Don't forget, the heat is also keeping the pipes unclogged. If you remove the heat a lot of the fats turn into hard mass that is extremely difficult to get rid of.
Reminds me of the story of what happened when traffic lights got "upgraded" to LED to save on energy. First winter all traffic lights got completely covered in snow and traffic ground to a halt. The "waste" heat as was not such a complete waste after all, it was continuously melting snow off the lights.
Incandescent lights still waste heat 90% of the time when there’s no snow. Adding some resistive heating to run when needed on LED lights is much preferable.
It's a story about carefully examining the differences when changing technologies and avoiding imprecise or dual use terms like "waste." This is something that should come naturally to a hackers forum, but a bit of marketing does get mixed in here from time to time, and people can't seem to help themselves from embellishing interesting new technologies into universally improved solutions.
So.. you get this back and forth between the two camps as they rush to take victory laps around each other.
It's not a new technology, though. The Vancouver system mentioned in the article has been in operation since 2010, which may be a short time in infrastructure terms, but similar systems that extract waste heat from sewage have been in commercial operation since at least the mid-1980's. The system in Vancouver is now being expanded to 9 MW of power output, but there's a similar one in Stockholm that first opened in 1986 that now has an output of 225 MW. That one uses treated wastewater though, while the one in Vancouver operates on raw sewage that hasn't hit the treatment plant yet.
District heating is at a different scale in northern Europe and especially in Scandinavia. The systems in North America are (with a few exceptions) typically local to a campus, a neighborhood or a small downtown area, serving maybe a hundred buildings with maybe tens of kilometers of pipes. The public heating utility in Stockholm alone serves 800,000 people, has over 3000 km of pipes and generates over 8,000 GWh of heat energy in a typical year. It's kind of the default way of heating most buildings, here. The main exception is single family homes where past eras of cheap electricity often made people prefer other heating methods.
Such systems don't have to be huge to be useful, but the bigger they are, the more opportunities you have to move heat from places that don't want it (data centers, industry in general, etc) to places that do.
> avoiding imprecise or dual use terms like "waste."
Reminds me of the Texas power grid failure due to being an energy market designed by ENRON (still is too!). Turns out the opposite of “efficiency” isn’t “waste” it’s “redundancy” which their design is very efficient at getting rid of.
Well, moving away from "efficiency" can take you toward redundancy, but it doesn't have to. Some waste is just wasteful. It's important not to oversimplify, otherwise you wind up with counterproductive memes like "that project is justified because it will create jobs" or "the free market is the best solution for every problem" (it's a great solution for some problems, but ignores externalities).
Unfortunately the English language makes me choose between completely conveying nuance at all times and being understood and upvoted (people prefer short and pithy over long and subtly more correct).
I agree with your followup 100%. I didn’t write it in my original as it would have watered down my main communication goal.
Whenever says something is being made more “efficient” I ask them what they are trading for what.
Reminds me of the story of what happened when traffic lights got "upgraded" to LED to save on energy. First winter all traffic lights got completely covered in snow and traffic ground to a halt. The "waste" heat as was not such a complete waste after all, it was continuously melting snow off the lights.
So, just add heating element when you need it? It's probably more efficient to use LED lights than to have incandescent light bulb for the few days of the year when you need it.
If you need to keep the heat with the wastewater, don't harvest it unless there's excess heat to be harvested.
As an amateur electronics engineer... I think we can do better. I have not checked what is the actual solution, here is what I figured out on my own:
First of all, the yellow light bulb might not be enough to reach with the heat to the red and green lights. And since it is only turned on for a brief moment, might not be providing enough heat. And will still be wasting heat when it is not needed.
Even heating it up when it is cold is a waste of time. I can only guess at the number of nines, but 99.9% of the time you can get away without heating even when it is very cold, because the conditions are not right for the snow to accumulate (read -- it is not snowing, the temperature is not close to zero degrees celcius and the wind is not blowing in the right direction).
The only time the heat is really needed is when the lights are obscured with snow.
