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Old 04-17-14, 12:38 PM   #1
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Default DIY drain water heat recovery setup using PEX tubing

Another nice piece over on builditsolar. This is a pretty simple setup. The guy just shoved a bunch of PEX into a drain line. The results aren't crazy good, but it is simple. No doubt this design could be improved upon, but for a simple cheap setup it is something.

DIY Shower Water Heat Exchanger

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Old 04-17-14, 05:57 PM   #2
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Looks like a recipe for a forever clogged drain to me.
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Old 04-17-14, 11:36 PM   #3
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The standard approach is to take a long straight section of the drainage pipe, cut it out and replace it with a copper pipe(the full length of the pipe I'd imagine, 6 feet or whatever they come in), then you take flexible copper of a thin diameter and bend a coil around the copper pipe with the cold end towards the bottom and coiling up the pipe for the best heat transfer and insulate the crap out of this wrapped up pipe to not lose too much heat. You then run the pipe to the cold side of your water heater insulating it with the thickest pipe insulation you can find.

When I'm looking at this being done, it seems pricey and tons of work, not to mention that the more you add to a plumbing system, the more chances you have to develop a leak. Copper is pricey and people on this forum are suggesting to use PEX pipe because it transfers less heat than copper so I'm not sure that alternative works that well. I'm on board with jeff5may and his comment about the forever clog. I suppose as long as this is in a straight run down pipe and you have a screen in your drain for hair and gunk and this is only catching water from the shower there is less chance of it clogging up if it remains clean water. You'd need to be sure that you've got a safety margin for flow for when soap scum gets goo on those pipes. Seems high maintenance but maybe you can get away with it. I just wouldn't recommend it unless everyone using the shower has diligence to be sure that that drain can remain as clear as possible and to have a hair catcher in place that gets cleaned out often.

I'm still not sure what savings you'll get. I've caught the water from showers in the tub with a stopper and when I was done with an 8 minute shower I would grab my IR gun and shoot the tub of water and it was already 80 degrees when I was taking a shower with 120 degree water. Now take the 80 degree water and figure you are probably going to be getting 60 degree water back to the water heater at its hottest because how much can you really transfer? So you are getting maybe 15 degree rise back to the water heater and you are still heating it another 60 degrees in the tank or tankless water heater.

I think this is a case where you go with a Niagara Earth 1.25GPM head or something else 1.5GPM or better of your choice. Then get everyone in the house to practice low energy showering practices(not taking forever in the shower or showering in a get wet, shut off, soap up, then turn it back on to rinse off type regimen). Then do the math on how much energy you use for showers and how much you realistically feel like you will gain doing this. When my house is cold(between 40-60) I take a shower that is 8 mins max because that shower warms me up and is cheaper than heating the house to 65-72 or so. In the summer when the house is warmer, my showers use between 2-5 minutes of water and I can actually turn the thermostat to vacation and I can go an entire 2-3 month period without turning the burner turning on, just heat from the tiny pilot flame that between it and kitchen cooking amounts to 4 therms($3 of gas). Dishwasher with the heated water function and drying uses 1kwh so I don't worry much about that and the 5.8 gallons it uses per load.
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