This is an idea of Nick Pine's to build a simple, inexpensive greywater heat exchanger that captures the heat in water draining from showers etc, and uses it to heat the incoming supply water. Potentially, a high percentage of the heating that normally goes down the drain could be recovered.
Nick's usenet post "greywater in a drum revisited" explains the concept:
PT Industries (1 800 44 ENDOT) sell 1/2"x400' coils of 125 psi (tested to 600 psi) NSF-certified HDPE pipe (0.752" OD and 0.622" ID, with a nominal minimum bending radius of 20xOD = 15") for $67, list. Ferguson's price is $48. I figure 800' in 3 vertical spirals in a 55-gallon drum with a liner and a removable bolt-ring top would have 143 ft2 of surface and hold 12.5 gallons, with an NTU of 6.8 and a burst efficacy of 87%. The layers could use some separators to prevent nesting, eg some lengthwise slices of pipe. This single-wall heat exchanger is not NSF-compliant as is, but endurance testing and leak detection and safety devices could change that. It has enough volume to do better over a day, with water usage in small bursts, eg a 10 minute 1.25 gpm shower. How could we calculate that? I tried hard to make this with 300' of 1" pipe, but it kinked too often when two of us (looking like Laurel and Hardy :-) tried to spiral it carefully into a drum on a warm day... All the holes would be in the drum lid. Cold water would enter the spirals at the bottom via a dip tube and exit from the top. Greywater would enter the bulk of the drum at the top via a 1.5" PVC dip tube with holes to let it find its own thermal level and leave from the bottom via another tube, with 1-2 psi in the drum to get it back up into a sewer pipe near the top of a basement ceiling (The GFX requires lots of vertical drop.) We might put a thermostat on the lid and an electric heating element through the lid to make a standalone water heater.
Small Test To determine How Much Water Temperature Drops From Shower Head to Drain
Some pictures of the prototype as it develops:
Setup for the GW Stratification Test - to see if the perforated GW entry tube distributes the GW to the correct temperature stratum in drum -- 05/04/21:
Overall setup -- green drum is the GWHX (others drums are just supports).
GW comes out of elevated 5 gal bucket on right, flows into distribution tube (the one you sent), and
exits out the tube near left edge of barrel. The exit tube extends down to near bottom of barrel
inside the drum. Entry flow rate is controlled by the ball valve just to left of 5 gal bucket.
Shows GW entry tube, and exit tube. The little tube below the GW entry tube has the 5 temperature
sensors mounted on it -- 4, 10, 16, 22, and 28 inches from bottom of drum.
With the interim insulation in place. There is some loose FG under the drum, between the
two 2X4s that keep drum off concrete. There is also a loose bat that goes on top when when done..
Before the test, I added cold water to the bottom 15 inches (or so), then added hot water using a T shaped wand with holes drilled in it to make water come out in many places, and approximately horz -- idea being to not mix the water up much -- seems to result in a pretty good temperature stratification in drum.
First Stratification Test (quarter inch inlet tube holes)
2nd Stratification Test (quarter inch inlet tube holes)
Single Coil Test
3rd Stratification Test (half inch inlet tube holes, and short tube inlet)
GWHX Pipe Coil Layout (first cut at pipe coil layout)
Three Coil Prototype (first cut at the three coil version of GWHX) 05/31/05