Yeah if I build this thing it's going to be fairly large. I have 60 feet of sewer pipe. To be worth anything I know it will have to be at least 20 feet long.
Only 2 things limit the size :
The physical length of 60 feet.
The availability of 50 cent a pound scrap stainless.

I expect efficiency to only be between 20% and 60%.

Even if all it does is 10% efficiency and prevent the pipes from freezing abs bursting then it will have been worth it.

I don't think that copper would be strong enough. All industrial heat exchanges are made of stainless.

I picked up 30 to 40 feet of 0.083'' wall stainless today from the scrap yard.
I did not get very much 2.5 inch pipe and I am pretty sure the 2.5 inch pipe I got is not 80 wall.
So there isn't really any reason why I cant. I thought the main hang up was going to be finding enough 80 wall 4 inch pipe.

Before you go to all this trouble, PLEASE verify your waste water heat value. You claim it is between 80 and 90 F.

Waste water recovery works best when the water inlet temp is cold, preferably below 45 F. I believe your water inlet temp is about 65 F.

Do not get the peak temp of waste water, get a 24 hour average. Not hard to do. THEN, design the heat recovery system.


Steve

Here the city some how has gotten away with burring the water line less than 2 feet.
So during the winter we get water that is ice cold. Its not freezing but close. The natural gas water heater acts more like an electric, as in it cant keep up.

A 24 hour average is kind of useless since water is not flowing out 24 hours a day. I could run each house hold device, pop the top off one of the sewer clean outs and point my flir at the out going water.
My wife takes raging hot showers, that one would be the most interesting to see going out.

The "piggyback" hx you mocked up will do fine on horizontal drain runs. It should live 100 years or more as a drain line. Just make sure the fresh water channel has a way to burp itself. Your heat transfer surface lies directly where air will accumulate if allowed to.

In an industrial hx, they use stainless because it is stronger and less expensive than copper. It is not an awesome heat conductor, so when the unit has to be compact, they use cupronickel (copper stainless) or (you guessed it) copper for the working transfer tubing. In modern industry, less expense translates to more return nearly always.

All you need to do (to measure the outgoing sewer water temperature) is to buy one of the inexpensive digital temperature recording devices (see below), put it in a plastic freezer bag and weight it down in the sewer outlet with a string attached. Then retrieve it in a few days.

It will show you the maximum, minimum and average temperature of the water over the data collection period.

Here is a simple RC-5 recorder that works well for me.

RC 5 USB Temperature Data Logger Temp Recorder Internal Sensor 32000 Points New | eBay

When you speak of "freezing cold" winter water - is this a temperature or a subjective assessment? For example, if I get into the shower and it seems "freezing cold" it might be 80 F.

Metrics make all the difference in planning . . . .


Steve

I also picked up a high frequency starter/stabilizer box.
This will allow me to light the arc with out touching the base metal with the tungsten electrode.
Now I also have a gas saver kit for my WP-17 torch.

http://ecorenovator.org/forum/attach...1&d=1459366718

The idea is I will use the gas saver and a strip or angle iron held in place with a vice grip to guide the torch along weld, running hot, fast and perfectly straight creating a weld that looks like it was done with some kind of automated laser machine.
I want the welds I put on the actual pipe I am going to use to look a lot better than my quick and dirty test.

To split all the 2.5 inch pipe I think I will use a cut off wheel. It will be slow and will suck a lot but that will give me straight cuts with out warping the pipe. I already know if I use my plasma cutter it will bow the pipe. I may test filling the pipe with kitty litter and cutting it but I still expect the pipe to bow.

I am thinking my first live prototype will be a shower water heat exchanger. it has been a while since I have been up under the house, and if I remember correctly the shower drain pipe is 2 inch pvc goes for about 3 feet before it goes into the main line. The cold water line is also very close by.
It would be easy to make heat exchanger section and plumb it in there.

Keep track of how much time you spend on this, I'd be curious.

NOte: have done some projects myself that I figured my 'return' was about 12 cents per hour - but it was 'fun'. <G>

The thing with this project is once it's done it should work night and day for decades.
So far the only time I spent on this was building the mockups in the pictures, maybe 2 hours while I was at work one night when I had to stay late because no one on night shift bothered to show up to work. As long as I keep moving and do things the are visually stimulating no one bothers to ask what I am doing.
So in a strange way the time I spent on this project far has pretty much paid for most of the materials I need to build it for real.