So here is a better design: have a photo cell inside the light to detect if the light is reflected back to the photo cell. The cell is isolated from the rest of the inside but has a clear access to a small portion of the glass in front.
When snow starts accumulating on the glass, the traffic light will start getting scattered in all directions including getting reflected back inside the traffic light and into the photo cell.
You can easily discern between light from the outside from light from the traffic light by seeing if it follows the pattern. Ie. constant light from outside regardless of the light turning on or off means it could be something like a sunset shining into the light and we need not worry about it.
What we are looking for is a clear difference in light shone into the cell when the traffic light is turned off vs when it is turned on.
If I can do that there is probably other, smarter people who can figure out even better solutions.
> Tucked under a Vancouver bridge, an energy centre sits on top of the existing sewage pumping station so heat can be captured before sewage reaches the treatment plant
Seems like if they’re doing this right before a treatment plant that they’ll be able to localize the problem and hopefully handle it effectively. Though it would be more assuring if they also talked about some of possible problems.
I feel like there must be better options than resistive heating to melt the snow.
For example instead of a separate visor for each light, use a single visor that covers all three lights. And maybe angle the whole thing forwards slightly so that there isn't even a vertical surface for snow to rest on - gravity would simply cause it to fall off.
It's wild to me how they're strolling through the sewer tunnel, with the voiceover explaining how it was put under ground to shelter people from all the disease carried in it... and they're wading through it, no respirator or any other protection of the sort.
Surprisingly safe given they're neither drinking the effluent nor washing open wounds with it.
The Great British Sewer Project (version one, not the recent and ongoing rebuild) was all about safe water, the disease came from a contaminated water for drinking and washing pump and not from "the Miasma" (unseen spirit forces wafting about in dank air).
The "put underground" part was mainly about isolating waste water from washing and drinking water, reducing the stench in the air was a related benefit of course, but that wasn't the vector of infection.
Just a few minutes into the video, the reporter gets some splashback in his mouth. That direct hit aside -- there are multiple "tributaries" actively dumping waste into the main tunnel. With all this moving liquid, there's bound to be some of it aerosolized into the air, especially in an enclosed space like that.
It says in the article that it sits on an existing pumping station, so that's probably the ideal place to avoid this problem, since those pumps would often be designed to grind/mascerate the incoming sewage anyway.
I'm amazed at how little district heating is used in northern climates like Canada and the northern US. In dense areas, district heating using waste heat from power plants should really be required by urban planners, or at least it would make a huge carbon footprint difference if we invest in it.
On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)
It's a cost thing. Prices for energy have been so cheap that investing in recapture capex didn't make sense. With energy prices going up, it does make sense.
A funny anecdote (or sad?), after gas prices spiked in much of Europe due to the war many manufacturing facilities were looking to cut energy use, they found a lot of waste. Many places were able to reduce their gas usage by as much as ~33% if I recall correctly.
I can't find the original source, but here's [1] one that claims 23% lower consumption across Germany in 2022 adjusted for temperature.
Reducing consumption as a response to increased prices doesn’t mean the difference in consumption was originally “wasted”; it only means it was not worth the higher cost.
Which could be a coordination problem, though. Where initial high costs are due to a lack of scale whereas if everyone participated, costs would come down fast.
I think you're misreading me. The fact that there was that much waste that could have been found and eliminated earlier is an affront to the health of our planet. At no point did I suggest freezing people in their homes in the middle of winter to die.
Cutting consumption because the price went up doesn’t necessarily mean the difference in consumption is waste. If you keep increasing the cost of energy, eventually people are going to freeze to death in their homes because they won’t be able to afford the heat. That doesn’t mean the energy required to keep them alive was wasted; it just means they couldn’t afford it anymore.
Germany is heading into a recession because the high cost of energy is destroying their manufacturing sector. They aren’t just making the same amount of stuff with less energy; they’re making less stuff and becoming a poorer country.
