Thermodrain wastewater heat recovery
Will the Thermodrain boost my electric water heater efficiency beyond 100%?
I'm considering installing a Thermodrain wastewater heat recovery system. You mention on your Electric tankless water heater page that electric hot water systems - tank or tankless - are 100% energy efficient in terms of their ability to convert electricity into heat. Since this special drain can capture 48% of the waste heat in hot water as it goes down the drain, would my hot water tank become 100% energy efficient if I install one?
Answer from Green Energy Efficient Homes
Unfortunately it is not physically possible to exceed 100% energy efficiency, because doing so would break Newton's second law of thermodynamics, which states that energy cannot be created or destroyed. That's exactly why electric hot water heaters are 100% efficient at converting electricity to heat: there's nothing else for that electricity to become. You may see claims of efficiencies greater than 100% for heat pumps or geothermal heating systems, because they use electricity to pump heat in from the outdoors, and can typically draw in 3-4 units of heat for every power-equivalent unit of electricity consumed.
But consider that, while an electric water heater may be 100% efficient at converting electricity to heat, your home is not 100% efficient at recovering that heat and putting it to good use. When you wash dishes in 120F water, much of the heat from that water goes down the drain when you pull the plug. (Some of it will warm the air inside your house, which is a good thing in winter but a bad thing in summer!) If you can recover some of the heat from the waste water and put it to work, for example, by transferring that heat to the input water for your hot water tank, you will be less wasteful than just letting it run down the drain.
A Thermodrain system is a section of 4" copper drain pipe that can replaces a part of your drain stack. The outside of the drain consists of a thin coil of copper pipe that travels around the stack in a tight spiral. You connect the bottom end of this spiral to your water supply, and the top end to the water input line for your hot water tank or tankless water heater.
When water travels down a drain pipe, it tends to cling to the walls of the pipe rather than fall straight down the center. If the water going down the drain is warmer than the water that came from your water supply, the heat from the waste water travels through the copper wall of the stack and into the water in the outside spiral pipe. When you take a shower, for example, you are drawing hot water out of your tank, which draws cold water in and cools the tank temperature down (which is why a 20-minute shower usually ends with lukewarm water and cries of anguish). But if the hot water is tumbling down a Thermodrain (or a Power Pipe, which is another wastewater heat recovery system), the heat from that wastewater is transferred into the water going into your hot water tank, which will reduce the amount of electricity it takes to heat the water, and also probably give you a few extra minutes of shower time.
In other words these wastewater heat recovery systems don't improve the efficiency of your water heater at all; but they give the water heater a head start by preheating the water before it gets into the heater.
If you're reasonably competent at doing home renovations, you can put a Thermodrain in yourself provided you can get to your stack and can run water lines to and from it that connect to your water supply and your hot water heater. First, build a strong support to hold the upper section of your stack in place. If the stack is an old one made of cast iron, it may way several hundred pounds and you don't want that crashing down on you when you cut out a section below it. Remove a section of drain stack and replace it with the Thermodrain unit, coupling the Thermodrain to the stack per the installation instructions. Run the cold water line from the water source to the bottom input line of the Thermodrain, and the line to your hot water heater from the top of the Thermodrain.
My brother put a Power Pipe in his basement when they rebuilt the upstairs bathroom. He measured the temperature of the copper pipe where the water goes in at the bottom and out at the top, when someone was taking a shower upstairs. He found that the temperature gain between the two pipes was as high as 18 degrees Celsius or about 32F, which means his water heater has significantly less work to do to heat its input water.
Here's an analogy that might help explain the distinction between trying to achieve system energy efficiency over 100%, and trying to reduce the energy required to run a system. My electric bicycle - a Bionx - has a battery that on a full charge will get me about 15 miles, if I contribute my fair share by peddling. The battery contributes electricity to help me get up a hill. When I start down the other side of the hill, I can use my regular brakes, which convert the kinetic energy of the bicycle (the energy of the bicycle moving) into thermal energy, because the friction of the brake pads against the wheels, while slowing down the bike, creates heat. Or I can switch the electric motor into regenerative braking mode, which uses the momentum of the bicycle to recharge the battery. This doesn't make my bicycle more than 100% efficient - it just means I am recapturing some energy on the way down the hill that I consumed on the way up. In the same way, a Thermodrain doesn't exceed 100% efficiency, it just reduces the share of heating effort contributed by the costed heat source (electricity, natural gas) because part of the heating effort is done by heat recovery from a waste source.
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