The neglected clean heat we flush down the drains | Hacker News Reader

The neglected clean heat we flush down the drains

bbc.com

23 pointsljf14 comments

You can also do this at a household level. It is called heat recovery drain pipe, basically a heat exchanger that replaces a vertical section of drain pipe. The wastewater heat is used to pre-warm the fresh water going to the water heater. Water falling down a vertical pipe tends to spread out and cling to the inner surface, giving you more surface area for heat transfer.

Last time I priced this system out the installed cost was going to be about $1000, at that price point the payback time is pretty long for a single family home but much better for an apartment building.

https://www.homedepot.com/s/power-pipe?NCNI-5

It would seem like a water heater using heat pump tech (these exist) could use this as a source of heat to scavenge in addition to the surrounding air.

I've also wondered why we eject the hot air from running an AC to the outdoors, while we then pay to heat the water in our hot water heaters. That seems like a no-brainer to use that to help heat the hot water.

Yep. Most homes have multiple sources of waste heat. Clothes dryers, ovens, AC, refrigerators... The heat from refrigerators is vented into the home, which is inefficient if you are running the AC.

I've been wondering for a long time why we don't have whole-house heat recovery systems.

> I've been wondering for a long time why we don't have whole-house heat recovery systems.

Every house in the Nordics built after 1985 or so has this.

In theory, you can have an AC or heat pump that shares a refrigerant loop with your fridges and freezers. But the efficiency gains are extremely marginal and it adds a lot of extra trouble. Right now you can just buy a fridge or a freezer, plug it in, and it just works. You don’t have to plumb refrigerant through it. Which is good because it reduces the risk of refrigerant leaks.

I spent a while looking to see if split-unit refrigerators exist-- not really (except giant commercial room size stuff). The reason is that mostly refrigerators are so well insulated that their waste heat is extremely small making it worthless to do much of anything with it.

As a recent homeowner, I've looked into some of this stuff, and my conclusion across the board has always been - it's never worth it.

Better airtight windows? Not worth it.

Heatpump? Not worth it.

Recycling/whatever? Not worth it.

Solar? Rarely worth it.

The basic math is that if you have a payback period of over 20 years then you'd do just as well to put that money in the market and then keep paying your extra electric bill. And on top of it, that's assuming your new thing (solar/whatever) works 100% perfectly with 0 effort or thought for 20 straight years.

I really want the math to work out on heatpumps and such, but when they are 30k installed it's just not there. I don't know the exact answers, but it does feel suspicious.

(Edit: Caveat, I want to acknowledge that there is a totally valid moral reason to do all of this, and I don't intend to negate that.)

Some of this stuff is only not worth it because it's insanely overpriced.

DWHR is a fine example of this: the device is just a couple feet of double walled copper pipe. It should be dirt cheap, but instead they sell for $800.

The real limit for DWHR is that you need a vertical drop to get sheet flow, a horizontal section would be much less effective. Also there is a moderate plumbing complication from the fact that your 'cold' water in the shower also needs to be sourced from the DWHR to not have a massive heat leak.

With the high price of the DWHR and the plumbing complications it'll never pay for itself except with the highest shower usage in the coldest climates.

That said, cost shouldn't be the only consideration. Even disregarding environmental concerns a building that is more passively efficient will work better during outages/energy shortages.

Dang! The only quote I got for a heat pump that was over to $20k was the geothermal source variety. I'd make sure you check with a few vendors and stay up on possible rebates in your area.

My solar is worth it. $20k to generate all the power my home will use for the next 30 years. Net metering really helps make the numbers work.

Glad that's the case. I do find that in certain areas, with certain electricity costs, solar can be worth it.

But I also find that a lot of people are miscalculating their break-even point.

If I invest $X, the default return is 5% forever for infinity years with 0 maintenance.

So if invest $X in solar, and return 5% for 30 years and then the system breaks, then I lost money compared bonds/whatever.

(Again, most of solar investment drives technology improvements and is a great way to help the world become more sustainable)

Seeing how sewage treatment plants need to heat up the sewage for the treatment processes to work it’s a zero-sum move at best.

The movement of heat is proportional to the heat difference. So if you're going to move the sewage across miles of basically uninsulated pipe, you're better off taking the heat out at one end (and using it very productively, essentially all the heat removed by DWHR is used!) and then putting it back in at the other, then you are leaving it in the pipe to have 99% of it lost to the environment.

DWHR isn't very efficient in and of itself, but that is because the short contact time and small temp differences doesn't permit it to remove much heat from the waste stream. But what it doesn't remove is still there, so that for of inefficiency isn't relevant to your comment.

I think you are assuming that there won't be any heat loss wbtween the home and the plant

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