Or even from a non-fossil capitalism point of view, a carbon tax that equaled the externalities would lead to higher efficiency and give people a permanant financial incentive to find these efficiencies.
I remember reading on a plumbing blog (or YouTube or something) about someone who installed one of these - high end house, massive shower, owner who liked to take hour long hot showers or something.
In that case, it paid for itself pretty quickly.
For "normal use cases" the cost savings over time just isn't there, when you compare the added complexity. You're better off saving heat with on-demand heaters or HVAC recirculators, etc.
Places where it could make sense already use them (I believe car washes recycle water and heat in some locations).
There's nothing to clog up. The drain is a normal drain; the innovation is that the cold water source pipe winds around the drain, transferring some of the heat from the outgoing water to the incoming.
Heat transfer requires time, which is why I've always assumed that such a system would have to use coils somewhere.
In the system under discussion, all the heat-transferring parts had better be metal, and it won't be worth a damn except when handling a hot shower or a draining bathtub.
> On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)
$851 just for the heat exchanger, not including piping and labor. It will take a lot of showers and laundry to pay that back. I’d probably add one to a new house build but would pass on retrofitting one into an existing home unless I was doing a complete plumbing remodel. Similar to daylight harvesting LED lighting, it’s possible you’ll never see a payback on the equipment and labor.
> On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building.
I have had one of those installed in my home recently. A QB1-16 from Q-Blue https://www.q-blue.nl/en/products/q-blue-showersave/
Supposedly it saves about 50% of the loss heat. Since I dislike hasty showers, I figured that would save me plenty.
> I'm amazed at how little district heating is used in northern climates
In my country we can pick our own electricity and gas provider, but for district heating you're tied to the provider for your area. Consequently, people with district heating get fleeced. When buying a house, I gave preference to the ones without district heating.
David MacKay discusses this in Sustainable Energy Without the Hot Air. He concludes that, while it's on first glance appealing to want to use waste heat from power generation, heat pumps are strictly superior to combined heat and power except for in a few specialized circumstances (e.g. industrial uses that require high temperatures). The book is fifteen years old now, so I suspect the math even more strongly favors heat pumps than when it was published.
But district heating is a distribution mechanism, not a heat generation mechanism. Cogeneration plants are one way to power it, but you can also use waste heat from industrial processes, very large scale heat pumps, or burning household waste. All of these are in common use in places where district heating is common. The public district heating utility in Stockholm claims to have the world's largest heat pump installation, which has the capacity to extract 225 MW of heat from treated sewage, in the process generating both heat that's sent into the district heating network, cooling that's sent into the district cooling network, and finally a small amount of electric power by releasing the treated water into a lake via a turbine. This facility opened in 1986. The utility also has various other facilities, both cogeneration plants that burn biofuel (mostly byproducts from the lumber industry) or household waste, as well as other heat pumps that extract energy from seawater.
District heating/district cooling has three advantages: you can change the heat source centrally without having to refit every single dwelling, there are economies of scale in the heat generation, and you can take advantage of heat or cooling that's just in the wrong place and transport it to where it's needed.
5th generation district heat systems can transport water at roughly the same temp as the underground and use distributed heat pumps to bring them up to the required temperature at point of use. This prevents heat losses along the way.
I’ve often wondered if late if rather than a “wet wall”, if houses should have a “hot wall”. Could I run a thermal loop past the fridge and a couple other appliances and either shunt it outside in the summer or distribute it around the house.
Total BTU aren't that great. Would (somewhat) work for a very small apartment, for any big house you would have a better experience making a proper thermal insulation.
Given that people are pointing out that heat recovery from washers doesn’t really work because they alternate between filling and draining (so there is no counterflow except incidental from other house activities), I’m not so sure that would be true.
Particularly if you use heat pumps instead of basic plumbing to achieve the transfer. Dump heat into a glycol line with a reservoir.
Seems to me like the electric power grid is an already existing, efficient energy distribution mechanism. Converting waste heat to electricity is probably a lot cheaper to retrofit than adding a new distribution network to places that don't already have one.
