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Comment by someweirdperson | Hacker News Reader

A mixture of hairs and fat going through a heat exchanger doesn't sound like low maintenance. Luke warm water remaining in the exchanger after a shower feels like a great way to cultivate bacteria, though on frequent use that's probably no issue.

Tapping the waste water as heat source for a heat pump seems like a better idea.

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Typically the drain pipe for such heat exchangers is a normal drain pipe — large diameter, no kinks, etc. — and made out of copper. A larger pipe surrounds it, bushings and T sections at either end of the segment. Cold water flows into one T section, flows around the drain pipe and extracts heat, then out the other T and into the cold water input of the hot water heater. There are a million YouTube videos on how to make one, and there are now readymade commercial products to buy, too. The tricks are getting at your shower drain and getting the cold water input piped over to it.

I get the desire to do stuff like this but the numbers never work out. Wrapping a large diameter drain pipe with copper is hella expensive and a terrible heat exchanger. It would take a century to recover the cost of the copper plus installation. Also It's best not to screw around with waste water. You want that stuff to get to where it needs to go. There are enough problems with it even when you're not messing around and even one backup during the its lifetime would easily cause enough damage to offer the entire thing.

It’s not the whole drain pipe. It’s maybe 18”. Encourage you to google it.

Why exactly 18"? That's some specific installation. You can make it any length you want. I encourage you to not be so smug.

Do you mean t section? That to me means a junction with one side going off at 90, like a T. I don't get why you'd do that in this situation though.

Check out the link posted. It's a pretty clever solution that wouldn't require any more maintenance than a typical drain. It would be not nearly as efficient as a typical heat exchange, but would also be low maintenance.

http://www.heatsnagger.com/product/hxdrain-dwhr-for-shower/

This seems advantageous if the water heater is right next to the drain, like a lot of European systems.

In massive American homes the water heater could be hundreds of feet away from the shower.

I figured that the most sensible way to install theses is in the cold water path directly in the shower. That way (part of) the heat is recirculated in the shower. The savings then come from having to run less hot water and thereby having to spend less on water heating. This works even if you have a central store of hot water (as I think is more common in modern European homes these days).

If the heat is already higher temperature than the target environment, then a heat pump is just a fancy resistor. Plus this doesn't obviate the need for a heat exchanger.

Better to just have a filter on the drain similar to a dishwasher.

No, the source is the waste water, the target is the fresh water. You could either use a heat pump to move the heat from the colder waste water to the hotter fresh water, or use a direct heat exchanger to pre-heat the freshwater while chilling the waste water down, to then put a resistance heater between the heat exchanger's freshwater outlet and the shower head's input.

But it should be more efficient to run a heat pump against the heat exchanger's inefficiency/delta then to let it happen and compensate with an ohmic heater.

Yes, the wastewater discharge with a 100% efficient heat pump might actually leave colder than the fresh water comes, due to heat loss to the room exceeding the work required to pump the heat against the exchanger's losses.

Ambient temperature water is entering the system somewhere. You'd put your heat exchanger there.

The replacement for the lost heat would happen to the equilibriated water and could happen with a heat pump. In principle (if your heat pump and exchanger is perfect) this generates no entropy so is more efficient than putting things with finite temperature delta in contact. In practise it's probably just simpler and requires a smaller heat pump.

Whether you use the waste water (which will still be a degree or two above as exchangers aren't perfect) or whatever your normal source is (ground source may be warmer) would be about cost and making the heat pump work for not-the-shower too.

Is it a fancy resistor? If you have a cop above 1, you're extracting more energy than could be got just using straight heat exchanger. I don't see why that would break just because you go above an arbitrary temperature (arbitrary because the output temp of the heat pump isn't the ambient temperature of the room)

Or am I missing something?

The heat is conserved. If the waste water is hotter than the incoming water it will conduct on its own (and if they're arranged in back flow the equilibrium will be pretty close). Hurrying it up with a heat pump only makes it happen faster and adds the energy you put into the heat pump. You could cool the waste below ambient, but that's no different to cooling anything else outside.

For simplicity, if your hot water system was already a heat pump, you could run it past/into the cold side. No real efficiency gain vs exchanging it with the cold water (actually a loss in theory) but might be cheaper.

Ok I've worked it out myself. I was inventing a magic free energy machine.

I was thinking a cop of 4 would be able to extract 4 units of energy to the heat exchangers 1. But there's only 1 unit of energy in it to extract, which the heat exchanger is already extracting basically for free.

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