Automatically Preheat Water to Save Energy

showerUsually when we are talking about plumbing fixtures for green building we are dealing with something that conserves water. But some plumbing devices can contribute to energy savings, as well.

When you are in the shower, the hot water from the shower strikes your body and transfers some heat before it falls away. But most of the heat in that water simply goes down the drain. Reportedly, 80 to 90 percent of the energy used to heat water for the shower is lost down the drain.

A drain water heat recovery unit (DWHR) transfers heat from water running down the drain to cold water going to the water heater. This preheats the water so that the heater is starting with warmer water, and thus needs less energy. A DWHR unit can save as much as 25-30% of the energy used for water heating, and payback periods range from 3 to 7 years, depending on use patterns.

The principle is rather simple. Cold water coming in to the building goes through a coil that is wrapped around the main drain pipe. Hot water running down the drain transfers its heat to the incoming water and preheats it before it goes to the water heater. As hot water is drawn from the water heater, it draws in cold water to be heated. If that water going to the heater can be pre-heated, then less energy is needed to heat it to the required temperature.

This is true for both tank-style water heaters, as well as for tankless water heaters. But, because tankless water heaters can only raise the temperature of the water by a certain degree range, having the warmer input helps further boost the output temperature they are able to provide. Water heat recovery also makes a great deal of sense in conjunction with solar hot-water systems.

Because the DWHR unit is typically installed on the main drain stack, it is not typically something for a do-it-yourself installation. A plumber will have to cut out a portion of the drain pipe and install the DWHR unit, as well as routing the water supply to the water heater through the DWHR. This is one reason that makes this less of a retrofit option for many homes.  However, there are no moving parts, and once installed, this system will passively reclaim heat from the wastewater going down the drain any time warmer water is going down the drain.

Not every household is suited to using a The best application of a water heat recovery unit is in conjunction with a shower. Bathwater sits in the tub, so, with nothing going down the drain during the time the tub is filling, no preheating takes place. And then, when the tub is drained, there is usually no other hot-water demand, meaning that the heat is lost instead of being recaptured.

In new construction, where the water heater can be located close to the main wastewater line, a DWHR makes real sense. Also, for commercial applications in hotels, apartment buildings, and other places where large amounts of hot water are used, a DWHR can make a difference in the amount of energy needed for water heating.

Further information:

GFX Drainwater Heat Recovery

Rocky Mountain Institute Report

Image: (cc-by-sa-2.0) by Chad Miller via Wikimedia