To my knowledge there's no really effective way of converting low grade waste heat into electricity. Basically all heat-based electricity generation today relies on being able to boil water into steam to spin a turbine. But with heat pumps and district heating, you can recover useful amounts of energy even from quite small thermal gradients. That's why things like room temperature sewage and waste heat from data centers can be useful to these systems even though they couldn't be used to generate electricity.
Also, if you rely on local heat pumps, you still need to take the energy from somewhere. Taking it from the air is fine in warmer climates, but in colder climates the efficiency of air-source heat pumps starts to become an issue when it gets colder. You can extract heat from deep underground, but that doesn't universally work in densely populated areas because the ground doesn't contain infinite amounts of energy and it's possible to extract more than it can sustain. So, in cold climates there's definitely a use case for distributing heat using water pipes, with or without local heat pumps in every building (see the sibling comment about 5th generation district heating systems).
Thermodynamics prevent you from turning low grade heat into electricity with anything resembling reasonable efficiency. Moving the heat around on the other hand it reasonably simple.
Yeah; if just used for hot water, this doesn’t make much sense compared to a hybrid heat pump water heater. Here in California, with a well insulated/partially passively heated/cooled house, our hot water is well under 10% of our electricity bill (ignoring our EV).
In places where it’s really hot or cold, it would be an even lower percentage. The money that it would cost to trench and maintain the hot water distribution lines for a house would probably be better spent on heat pumps, architectural features or upgraded insulation (Even just $2000 of extra insulation does an amazing amount of good vs. bare minimum code insulation, and I doubt you can get district hot water installed for less than that.)
> I'm amazed at how little district heating is used in northern climates like Canada and the northern US.
It is used on most if not all Canadian university campuses. Canadian university buildings are also far less sprawling because they're all connected to a steam generation plant.
Similarly, I know Harvard and MIT, and also parts of Boston have centralized steam heat systems. Harvard uses theirs extensively still, at least in the old buildings, but I'm not sure about MIT. Boston's is fed by an old commercial steam plant on the outskirts of Chinatown. I think it covers a pretty limited area though. I never lived anywhere with it.
The problem is adding the infrastructure where it doesn't already exist. District heating is great when building a new city, but we don't do a lot of that anymore. Just imagine the cost of laying new pipes under an entire city. I'm sure someone smarter than me can do the math, but I'd suspect it'd take many decades to recoup the environmental and economic costs of installation.
> I'm amazed at how little district heating is used in northern climates like Canada and the northern US.
District heating is very expensive to build and difficult to retrofit. Doesn't seem very surprising to me. Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity. As far as I can tell, this is universally true among western nations.
> Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity.
Former trench digger here. The cause are usually two problems... first, the "underground line maps" ("Leitungskataster" in German) are usually not up to date because not everyone bothers to update them. Quite the "fun" if you suddenly hit a cable at 50cm when the map says you should be clear up to 2m!
The second problem is that Western societies almost always choose the cheapest bidder, which means no work on weekends or, heaven forbid, 24/7 as that would be too expensive. That means everything is slow as molasses.
Why is it cheaper to have one shift work on a road project that takes months, compared to three shifts working around the clock making the project take 1/3rd of the time? I've always vaguely wondered that when driving through construction zones that are seemingly devoid of workers most of the time.
I assume the workers are paid hourly, so wouldn't the cost be the same? Are second and third shifts more costly?
I think it's only cheaper because people aren't being compensated for the annoyance caused by road works that take forever. Months of detours, traffic jams, dirt, people struggling to get to their homes/shops/whatever, probably even extra accidents. If we were to put a monetary value on those, maybe the cost of working odd hours would be compensated.
It might not even be necessary to work 24/7. My street was recently dug up over its entire length to install new water mains and sewage pipes. It took nearly 2 years. Only 1 crew of some 5 people were working on it at any given time. Given that the street is roughly 1km in length, at least some of the work could probably have been done in parallel with multiple crews.
