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.
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.
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.
Read full article to learn about different models of drainline heat exchangers…