This looks in principle to be a good idea. It often occurred to me that a whole lot of heat was going straight down the drain after bathing or during showering. The construction of this device looks so simple – it would be easy to make one cheaply using 2″ copper pipe and a coil of flexible copper tubing. For best heat conductivity it would be a good idea to solder the spiral tube to the 2″ down pipe, and then wrap it up in a good thick layer of pipe insulation.

Some sort of valve arrangement to allow the pipe to fill, wait until full and then empty, would maximize the time that the waste water was in contact with the heat-exchanger – how about something like a ping-pong ball acting as a float which only opens the valve in the bottom of the pipe once the tube has filled. The 60″ length of pipe will hold about 3 liters of water – so about a minute to fill from the average shower.

I estimated that about 9kWh was needed to heat a tank of water to 60 centigrade required to fill the average bath. However the actual tolerable limits of bath temperature lie within a narrow band of body temperature. (about +/- 8 centigrade either side of 37 degrees) Dumping the 150 liters of water down the drain at 40 degrees centigrade when it could be allowed to cool to 15 centigrade seemed like wasting almost half of the energy nearly 4.5kWh

My first thought was to use an old refrigerator to pump the heat from the waste water back into the household. Unfortunately being a ground floor property would mean considerable excavation to make an insulated storage tank below the level of the bath outlet. Using the waste heat to preheat the cold water inlet to the water heater – especially when showering, seems an excellent idea.

In winter, direct water heaters really do struggle to produce an acceptable flow rate of hot water, as both incoming water and air for combustion is very cold. Bringing up the coldwater inlet temperature by perhaps only 10 degrees will allow a much greater flowrate for a given heat input. My previous boiler took 20 minutes to fill a bath to a reasonable temperature in winter. Using a second heat exchanger on the water heater flue gases (condensing boiler style) would help preheat the cold water inlet before reaching the main heat exchanger, it also allows the heat of vapourisation to be recovered from the water vapour in the flue gases.

I haven’t read the material on this yet, but I think I know what is being tried or proposed.

  1. Very strict Codes about the possible “mixing” of potable water with waste water… The heat exchanger may not be able to pass this “test” indefinitely… assuming a leak after 30 years, etc.
  2. A friend of mine wanted to try this scheme about 30 years ago, but I think was discouraged by the above “Code”.
  3. Don’t take out too much heat from the exhaust gases of the water heater… if you do, there won’t be enough heat left in them for them to rise up the chimney!

I like the idea, but perhaps there is a “safer” way. Perhaps I better go read the web site now too!

I have used a very similar system on my shower drain for years now, I found a  barrel of heat exchangers at the local scrapyard (New, with a few dings) and  used two of them in a “U” shape to replace the “P” trap in my Tub/Shower, (To  trap and slow the drain water, and to make some turbulence in the pipe) then  connected the input to the hot water heater through the coils around the  outside, and insulated the whole thing.  Don’t know the efficiency, but the water pipe to the hot water heater is now  warm to the touch instead of the 55 Fahrenheit it stayed year ’round before  putting it in. (Well water)  The only difference is my heat exchangers are only about 18 inches long,  darned if I know what they were originally used for, probably something to do  with air conditioning, they are solid copper in standard 1 1/2 inch pipe  size.and “Wrapped” with 3/8 copper pipe, all brazed together.  You could make something similar for a few bucks, and a lot of effort.