> Given that the street is roughly 1km in length, at least some of the work could probably have been done in parallel with multiple crews.
This is routine in Asia. Just look at this video remodeling a railway station in just a few hours [1].
For that to work though, the construction companies need to have the backing of the government that there will always be budget to keep these people employed - even for supposedly "low skill" work such as road construction, recruiting and onboarding costs are significant. But when government appears to be in budget crisis to everyone reading a newspaper, no reasonable construction company will hire more workers than they could also use for private projects (e.g. home construction, garden remodelings, ...) if the government decides to undergo yet another shutdown.
Late 2022 my partner needed to go to hospital here in the UK at something like 4am. On the way there we got slowed slightly by roadworks to replace a (normally) very busy roundabout. The signs all stated that the working period for the rebuild was something like 11pm to 5am for that single night.
It was an eye opener for me, it had never occurred to me that such late night roadworks was carried out to reduce daytime disruption to traffic.
There again, this is in a city that is currently expanding the district heating system that they installed when they built a new waste incinerator - so that the process of waste is: 'burn the waste, heat water to turn turbines to generate electricity, the waste steam is then fed through pipes in the city to hospitals, council owned buildings, and some select high rise apartments, then the water is returned to the incinerator building to be reused. Solid remaining 'slag' from the burnt waste is used to make non-fossil asphalt. And the incinerator building also has wind turbines and solar panels covering the roof.
> I assume the workers are paid hourly, so wouldn't the cost be the same? Are second and third shifts more costly?
At least for Germany, yes - by law, night and weekend/holiday work must both be approved by the authorities and compensated. Most union contracts here go way beyond the vague notion of the law and require significant compensation.
Overtime is more expensive. Shift differentials (paying more for 2nd and 3rd shifts) makes those shifts more expensive. There are more workers willing to work in the 7A-4P window than in the 9P-5A window.
The builders of my house (who were angling for, and got, PassivHaus+ certification for it), built a heat exchange for the drains on the second floor that warms the intake to the hot water heater. It works, but it's pretty limited, in practice it does pass some of the heat from a bath or shower or dishwasher run back, so that's something. Of course the timing matters, water drains pretty quickly, even from a bath, the there's no coordination to guarantee the water heater runs while that happens (other than the good chance that if we just pulled hot water out it will be running to replace it, but sometimes that isn't immediate).
The water heater is always refilling. Otherwise you’d have no hot water pressure. It’s just that the new water goes to the opposite end of the tank from where the output is attached.
Hmm, good point, I guess I hadn't thought about it, but that does mean whatever residual heat is in the water draining does get transferred to the water coming in to the tank, which if anything means the heater is less likely to come on.
For showers definitely. But as someone astutely observed, washing machines (clothes and dish variety) pull a bunch of hot water, and then dump it out a half hour later, and then may or may not pull more hot water afterward, but never while they are draining. I guess there's an expensive reservoir for that and that sounds like more water damage waiting to happen.
So the only heat transfer you'd get there would be whatever heat capacity the exchanger has. And since the thermal conductivity is high, all it's probably done is heat up the air inside of your walls a little bit.
The best alternative to me seems like heat pumps, although you might also be able to scavenge heat to feed into your radiant heating system, or the cold air exchange on your furnace. The former seems more likely not to cause problems on hot days.
The DoE quoted a figure of 30% savings on showers, which is pretty impressive. All houses should be built so this is standard. It's so much more expensive to retrofit it later especially if the layout doesn't have the shower over the water heater.
I was just about to post that same link. I suspect the reason you don't see them much is because installing one will often require more plumbing and more labor. Residential installations in the US are particularly sensitive to up-front cost, and tradespeople in the US tend to be conservative in choosing 'new' technology.
When I renovated my house, I installed such a heat exchanging pipe. It reduces hot water usage (=fossil gas) by about 40% and with my usage payed itself back in about two years. It's not a drop-in replacement, since you should use it only for the shower drain.
