Thermodrain wastewater heat recovery
Can my hot water be more than 100% efficient?
If electric hot water is 100% efficient and the Thermodrain waste heat recovery drain captures 48% of waste heat, will my system be 148% energy efficient?
Answer - Best Answer on Yahoo! Answers
You cannot exceed 100% energy efficiency because to do so violates Newton's second law of thermodynamics. You can't convert X units of electrical energy into X+N units of heat energy. (Heat pumps sometimes claim to have 300% or 400% efficiency, but this is slightly misleading; what they are really doing is using one unit of electrical energy to extract three or four units of heat energy from the air.)
The Thermodrain, or the competing product Power Pipe, are both energy recovery systems. The Thermodrain captures the heat from waste water and uses it to preheat water going into the hot water tank. That heat from the waste water means that you don't need to use as much electricity to heat the water. But it doesn't boost the efficiency of the electric hot water.
You can install a Thermodrain or Power Pipe where you have a section of drain stack and you can lead water lines to it. Cut out a section of drain stack (being careful to support the upper section, as sometimes they are not secured above, and a cast iron drain stack can weigh several hundred pounds). Replace this section with the Thermodrain or PowerPipe unit. Then divert the water entering your hot water tank or tankless heater to the bottom of the Thermodrain or Power Pipe, and send the water coming out the top of the Thermodrain or Power Pipe into the hot water heater. Thus the water entering the heater has been warmed by the waste water, which tends to cling to the sides of the drain pipe.
My brother has a Power Pipe in his home and he measured the water temperature at the bottom and top of the drain. The net temperature gain while someone upstairs was taking a shower was around 18 degrees if my memory serves correctly.
On the 148% energy efficiency question, an analogy might help here. I have an electric bicycle. I can go about 15 miles on a full charge. I use electricity to help me get up to the top of a hill (that's the heating of the hot water with the electric element). When I go over the top of the hill, and start to get going too fast on the way back down, I can either use friction brakes, which turn that kinetic energy into waste heat, or I can switch the electric bike into regenerative braking mode, which turns the electric motor into an electric generator, and captures that kinetic energy as electricity (which is then stored as chemical energy inside the battery). Have I achieved more than 100% efficiency? No, I've just recaptured some of the waste, and can use that to get up the next hill. In the same way, you have captured some of the waste from running your hot water down the drain, which means less energy up front for the next tankful of hot water.
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