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 are 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 consumed 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% 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. In the right application, drainline heat exchangers allow a significant portion of the heat from hot water going down the drain to be recovered.
How a drainline heat exchanger works
The process is pretty simple. A special section of copper drainpipe is installed beneath a shower (typically the largest hot water use in a home) or other hot wastewater source. 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 (though the tankless water heater has to be thermostatically controlled and, thus, able to deal with inlet water of varying temperature).
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: Vasile’s original company, WaterFilm Energy of Medford, New York, and three Canadian companies: EcoInnovation Technologies of St-Louis-de-Gonzague, Quebec, which makes the Eco-GFX; ReTherm Energy Systems of Summerside, Prince Edward Island; and RenewAbility Energy of Kitchener, Ontario, which makes the Power-Pipe.
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.
The Power-Pipe is a little different than the others. It has a header that splits the supply pipe into four smaller, 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 the 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 standard, smooth drain pipe.
Maximizing recovery efficiency
According to an article in Environmental Building News, heat recovery efficiency can be as high as 60% — 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.
For the highest rate of heat recovery, it is ideal 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 showers and close by, so that there is minimal length of supply piping from the heat exchanger to the water heater).
These systems are even more cost-effective in schools and commercial buildings that use a lot of hot water: school shower facilities, health clubs, 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.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.