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In the winter, I rather enjoy hot showers. I try to keep them short (5-6 min), but it's still a lot of heat going down the drain and out of the house - particularly with 3 other people who do not have the same timing compunctions. I was wondering if it would make sense to install a heat exchanger between the incoming and outgoing water lines (possibly with a valve to skip it during summer) to capture that heat.

I don't know enough about the efficiency of available heat exchangers to come up with a ballpark estimate for heat energy (and cost) saved.

Possibly relevant data: four adult household, roughly one shower/(day-person), average time 15 min. Water temperature of ~110ºF (~43°C), gas heated (recent heater).

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  • Any answer to this question is going to be a guess (with the information given), and can only apply locally as prices and availability of materials vary. – Highly Irregular Feb 3 '13 at 18:03
  • The most economically sensible option will always be to externalize your costs, which is why need a sustainability movement in the first place! – Jay Bazuzi Feb 3 '13 at 18:11
  • That might work for me, but externalizing costs isn't a society-wide solution - so the question remains as to whether it's a good idea for individuals to pursue this sort of upgrade. – AlexanderTD Feb 3 '13 at 18:22
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    take the temperature of the water in the waste pipe (or put the plug in while you shower, then take its temperature) - I think you might be surprised by how low it is – EnergyNumbers♦ Feb 3 '13 at 18:48
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    @HighlyIrregular while you're right that an answer that only addressed the specifics would be too localised, a good answer should address the conceptual issues - the components of the economic and physics calculations - and would therefore be useful to anyone asking a similar question, whatever their bathing habits. So this question is not too localised: but some answers could be. – EnergyNumbers♦ Feb 3 '13 at 18:54
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There are drainwater heat recovery systems in production. It is mostly made of copper and in principle it's a passive, opposite flow heat exchanger. The freshwater coming to the house circulates through thin pipes wrapped around a bulky waste-water pipe.

Any obstructions in the way of waste-water could cause clogging, that's why there is (probably) only one design possible. One example for all: One example for all - Power Pipe

This is economically sensible because there is no energy input needed to collect heat from the waste-water. You pay only to buy the equipment and to install it. It can be used on vertical and/or horizontal sewage lines. I'd wrap the exchanger into mineral wool or PUR foam after installment.
Installing a heat pump has very little advantage, because incoming water is usually only several degrees above 0°C. You cannot go lower to prevent freezing the sewage line.

The efficiency will depend on:

  • the length of exchanger - the longer the better
  • the intensity of flow of the water - the lower flow the better

Update

If possible, the incoming water should be split before the system. Then only the water to be warmed would be heated and cold water would always stay cold.

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    This would heat all the incoming water? Because you may also want to have cold water from the tap. – mart Feb 8 '13 at 11:14
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    @mart: Thanks for an idea, 1.) see my update. 2.) Even without splitting the incoming water before the system it would be bearable. This system heats water only if the draining/leaving water is warm. If you use use mostly cold water, you get the same temperature. But if there is somebody taking a shower, then all the water (cold too) would be warmer. So it depends on how cold tap water you require and when. – Peter Ivan Feb 8 '13 at 11:39
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    I'm surprised there is no discussion of the embodied energy of that much copper. Regardless of cost, how long does it take the energy saved from hot water to equal the energy needed to produce the copper? – half-integer fan Feb 8 '13 at 13:08
  • If you mean your question seriously, post it via sustainability.stackexchange.com/questions/ask. It might be constructive. – Peter Ivan Feb 8 '13 at 22:50
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    @half-integerfan perhaps aluminum could replace copper, but you loose about 40% conductivity according to Wikipedia. Then a clever design would be required to mantain a high efficiency. Also consider that the device can stay in place for 10s of years. – clabacchio Sep 23 '13 at 10:15

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