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sparkchaser 11-14-11 10:31 PM

Wastewater Heat Recovery
Hello all,
I've been looking around this site and didn't find a thread on wastewater heat recovery, so I guess I'll try and start one.

Several years ago I read about a heat exchanger for bathtubs and showers comprised of a copper 2 inch drain pipe wrapped with 3/8th inch copper soldered in place, then wrapped with insulation. As the shower is running, hot water goes down the drain, and COLD water supply is routed through the smaller tube on the way to the shower. Most of us probably don't run the shower at maximum temperature, so the COLD water which is flowing and mixing with the hot water (so it doesn't scald us) is now WARM water, thus requiring less HOT water. A variation on this theme would be a larger heat exchanger in the main drain pipe, with the smaller pipe going to hot water tank inlet.

So, does anyone have any experience with this concept? I know a few years ago there was such a product on the market but I haven't seen or heard of it since. Perhaps some of us can try this out! I have a houseful of girls so I stand to gain a lot from this! I'm thinking maybe 2 feet of 2 inch copper wrapped with as much 3/8th inch tubing, or 1/2 inch if I can get it to bend that tight, probably using a mandrel and putting the larger tube in a lathe while wrapping the smaller one on. Then solder, and wrap in reflective bubble wrap insulation, perhaps followed by fiberglass batt. I have room under my house to install it. I'm also thinking of trying a piece of 1/2 inch PEX in a U- shape taped to the bottom side of the main drain line, for about a total of 30 feet or so (remember u-shaped) then wrapped in reflective bubble wrap and fiberglass. Then tie that into hot water tank inlet. I already have the bubble wrap insulation and the fiberglass. Perhaps also insert two thermocouples for HX inlet and outlet temperatures. I really wish I had a data acquisition device.... hmm... next little project?

Patrick 11-14-11 11:21 PM

Graywater Heat Recovery (DHR) System: GFX

pinhead 11-15-11 06:56 AM

I am thinking of installing a system like this so after my shower I went to the basement to feel the cast iron drain pipe. It was cool to the touch. I am sure copper will be a better conductor of the heat, but I think 2' is much to small to aim for to see large gains. But if 2' is the only room you have it might be enough to see some gains. The production versions are often 6 or 8' long.

Xringer 11-15-11 07:02 AM

We have had a few posts on the topic before..

Maybe you can find some info, looking here:

I'm still interested in a DIY scheme, since my ground/city water runs pretty cold..
And, my plumbing needs some work downstairs anyways..

Ryland 11-15-11 07:26 AM

These type of heat exchanger's work best if they are vertical because the grey water clings to the walls of the pipe when it's flowing vertically, because of that and the requirement of having a long stretch of straight drain pipe that you can cut out and replace, I haven't heard of anyone installing one of these yet even though I saw a display with a heat exchanger like this nearly 20 years ago, but I agree, they could save a lot of energy.

As for 2" drain pipe, if it's just coming off the shower then the 2" is even over kill as I think plumbing code requires 1.5" or larger, with a smaller drain pipe you might run two fresh water lines around the outside to make it easier to bend and keep tight, of course if the drain pipe is laying horizontally then you might just solder your fresh water lines to the bottom of the drain pipe.

Xringer 11-15-11 07:30 AM

It seems like the price of copper has killed the market for these..

Here are some items for the bathroom's fresh air..

Heat Recovery Unit Fan Whole House Ventilation System items in Fans4Less Ltd store on eBay!
Dealer in the UK.. Seems like they don't ship to this side of the pond..?

strider3700 11-15-11 09:54 AM

I have a vertical 4' stretch of easily accessible 3" copper drain in the main stack just before it exits the house. this is only a couple of feet before the hotwater heater. I suppose I could hand wrap it with 3/4" soft copper and try it but yeah the cost of that copper is going to be pretty high.

Xringer 11-15-11 10:32 AM

I'm thinking of building a large PVC 'U' trap under my tub. (Maybe 4 to 6 feet tall).
Inside the PVC 'U', I would run a 1/2" flexible copper loop, fed from the cold water source.
The bottom of the 'U' would consist of four 45deg elbows.
I would put a 'T' at the very bottom of the 'U', to use as a clean-out. (Diamond ring trap). ;)
Since greywater is very low pressure, a screw-on cap or PVC valve would work for the clean-out.

The copper entry points would be at the top (using Tees), above the 'U' water line,
so the seals wouldn't have to be super tight. Just water-resistant, like RTV.?.

Don't know how efficient it would be, but it's got to be better than nothing.. ;)

I was just thinking about how these things work. In addition to recycling
the pre-heated water, some of your lost body heat would also be recycled..

AC_Hacker 11-15-11 12:18 PM


Originally Posted by Xringer (Post 17607)
In addition to recycling the pre-heated water, some of your lost body heat would also be recycled..

Your lost body heat would only enter the wast water stream if you are taking cold baths or cold showers, since heat only flows from hotter to colder, at a rate that is dependant on the difference in temperature.



AC_Hacker 11-15-11 12:45 PM


Originally Posted by Xringer (Post 17605)
Dealer in the UK.. Seems like they don't ship to this side of the pond..?

Look again, they do ship over here.

BTW you might also want to check out this Panasonic HRV

They have 'em on ebay.


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