[stevehull]

Drake,

I made up a preheat "gray water" heater for our shower almost 20 years ago. It has worked very well - maybe too well at times.

In short, I took a scrap piece of 3 inch copper and wound 1/2 inch soft copper around a dowel that was 2.5 inches (first prefilled the tubing with sand). The smaller diameter made sure I would get good contact between the two copper pipes. The entire thing is about 18 inches long.

I thn soldered the entire thing together with a huge amount of solder. Looks messy, but the solder made sure I got good conduction. A real PITA to get all the sand out, but it prevents the tubing from flattening when wrapping it around the dowel.

Adapted a series of copper and PVC couplers to get from 3 inches to standard PVC drain pipe diameter.

Placed the water heat exchanger on it's side connected to shower water drain through three inch copper pipe. Hooked up cold water to shower valve through heat exchanger copper tubing. Wrapped the thing with some scrap fiberglass batting.

The problem is that, even with a pressure balanced water valve, I still have to back off on the hot water during a shower as the water stream gets hotter and hotter as the cold water (heated by the drain water) gets warmer and warmer.

The newer pressure balanced values are SO much better now and I suspect that would not be a present issue.

My incoming cold water, from well is about 55 - 60F and the water going down drain is about 100F. It takes about 2-3 minutes to get the cold water line warming up due to the thermal mass of the gray water heat recovery unit and it eventually gets up to about 80-85 F or so. That saves ~ 1/2 the BTU's to heat water to about 105F.

Cost was not much - but I did have to scrounge to find the 3 inch copper pipe. Kept going to recycle center before I found a 4 foot long piece with a big dent in the middle. I bought it, cut off my 18 inch section and then recycled the remaining part back. Net cost about $18 if I recall. The soft copper was another recycle find.

Not hard to do, not expensive, but has substantial returns. The further away from equator you are (with colder water) will mean an even greater savings as the efficiency will increase.

Hope this helps,

Steve