The DWHR systems commercially available for domestic use can be divided into the following four groups:
The vertical system typically consists of a large diameter copper pipe through which the drain water flows. The water clings to the copper surface as a thin film, which allows good heat transfer into the copper. The cold water to be preheated flows in a smaller diameter copper pipe which is wrapped around the drain pipe. Alternatively, the cold water flows in a thin area between the drain pipe and a pipe with a slightly larger diameter fixed around it. This is also called pipe in pipe heat exchanger.
Vertical systems are available in various lengths up to ca 2 meters (6 ft). They need to be installed below the shower, although some horizontal displacement is allowed. Due to the height they are often installed in the basement. Of all types of DWHR, the vertical units are the most effective. This type is produced by a number of manufacturers, both in USA, Canada as well as in Europe.
Shower platform with integrated heat exchanger
The second type of DWHR system, shower platform with integrated heat exchanger, is not as common. They can be used whenever the vertical DWHR is not possible to install. They are typically more expensive and not as effective as the vertical systems.
Horizontal systems for installation under the shower
Horizontal units that can be placed close by or under a shower rely on easy access to drain and cold water, but then provide very easy installation. They are most suited as add-ons for shower cabinets and are presently not so common.
Tank based systems
The fourth type of DWHR uses an insulated tank where the drain water is stored temporarily, and through which the cold water is guided in a heat exchanger. This allows heat recovery also from other household appliances as clothes washer, and dish washer because it doesn’t depend on water draining at the same time as cold water being used.