance in terms of energy savings. It also requires most plumbing and installation effort and cost.
In the second installation option, the preheated water is fed into the cold water supply of the water heater only. The energy savings are due to the preheated water requiring less energy to get up to the temperature setting of the water heater. The flow is unbalanced, because the cold water flow is smaller than the drain water flow, and this reduces the performance of the WWHR to some extent as compared to a balanced flow setup. (The temperature of the preheated cold water could actually be higher but due to its lower flow the total power transferred is less as compared to the balanced flow setup.)
Feeding the preheated water into the mixing valve of the shower only allows the simplest installation: only shower drain and cold water input to the shower mixing valve need to be diverted locally. For this case, the flow is also unbalanced (cold water flow is 30-60% of drain flow, depending cold and hot water temperatures), which again reduces WWHR performance somewhat (15% is reported for a particular WWHR unit)
For a retro-fit into an existing building, the preferred installation option depends on the relative position of the water heater as compared to the shower, any available space under the shower, ease of access to existing plumbing etc., and should be considered for each case. A new-built house obviously gives much greater freedom in designing in an optimal WWHR system layout.
In principle, all four types of WWHR systems could be used in the install options given above, but typically the shower floor with integrated heat exchanger and the ‘under shower’ horizontal unit are most suited for a local install in the shower and a feed only into the mixing valve. On the other hand, the vertical and tank based systems are typically placed in a separate room (basement, floor under the shower, technical room) where a more extensive install into both water heater and shower mixing valve is a viable option.