GFX — Drainwater Heat Recovery

According to the U.S. Department of Energy, as much as 250 billion kWh worth of hot water is sent down the drain.  We pay to heat water, and then we pour it down the drain.  That’s not good.  Fortunately, a good percentage of the wasted energy is recoverable.  
GFX installed in the drain of a home pre-heated incoming cold water and lowers the energy needed to provide hot water for your home.

GFX installed in the drain of a home pre-heated incoming cold water and lowers the energy needed to provide hot water for your home.

Several manufactures make version of drain water heat recovery systems—and one class in particular seems best suited.  These are gravity film heat exchangers, or GFX.  Water flowing down a drain pipe tends to cling in thin a film to the sides (not fall down the middle as you might imagine.  We can take advantage this to “grab” the heat from the waste water and add it to incoming water.  The two streams are separated by two walls of copper, so your incoming water is not fouled by the outgoing water.  In the photo, you see an actual installation (from my basement!) of the GFX, with the cold water inlet in the blue box, and the preheated water outlet in the yellow box.

The savings you’ll see depend a lot on whether you use batches of water (like baths, dishwashers, clothes washers) or whether the water drains as you’re using it, as is the case with showers (these simultaneous uses deliver the best recovery).  Depending on how you use water, you could save between 20-40% on hot water costs with a GFX, all from a piece of equipment with no moving parts, that uses no electricity, and that should last 50 years.

The GFX system can be a good complement to high-efficiency water heaters and solar hot water systems.  There are some installations challenges in existing homes, and especially in home built “slab on grade”.   There are also a variety of installation considerations to optimize performance.  But for many homeowners–especially those who have teenagers with a proclivity for hour-long showers!–GFX can be an attractive option.


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5 Responses to “GFX — Drainwater Heat Recovery”

  1. kirstencorsaro Says:

    Interesting! You mentioned it works differently for a shower versus a bath– which one works better?

    • greenhomesamerica Says:

      It works better for a shower. Here’s how: the hot water running down the drain from a shower is happening the same time the cold water is being fed through the heat exchanger–giving you good simultaneous transfer. With a bath, while you’re drawing the water there is generally no hot water going down the drain and thus nothing to help pre-heat the incoming cold water. Similarly, when you drain the tub, there isn’t necessarily any cold water being fed through the exchanger, and thus not much opportunity to grab the waste heat in the drain. If your family primarily takes baths instead of showers, you won’t get as big a benefit. (But if your family has two teenage daughter like mine who take 30 minutes showers, you’ll get a huge benefit!)


  2. A Green Bathroom–Energy and Water Efficient « GreenHomes America Says:

    […] other opportunity on the drain side is “drainwater heat recovery“, using a device which lets you capture heat going down your drain and using it to preheat […]

  3. Energy-Wise Bath Remodels - Ted Cushman's Remodeling Blog Says:

    […] In this post, Mike highlights a few ideas: low-flow fixtures, energy-efficient LED lighting, and a heat-recovery drain. Take a look. Published Nov 12 2009, 11:10 AM by […]

  4. Solar hot water in New York « GreenHomes America Says:

    […] that we pay to heat water.  (And heat water that we generally let run down the drain–but drainwater heat recovery is another […]

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