No, it preheats the cold water connected to your shower. Warmer cold water means you need less hot water to achieve the same temperature in the shower.
In Sweden most of the population is covered by district heating. I think it is mostly a matter of population density, in Sweden most people live in 3-6 floor residential buildings clustered together close to public transport hubs. In Canada and US you have urban sprawl and a lot of rural population, so running district heating pipes to every single house is just not worth it.
To be fair people who live in detached houses in Sweden often don't have district heating either, but the % of the population living in detached houses is tiny.
I guess if everyone did that (residential heat exchangers on your house drain), there could be a real risk of the sewage cooling and congealing in the downstream pipes, causing a blockage.
This would work as a sort of energy storage - you could overheat your water reservoir during cheap power times and then transfer the extra heat to the entire city the rest of the day.
You would need a lake sized insulated reservoir for that to work. Too much water is used already by those systems, they don't typically want to add another storage. Typically they are gigawatt scale systems burning something because just using electricity is too much strain on grid, unless you have a power plant nearby (like cogeneration, which uses waste heat from electricity generation from fossil fuels).
Why? Just overheat the water circulating through the district water system when you can. People's thermostats will simply turn off sooner, leaving more heat in the water.
Then later, you underheat the water, and the city's various thermostats turn on a bit longer.
The risk is that you underheat too much, and people compensate by plugging in their own heaters during underheating. And underheating will occur during expensive energy periods, so this will exacerbate periodic energy shortages. But overall this could work to store a ton of energy - there's a huge volume of water in a district heating system's pipes.
Some places already used flooded mines or other underground reserviors for this purpose. As an added bonus, they're already warm and continually have heat replenished by geothermal heat.
I live in a building connected to the False Creek system. Some details that may be interesting:
- Most of the apartment blocks currently served are located in a former industrial area that was redeveloped for the 2010 Winter Olympics and gradually built up since then.
- All new buildings in the utility's service area are required to be connected to the system for space heating and domestic hot water. The city charges a connection fee and the building must be designed with the requisite plumbing to integrate with the system, but some space is saved by not needing a large central heating plant.
- The hot water is indeed very hot, and reliably so. I wouldn't be able to tell the heat source is sewage if the owner didn't mention it.
- In Winter, it's easy to identify which buildings which are connected to the utility, since they don't have have large steam clouds coming out the top.
> Drain-water heat recovery technology works well with all types of water heaters, especially with demand and solar water heaters. Drain-water heat exchangers can recover heat from the hot water used in showers, bathtubs, sinks, dishwashers, and clothes washers. They generally have the ability to store recovered heat for later use. You'll need a unit with storage capacity for use with a dishwasher or clothes washer. Without storage capacity, you'll only have useful energy during the simultaneous flow of cold water and heated drain water, like showering.
> ...
> Purchase prices for drain-water heat recovery systems range from $300 to $500. You'll need a qualified plumbing and heating contractor to install the system. Installation will usually be less expensive in new home construction. Paybacks range from 2.5 to 7 years, depending on how often the system is used, and the temperature of the incoming water, which is dependent on ground temperatures.
How do you actually implement this? Waste pipes are huge compared to supply pipes, and rarely take even remotely similar paths. In my home, which I'm repairing right now, I have one water heater but three different waste egresses from the house.
> A DWHR unit consists of copper pipe tightly wound around a vertical section of copper drainpipe. As water flows down the drainpipe, it clings to the inside surface of the drainpipe. The heat from the drain water is transferred through the copper drainpipe to fresh cold water flowing in the outer copper coil.
> The warmed water is then sent either to the hot water tank or other permitted end-use. In either case, the amount of energy needed to provide hot water is reduced. DWHR systems provide greater potential for energy savings as the number of simultaneous flows increases.
You wouldn't necessarily need this on every waste water egress. In my house, I've got one shower egress route that it would make sense on. The kitchen (I don't have a dish washer) doesn't run hot water for long enough for it to be reasonable - nor the 2nd floor bathroom with just a toilet and sink.
