This simple system for recovering heat from wastewater makes a lot of sense—especially for families and commercial buildings that produce a lot of hot water.
Power-Pipe drainline heat exchanger. Heat from the hot water going down the drain pipe is transferred to water passing through the smaller-diameter pipes.
Photo: RenewABILITY Energy 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. Im 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.
SUPPORT INDEPENDENT SUSTAINABILITY REPORTING
BuildingGreen relies on our premium members, not on advertisers. Help make our work possible.
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.
Several versions
There are four manufacturers of drainline heat exchangers that I’m aware of: Vasile’s original company, WaterFilm Energy of Medford, NY, 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" copper pipe coiled around a length (typically three to five feet) of 2"- or 3"-diameter drain pipe.
The PowerPipe 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.
Ideal for heat recovery is if all household members use the same shower (or have several showers drain through the same vertical length of drainline. It helps if the water heater is in a basement (or beneath the shower(s) 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.
Cost
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.
I agree that the savings may be questionable in single-family homes; these make the most sense in facilities where multiple showers or other hot-water uses occur, such as gyms, laundromats, and commercial kitchens. For uses in which the draining of hot water is not coincident with the use of hot water (such as draining a bathtub or clothes washer), there will be very little if any savings. Reasonably good payback is apparently being achieved in multi-family residential projects.
I tend to agree with Robert Riversong, as with the high cost of purchase and installation I have my doubts as to what the payback period is likely to be.
That said, this is a good system for single-family dwellings; but for a large apartment block or commercial building I would definitely be looking at a heat pump system using the gray water collection tank as the heat souce, since it will be collecting mainly warm water (the gray water being treated and used for WC flushing).
I installed a PowerPipe in our recently completed single family home and we love it (6' unit that is 53% efficient). One recommendation from the manufacturer that we followed was to run the water coming out of the heat recovery unit to both the hot water tank and the cold side of the shower. That way we mix hot with warm at the shower head rather than hot with cold. With a low flow shower head we can feel the water temperature go up a minute or two into the shower as the warmed water reaches the shower head. And no, we're not luxuriating :). It is a simple and brilliant piece of technology.
Unless people take very long showers with high-flow shower heads, I am skeptical about the benefits and payback in a single-family residential setting, particularly if the DHW system is not optimized by design for heat recovery.
And I can't help but wonder whether this might trigger the Jevons Paradox, in which increased efficiency results in greater consumption ("I can luxuriate in a long shower because I'm saving so much hot water.").
I saw this technology used in a residential college building about 10 years ago, I think. What a simple and effective concept. I love the quotation in EBN "it just sits there and saves you money."
Add new comment
To post a comment, you need to register for a BuildingGreen Basic membership (free) or login to your existing profile.