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Old 02-28-16, 06:58 PM   #1
oil pan 4
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Default sewer water heat exchanger

The average temperature of household waste water is typically between 80F and 90F.
All you have to do to figure out how many BTUs you are flushing down the toilet is to get a water bill figure out how many pounds of water you used, subtract ground water temperature from average waste water temperature. Then multiply the temperature differential by pounds of water.
I bet it comes out to be a pretty big number.
What if we could put some of that heat back into the household?
If some one were to live off grid this could massively reduce their water heating requirements and save a lot of propane.

Here is my idea:
Get a good long length of 4 inch, 0.080'' wall 304 stainless steel food grade pipe, because most home poo pipes are 4''. Then take a length of 2.5" or 3" and split it down the middle and weld it on to the bottom of the 4 inch pipe, then weld water line hook ups to the ends of this bulge. So as the warm waste water flows out cool fresh water comes in the waste gives up its heat to fresh water.
I have about 100psi of water pressure so I want 80 wall pipe, if you had well water at a much lower pressure then standard 1/16'' pipe would be fine.
Did I mention that I may try to build this?

To build this you need lots of 4 and 2.5 or 3 inch pipe.
I have been told this 4 inch 80 wall pipe runs upwards of $20 per foot retail prices and 2 or 3 feet isn't going to be enough, this is a heat exchanger so it has to be long, as long as possible.
Lucky for me I can get a lot of pipe and cheap as dirt from the scrap yard. I already have about 40 feet of food grade stainless 2.5 inch pipe I have some 0.080'' wall 4 inch pipe but its slated for another project, I should be able to get a lot more.

I don't have to use 0.080 wall pipe but I kind of want to.

Other hazards to look out for:
When you weld stainless in the presence of oxygen you can create hexavalent chromium if you don't know what you are doing. If you don't know already what hex chrome is then you should not be trying this.
The welds have to be air tight.


Last edited by oil pan 4; 02-28-16 at 08:40 PM..
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Old 02-28-16, 09:23 PM   #2
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I bet it comes out to be a pretty big number.

Hmm, more like pretty small. Run the numbers.
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Old 02-28-16, 09:54 PM   #3
oil pan 4
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According to the EPA the average American family uses 400 gallons per day.
That is 99,600 pounds of water per month.
Heat that up 30'F for a total of 2,988,000 BTUs.
Or the energy of almost 1 gallon of gas per day, every day for years.

Seems like a pretty decent sized number to me.
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Old 02-28-16, 11:37 PM   #4
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I'd probably do what you are doing if I had 'free' stainless steel (or copper) pipe and disregarded the salvage value.

Using your epa numbers, , you are leaving out an awful lot of factors.
https://www3.epa.gov/watersense/pubs/indoor.html

70% used indoors, so that drops the 400 to 280 gal

Leaks and toilet are not heated water, epa says those are 40%, leaves 120gal day

epa says clothes washers are 22%, will assume warm water left at end, I'll grant your 30 F there, but we wash in cold water. 1/2 of faucet taken as cold water, 8%, leaves 120-32 = 88 gal per day that may be warm enough, say 100F (70+30) to recover heat.
Even engineered heat exchanges for liquid to air need about 10F differential,

so, max you get is 20F * 88 gal *8.3# = 14,608 BTU per day, about a pint of fuel, or say 45 cents a day if you have an elec WH, less if nat gas heater. A long long time for ROI, if ever, just based on scrap value of SS pipe.
Take away the clothes washer or use a modern one, the number drops to near zero since the water going down the drain from a shower is not sure 100F unless you have alligator skin.
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Old 02-29-16, 12:24 PM   #5
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There are other appliances like the dish washer that use hot water, most only have 1 water connection and its recommended it be connected to hot water. Then if you connect them to cold water then they just use their built in heating element to heat the water.
Toilet water can be heated in some cases to prevent moisture from condensing on the toilet running down the toilet causing mold or wood rot.
For me I get oil, grease, hydraulic fluid, diesel on clothes so the real dirty stuff gets washed in the hottest water possible.

I already bought the 316 alloy 2.5 inch tube last year for 70 cents a pound.
When the scrap 4 inch pipe becomes available it should only be around 50 cents per pound because its only 304 alloy. 8 to 10 inches of 0.080 wall 4 inch pipe is about 1 pound. If I cant get enough 0.080 wall then I will use 0.062 wall, which should be closer to 1lb per foot.