(edit)
There are spots where (in my house for example), this could be more useful. For example, that 2nd floor bathroom I've looked at a tankless water heater for the sink (so that I don't have to pull hot water from far away and then could simplify to only need to run cold water). Capturing the hot water there and returning the heat there there may make some sense... though for just a sink that would be a long ROI time.
Dynomight suggested [1] recovering the heat from hot showers by showering with the drain stopped and letting the water cool to room temperature before draining it.
I remember thinking it was interesting at the time, but I have not tried it (my shower has a low lip so it couldn’t hold very much water).
Being a cheapskate, I did this when I lived in Chicago winters. My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.
This also has the benefit of potentially raising the indoor humidity, which is otherwise going to be quite dry.
> My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.
Is that a bad thing? I honestly ask. I'm twisting and turning the idea in my head and can argue in both direction.
I guess it can be a bad thing if you live by letting the interior go to cold frequently and then try to heat it back to room temp. For example if you would turn off the heating for the day, and then try to heat it back up from near freezing to room temp. That would be bad in itself, and the the extra mass would make it worse.
But if it is just maintained near the room temperature point then it is not that clear to me if it is bad. Would be happy to hear either way.
The more I think on it, the less certain I am one way or another.
From a real-world standpoint, I would lower the thermostat say 15 degrees F during work hours, so re-heating that water when I return seems like it would be a definite negative.
It might help to think of the tub of water as a heat battery/capacitor: the energy you put in is the energy you get out. So, yes, it will slowly absorb energy when you are heating the room. However, it will also release an equal amount of energy as the room cools down. On that front I'd say it equals out.
Technically the tub of water will change how fast the room changes temperature. Because the difference between the room temperature and outside temperature affects the rate that energy escapes the room, this could affect how much energy the room emits to the outside. For example, imagine that you had so much water that the room only went down to 65 degrees F throughout the day; that means that you put enough energy into the bath battery to heat the room to 65 degrees F throughout the day.
The other way that you would be consuming extra energy is by evaporating the water. At <70 degrees F and little air circulation, I don't imagine that evaporation is consuming a lot of energy. A bath tub of room temperature water doesn't seem like it would be very effective as a swamp cooler.
My guess is that a bathtub of room temperature water wouldn't measurable affect your heating bill.
If you drain the water when it’s at its coldest—right after you get back from work, say—that larger temperature swing might actually work in your favor.
Capturing heat before we flush it down the train is probably preferable to extracting it after, given that delta T will be higher. There are shower drain heat exchangers which can recover some of the heat in the water going down the drain.
Not much time to capture heat in a flowing drain, many small heat exchangers will be less efficient and more expensive than one large one, very hard to fit in a small space without utilising a compressor, plus additional plumbing - the captured heat pipe would have run all the way back to the water heater, losing heat all the way.
Since we premix the hot and the cold water and then spray it over a human body, 30% recovery rate sounds impressive honestly. I doubt you can go much higher than that even with the counter flow design they undoubtedly already have.
I wonder what the breakdown is by usage. I imagine the combination of: dish washer, sink, and clothes washer, might make it appealing to collect it all at once.
Clothes detergents have been able to effectively clean in cold water for decades. The only reason people still wash using these detergents in hot water is ignorance. It's such a huge waste. Please stopping washing clothes with brand name detergents using hot or warm water. It costs more money, does more environmental damage, and doesn't even make your clothes marginally cleaner.
Ignorance? I doubt it. There's a variety of reasons to wash at least some items in very hot water. Killing dust mites for people with allergies sounds like a good reason. Getting rid of faecal contaminants sounds like another good reason. It does way more than making clothes "marginally cleaner".
Not commenting to argue either way, as I have no idea what the truth is. But, I’m surprised that there can be such strong, confident, and basically contradictory opinions on this topic which seems to me like it ought to be a simple matter of fact.
As someone who has done laundry before, I feel confident in chiming in here: Anecdotally, hot water is almost always better for reducing smells in my clothing. It helps some fabric types more than others, but it doesn’t hurt any of them.
