Q: How do wastewater heat recovery systems work, and are they practical to install in a new home?
A: Steven Baczek, a residential architect from Reading, Mass., who specializes in designing durable, low-energy homes, responds: Whenever we shower, we infuse water with energy to heat it, use it for about two seconds, and then send that warm water—and all that energy—literally right down the drain. It is estimated that somewhere between 80% and 90% of the energy used to heat water ends up going down the drain with the wastewater.
But it doesn’t have to be that way. With a drain water heat recovery (DWHR) system, we can recover some of the energy lost as that hot water drains away. I try to talk clients into installing DWHR systems on all of the projects I work on these days. And because DWHR systems have no pumps or moving parts, require no regular maintenance, and add very little to an overall budget—all while offering measurable energy savings—my clients are quickly on board.
A DWHR system consists of a central drain pipe, usually copper, and a series of formed-coil pipes tightly wrapped around it. This type of system is sometimes referred to as a “double-wall heat exchanger.” The double-wall design ensures that the draining greywater never mixes with any of the incoming potable cold water.
Because the system is mounted vertically, the warm greywater clings as a thin film to the inside of the drain pipe as it flows down and through. Heat from the wastewater is then recovered and transferred to the cold water coming in through the coils. Some companies boast a temperature rise as high as 25°F. While that may seem to be extreme, there is no doubt that the system does have a positive impact on the amount of energy used in heating water.
To plumb the DWHR device into a home’s wastewater system, there are basically two setups. The first option runs the incoming warmed water directly to the water heater. On the drain side, the heat exchanger can be hooked up to one shower drain exclusively or possibly to a number of drains from fixtures and appliances that converge into a single drain. The perceived advantage of latter approach is having multiple drain outlets to recover heat from.
The most critical consideration to this setup is the length of drain systems before they converge into the DWHR system. Longer pipes allow wastewater to lose more of its heat before entering the DWHR system, but continuous pipe insulation on the drain can help to counteract this loss. According to the experts, this arrangement seems to work best with at least three full-time occupants in the home.
The second configuration is more of a concentrated, or point-of-use, setup. It places a DWHR system directly downstream of the shower drain. The coiled pipes from the DWHR system are then plumbed into the cold water supply for the shower. In this setup, the recovered heat is transferred directly to the cold water supply for that shower, which means that the water enters the mixing valve at a higher temperature, and less water from the water heater is required to raise the overall temperature to a comfortable showering level. The biggest advantages to this highly concentrated system are that heat losses from long lengths of drain pipe are completely eliminated, and the plumbing layout is straightforward.
There are many DWHR units on the market and costs can vary depending on the sizes of the pipes you choose. Most come in at less than $1,000. Most companies say that you can recover the cost of a system in as little as two years depending on the energy costs in your area and on how much hot water you use. But regardless of the payback time, it’s great way to save energy in a home, and the “set and forget” passive operation of the units makes them non-intimidating for your homeowner clients.
Illustration by Tim Healey