Capturing heat from water down the drain
Over the past few weeks I’ve written about various strategies to produce hot water efficiently. We’ve seen that tankless water heaters are more efficient than storage water heaters (though not without their drawbacks), and we’ve learned that heat-pump water heaters produce two to three times as much heat per unit of electricity consumption as electric water heaters that rely on electric resistance heat.
But the unfortunate reality is that even with the most efficient methods of generating hot water, we still lose the vast majority of that heat down the drain. Domestic hot water is a once-through product. I’ve seen estimates that 90 percent of the heat in hot water is lost down the drain. Dan Cautley, an energy engineer with the Energy Center of Wisconsin, says that drain water "may be one of our largest untapped resources."
It turns out that we can do something about that. Drainline heat exchangers allow a significant portion of the heat from hot water going down the drain to be recovered -- in the right situation.
How a drainline heat
The process is pretty simple really. A special section of copper drainpipe is installed beneath a shower (typically the largest hot water use in a home). This section of drainpipe has smaller-diameter copper piping wrapped tightly around it. The cold-water supply pipe leading into the water heater is diverted so that it flows through the small-diameter copper pipe.
When hot water is being pulled from the water heater to supply the shower, the water going into the water heater is preheated by the wastewater going down the shower drain. If it’s a tankless, rather than storage, water heater, the incoming water temperature will be higher, so less energy will be required to get it up to the needed delivery temperature -- thus saving energy.
The man who invented the drainwater heater exchanger, Carmine Vasile, called the product a GFX, for "gravity-film exchange," recognizing that water going down a vertical pipe forms a film that clings to the inner walls of the pipe where the heat can effectively be transferred through the copper to the supply water.
There are four manufacturers of drainline heat exchangers that I’m aware of: the original and three Canadian companies. Most of these have a single 1/2-inch copper pipe coiled around a length (typically three to five feet) of 2-inch or 3-inch diameter drain pipe. These manufacturers are WaterFilm Energy of Medford, N.Y. (www.gfxtechnology.com), which developed the technology; EcoInnovation Technologies of St-Louis-de-Gonzague, Quebec (www.ecoinnovation.ca), which makes the ECO-GFX; ReTherm Energy Systems of Summerside, Prince Edward Island (www.retherm.com); and RenewABILITY Energy of Kitchener, Ontario (www.renewability.com), which makes the Power-Pipe.
The last of these is a little different than the others. It has a header that splits the supply pipe into four square-cross-section pipes that provide more surface area for heat transfer.
Most of these manufacturers offer various lengths and diameters of drainline and can accommodate different supply pipe diameters.
No moving parts,
nothing to wear out
The beauty of drainline heat exchangers is that there are no moving parts, nothing to wear out, and nothing to get clogged. Only fresh water goes through the small-diameter supply pipes; any hair or other materials pass through a drain pipe that -- on the inside -- is identical to a standard drain pipe.
According to an article in Environmental Building News, heat recovery efficiency can be as high as 60 percent -- which can effectively double the water heating efficiency. Just how much benefit a drainline heat exchanger will provide will depend on usage patterns and how the plumbing in a house is configured.
Ideal for heat recovery is if all household members use the same shower (or have several showers) that drain through the same vertical length of drainline. It helps if the water heater is in a basement or beneath the shower and close-by, so that there is minimal length of supply piping from the heat exchanger to the water heater.
These systems are incredibly beneficial in school shower facilities, health clubs, and other facilities with significant shower use. They also make sense in other commercial facilities with significant hot water use: Laundromats, commercial kitchens, etc.
Installed in a new home, drainline heat exchangers typically cost $500 to $800 (including installation). Costs in multifamily buildings should be lower. In some states there are rebates available for such systems, though I’m not aware of any in Vermont.
Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to firstname.lastname@example.org.
TALK TO US
If you'd like to leave a comment (or a tip or a question) about this story with the editors, please email us. We also welcome letters to the editor for publication; you can do that by filling out our letters form and submitting it to the newsroom.