I am thinking this is going to be a bigger energy saver for winter time.
Ground water comes into the house any where between 40'F and near freezing, even our natural gas hot water heater cant keep up during winter.
The summer time water temperature differential is only around 30'F, but during winter it will be nearly double that.
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Old 03-01-16, 01:46 AM   #6
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What would you heat with the waste water?
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Old 03-01-16, 06:44 AM   #7
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The most efficient implementation of this strategy is to preheat incoming potable water supply on its way to the water heater. To account for time lag (as with clothes or dish washing), it is more effective to wrap a "worm" tube around the drain pipe section. The longer the length of time the water spends in the worm, the better the usage vs discharge lag is dealt with.

The other common method is to preheat the cold line of a sink or shower with the drain water at its point of use. There have been many builds documented where the shower floor was embedded with hydronic tubing, warming the incoming water with the shower drain water. Some were constructed the same as heating slabs,and some had intimate, direct contact between the drain water and the incoming tubing. Both methods worked well enough to notice the preheating effect.
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Old 03-01-16, 11:18 AM   #8
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I don't see any point in wrapping a worm around the sewer pipe. All the out going water in the sewer pipe is almost never deeper than 1 inch and does not stay in the pipe for very long unless you have problems.
If a worm were wrapped around the pipe only the edges of the 2 pipes would be in contact, through the walls of the pipes.
The way I propose is more like a heat exchanger.

Quote:
Originally Posted by gtojohn View Post
What would you heat with the waste water?
Incoming utility water.
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Old 03-05-16, 01:18 AM   #9
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Time for a visual aid.
Here are some small cross sectional mockups I did.
Looks like 2 inch pipe on 4 inch pipe wont work. Its going to have to be 2.5 inch pipe.
I used actual 0.080 wall 4 inch pipe and standard 0.062 wall 2 and 2.5 inch pipe.



2.5 inch pipe on 4 inch pipe:


2 inch pipe on 4 inch pipe:


Some welds. Miller maxstar, 70 amps, 2% Thoriated tungsten, very pointy, 20SCFM.


To further improve the welds I am thinking about getting plasma cutter torch wheels and putting them on the tig torch that way I can roll the torch along the weld and get a super smooth evenly heated weld almost as if a laser wielding robot welded it.

Next step will be to build about a 1 foot long functional prototype, that wont be put in the ground, just torcher tested to failure.
Where I will do actual back purging the fresh water pipe and develop a good way to split 2.5 inch pipe down the middle and fit up the welding joint a little tighter.
Then I can measure the volume of the 1 foot section of fresh water pipe and see how much of a hot water reservoir I have.
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Old 03-05-16, 05:37 AM   #10
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Oil pan,

Hate to be a wet blanket, but your return in $, heat or whatever is going to be negligible. Up north, where entering cold water is COLD, then heat recovery from waste water can be rational.

I built one for my shower when living in Michigan about 30 years ago. The cold water coming in was about 40 F and the drain water in the shower was about 80 F. I plumbed it so that the recovered heat, preheated the cold water to the shower valve. After about ten minutes in the shower, I noticed I could turn down the hot water valve just a tiny bit.

It worked - but not to my expectations.

The key is understanding the difference in water temperatures. Assume your incoming water temp is about 60 F. Assume that waste water is about 80F. The difference that is there (delta T) to recover is about 20F or so. The recovery is no where near 100% so let's assume 50%. That means that a 10 F difference is there. Now multiply by the average number of quarts per minute going down the drain when this is working - let's call it two gallons (8 quarts) per minute. The number of BTUs saved is tiny! About 80 BTUs per minute.

As an example, the heat from one burning match is about 1 BTU.

Now go to a more northern place where incoming water temp is about 40 F (all else same). The delta T is now 40 F - double that in your area. The recovery is also double, but still small.

I love the idea of recovering heat, but sometimes the cost (or % recovery) to do so is just not there. Waste sewer water might only be 60 or 50 F.

But as an interesting and fun project? There I am with you 100%!!

You will need to mount your recovery tube in a vertical orientation so the water swirls down on the pipe surface (water "skin effect"). Copper will be a far better heat transfer media compared to other materials. Even better would be to make a coaxial heat exchanger with ripples or dimples on the exchange metal to expand the available surface area for heat exchange. A long pipe (10 feet) would also help to get efficiency. But count on a maximum of maybe 50%.


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