Drain Heat Recovery Device: GFX
by TreeHugger 
on 04. 7.05
Approximately 80-90% of all hot water energy goes down the drain, carrying with it up to 955 billion kWh of energy in the US alone. The GFX is a device that reuses the heat from hot water going down your drain, and uses it to preheat water going into your hot water tank. Its operation is based upon a natural phenomenon whereby surface tension and gravity create failing water films. They say it'll recycle 60% of your heat, and it has no moving parts or parts to clean, so nothing to maintain or pumps to keep running. :: GFX Technology [by Justin Thomas]
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In engineering terms the principle employed here is called "counter-current extraction". Its used in many chemical processes and heat conservation devices. The 60% gain probably is measured across the device itself. If you include the loss across the pipe that carries the pre-heated hot water tank intake, from the exchanger all the way to the hot water tank inlet, its probably a lot less. Makes the most sense for use with "on demand" hot water heaters.
In other words, it's the water equivalent of a Heat Recovery Ventilator (HRV) system.
Exactly. But you have to figure in the life cycle burden associated with installing double the amount of pipe, so as to run the pre-armed feed water all the way back to the water heater inlet. That's another reason to favor on-demand water heaters, which are right under a sink or next to a shower and can have the recovery coil built right in. With air exchange system right next to a furnace thats not a problem.
Sounds good. Which under-the-sink water heaters can accomodate a recovery coil?