# drain water heat recovery help please



## gibbsgirl (May 1, 2013)

hi all,
I found a product from home depot called a "power pipe". it is for water heat recovery to make the water feeding out of your sewer (like hot shower water) run next to the pipe feeding cold water into the hot water heater.

the idea is that it raises the temp before the water gets to the hot water heater so it takes less power to raise the water to the temp the hot water is set to.

anyway. I would like to do something like this since we are in the process of choosing a new hot water heater anyway. Here's my problem though.....

The sewer pipe we have is old 6 inch iron pipes. Not sure if they are original (house is from the 1930s). Doesn't need replacing. It's right next to the basement wall. Looks like hiring a plumber to change it to the pvc pipe that the "power pipe" is meant to work with would be expensive and silly.

Really trying to figure out if there is a different product that would work or a handy person with experience who could offer some info about another way to do this. thank you for reading.


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## Harry Chickpea (Dec 19, 2008)

The general concept is called tempering or preheating. There isn't any appreciable usable heat in the water going down a shower drain and the potential for contamination is enough that the idea isn't worth doing. Consider - the water in a shower hits your body at about 95 degrees, loses a lot of energy in converting into vapor that coats the bathroom mirrors and heats the room, and exits to the drain at maybe 80 degrees. Incoming water is at ground temp more or less, which averages around 55 degrees. As a rule, you can only expect a near perfect thermal transfer device to give a 60% recovery. With the thicknesses required for pressurized water in contact with a sewer pipe, expect 30% efficiency. 80 degrees minus 55 degrees is a delta T of 25 degrees. 30% of 25 is 7.5 degrees of pre-heating. If you continue out the math and cost of electricity, you might save $5 per year. The cost of that Home Depot product is $600. If you figure the interest on $600 the device will never pay for itself in your great grandchild's lifetime. The old-timers hung a galvanized water tank near furnaces to pre-heat the water going into boilers and limit the thermal shock. Those tempering tanks did work and used the waste heat of the boiler room effectively. If you want to preheat water, look into solar batch water heating.


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## Jim-mi (May 15, 2002)

The concept is fine. Seems much more suited to new construction where pipes can be placed where needed. Add into that cost of the unit, the problems/cost of an install.

The math doesn't work out in your favor............


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## SolarGary (Sep 8, 2005)

Hi,
I think its a good concept.

There is a bit of material here on it:
http://www.builditsolar.com/Projects/Conservation/conservation.htm#GWHX

Some more DIY solutions if you search down this page for "DIY grey water heat exchanger"
http://www.builditsolar.com/Experimental/experimental.htm


The DIY version that is shown there could be used (maybe) with the iron pipe. It has a slight risk of contamination of the incoming water, but I think the risk is very very small.

I got curious a couple years back about how much heat you send down the drain in a typical shower and measured the temperature of the incoming water and the water going down the drain for a typical shower.

Its all detailed here: http://www.builditsolar.com/Experimental/ShowerTest.htm

Water comes in at about 105F and goes down the drain at about 93F.

A perfect heat exchanger could recover 83% of the energy in the heated water. It seems like recovering about half the energy might be a practical target with a real heat exchanger.

Gary


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## Harry Chickpea (Dec 19, 2008)

Your temps and calcs seem a little better than my off-the-cuff ones, but I still see a solar batch heater as a much more cost effective and less problematic alternative. Some of the designs are just asking for trouble - I have to remove hair and soap globs from our shower drain periodically, I shudder to think of how that pex exchanger would get coated with khuh-khuh over time. Also, in some areas and at resale, the local code inspector could have a big frown.

What I notice about the Home Depot ones is they look suspiciously like the condenser on a moonshine still. I wonder if that is the real off-market purpose.


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## SolarGary (Sep 8, 2005)

Harry Chickpea said:


> Your temps and calcs seem a little better than my off-the-cuff ones, but I still see a solar batch heater as a much more cost effective and less problematic alternative. Some of the designs are just asking for trouble - I have to remove hair and soap globs from our shower drain periodically, I shudder to think of how that pex exchanger would get coated with khuh-khuh over time. Also, in some areas and at resale, the local code inspector could have a big frown.
> 
> What I notice about the Home Depot ones is they look suspiciously like the condenser on a moonshine still. I wonder if that is the real off-market purpose.


Hi,
I like the batch solar or a simple drainback solar heater as well -- they are probably not much more work to install (maybe less), and will get better results in a sunny location. But, a lot of people don't have a site with a good sunny spot to put them in.

Interesting thing is that you could do both the batch solar heater and the grey water heat recovery and end up heating the shower water for free (with solar) and then recovering heat from the shower drain to use for something else -- like space heating.

I left a question for Tyler, who did the PEX drain water HX, if he has checked for soap build up - -- on this page: http://www.builditsolar.com/Experimental/PEXGrayWaterHX/PEXGrayWaterHX.htm
Hopefully he will get a note about the comment and answer.

Gary


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## gibbsgirl (May 1, 2013)

thank you so much for the replies. i'm really excited to have my hubby take a look at the links with me. i'm not very good with shop-talk/tech know how. but, he can help me if I track down info like this to sort out how to do what's worth doing and what is snake oil. i'm just the idea girl.

I love this web-site and really appreciate the time so many people are willing to take to share knowledge and experience with people like me. 

I want to eventually get solar power. so my pet project for the last couple years has been to lower our utility usage as much as possible. i'm please to say that a few years ago we were consistent using 2200-2400 kwh a month. Now I have many months where we are between 600-1000. yippee.

I know that in the long run, I am moving towards a big savings in the size of solar power system we'll need by finding the things that are lowering our needs. you're help on this thread is awesome because this is the first time i'm attempting to be more efficient with a heat pump water heater and possibly this drain water heat recovery idea. so far I've only attempted to reduce our usage.

thanks again for taking the time. please feel free to keep sharing any ideas or experiences that come to mind.


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## gibbsgirl (May 1, 2013)

ps on water usage we used to use 6000-8000 gallons a month. overtime we're now consistently about 3000. family of 7. better than some folks do, worse than others. i'm just pleased that my family has accepted the challenge of trying with a reasonably willing spirit. not done yet improving too. seems like every few months I can see we've made more progress.


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## gibbsgirl (May 1, 2013)

well, I have not yet figured out a drain water heat recovery idea that we're ready to try yet. But, we did get the water heater replaced. Boy, what a difference that appears to have made!!!

The whole reason I started looking into heat recovery was because I found out we needed to replace the water heater anyway.

The original water heater had a tag that sad 1997 on it and that it used 5200 kwh a yr. I bought a heat pump hybrid water heater to replace it. The store in town had regular electric resistance only heaters for as low as $300. The one I picked out was $1350. The tag on it said 1500 kwh a yr.

I think when we removed the old heater it was using a lot more than 5200.

Been watching the meter the last few days since the change. I will have to keep track of it the next couple of months to see for sure what it does. But, I have the first two months of utility bills from here to compare to. If I had to ball-park it, I think we are already using about 30 kwh a day less. Yikes!!!! That is so much. I don't even understand how that is possible. But, whatever the actual drop in usage ends up looking like, I'm so grateful that it is getting lower now than what the old one used.


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