Grey water heat recovery?
 

Anyone doing grey water heat recovery to preheat incoming domestic water? What's your system? Seeing results for effort?

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

Nice idea for for point source reclaiming(just shower). I'm thinking whole house(at least all hot waste water) tub/shower, dishwasher into a overflow holding tank xchgr. Maybe all grey water as none of it would be colder than incoming well water. Could hook tank to small solar collector for fun.

We just leave the plug in the bath until it drops to room temperature. This easily raises the bathroom 10C, and the surrounding area a couple of degrees. The water, once cold, gets dropped to a holding tank to flush toilets.

Steve, that's inspiring that it turned out that easy and cheap to do yourself. If I did that I think I would hook up the heated water so it would go directly into the cold water entering the water heater. That way you'd save the heat energy for later if you didn't want to use it immediately. I realize your situation is different because you have an on demand water heater setup.

How did you get the sand out?

nexsuperne, can you elaborate on the water recycling to the toilet system? Very interested. We also leave water in tub to cool but have separate shower also an dish washer that drain hot water.

This looks informative:

Drainline Heat Exchangers - BuildingGreen

Generally, these device are connected to the main COLD water for maximum efficiency. So, when you take a shower, they warm a little the cold of the shower and improve the temperature of the cold water entering the tank.

For max efficiency, you want a maximum of cold water entering the heat exchanger.

re plumbing the gray water recovery unit into a cold water intake tank.

Yes - that is something I would absolutely do differently today. At the time, I literally had the cold water line to shower running in basement right next to the shower drain.

On the positive side, it is very tangible on how much energy I am saving as I have to back off the amount of hot water by lowering mixing valve.

Today, I would put a similar device on the dishwasher drain (not the clothes washer as we use cold water) and each shower. And yes, all prewarmed water into water holding tank before demand hot water heater.

The temperature of the holding tank water may not go up much, but those saved BTUs remain.

The sand in the soft copper was a pain, but lots of shaking and tapping got it all out. First, I tried to blow it out with water pressure - not a good idea as it just packed in the sand. To dry it out I had to put the unit in the oven at 200 F for about a week :mad:.

I have seen other hot water recovery designs and one I like is a copper based counter flow "tube within a tube". I will try to find that and post it. The problem with that design is the lower amount of contact area for conductive heat flow. But a LOT simpler and quicker to make (no filling soft copper with sand and coiling it).

Just a thought, I understand the intent of the sand is to avoid flattening the copper but would it be so bad if it did flatten out against the OD of the large pipe? You might slightly affect pressure drop in the line, but you would get a larger contact surface on the drain pipe. The link attached above looks almost like square tubes wrapped around the large drain pipe.