If you can afford a home, electricity, water, detergent, and clothing, I personally cannot imagine a single reason not to use hot water.
If you really want to get something very clean then add 1/4 cup of Borax and use Extra Hot if your washer supports it. Source - me - I wash household blankets the kids make super dirty sometimes. If the material can handle the heat (no shrinking or delicate materials issue) then the Borax really does super charge the detergent effects somehow.
I don’t doubt there might be something environmentally problematic with Borax but I just am not aware myself.
I had a girlfriend years ago who insisted that cold water worked just as fine as hot water. While I didn't write down any observations, I did do several loads of laundry over the next couple weeks where I adjusted water temp as a variable, and found that things typically smelled more fresh when they were washed in hotter water.
Maybe it's placebo, maybe it's my detergent, maybe it's something else, but for me, personally, I always wash non-delicate laundry with hot water because at the very least, I feel like I smell better, and that's worth it to me! :)
My dishwasher uses 1kwh of power per load, thats hardly worth capturing
Shower tho… esp if you purchase ready made shower capsule that does not require external hot water supply and produces it’s own from recycled water? That might be the one.
That photo of steam coming from the manhole cover in NY is pretty goofy. NY has 100s of miles of super-hot steam pipes underground. You'll see that same steam come up from manhole covers after a good rain in August. I'm pretty sure it's not from wastewater pipes.
Doesn't even have to be hot, it just has to be hotter than the air above ground, and relatively humid (how much depends on the temperature difference). You can also see rivers steam when it gets very cold, and they're often only a few degrees above freezing at that point.
Is all the steam from hot wastewater? With all the things going on underground in cities, I've assumed there must be heat (i.e., from basic thermodynmics), and of course it needs a place to escape.
In some places, India for example, hot water is sometimes supplied by a smallish reservoir at the showerhead. The reservoir's normal state is full of tap-temperature water. For a hot shower, you turn on the reservoir's electric heater; it warms up (pretty quickly - think of electric kettles, and I don't want to shower in 100 deg C water) and you turn on the shower.
I've always wanted something like it at home; it seems much more efficient than a central hot water heater that's always warming the water. (I've also read about at-the-spigot devices that heat water like a coffee maker, with water running past something hot.)
I wonder how the heat capture of wastewater could work with it. It becomes a sort of closed loop hot water device, with heat exchanged with showering humans, the capture device, and the electrical device to top it off.
I don’t know what current wisdom is, but when these were new they (eg This Old House) would suggest putting one at the master bath and maybe the common bath, then run one for the rest of the house.
Effectively daisy chaining two of them for piping hot showers, but still warm enough for the washer and dishwasher.
Pretty sure the deal was that none of them could really pull a couple gallons a minute at full temperature at the time.
Home plumbing doesn't generally do multiple pipes. I wouldn't know if that's a code thing or just a complexity thing.
But my understanding is that the heat output of the inline heaters is proportional to flow rate, so you aren't achieving much by running them in parallel instead of series.
It's not that the early devices couldn't handle 3gpm, it's that they couldn't heat 3gpm, so you got really warm water but not hot water. No idea where things stand now.
I’m really not entirely sure. I haven’t been around one since I traveled overseas and that’s been a while now. My new place has a new furnace and the old one we never got around to doing improvements so I just remember the old complaints.
Thermal conductivity seems likely. How many feet of pipe can you cram in there, especially if you have to insulate so you don’t catch the building on fire?
Or can you make a 3d counterflow system so you don’t have to insulate at all?
ETA after further reflection, I believe they generally worked better in Europe and that would most likely be down to 220 vs 120, so you may be right about power density.
There's an installation near Cambridge that uses the heat to warm industrial greenhouses. It has a methane powered generator which runs the heat pumps and releases its exhaust CO2 into the greenhouse to promote growth.
Clean water flowing in the pipes to a house will often be warmer than outside air during winter and have a higher heat capacity than air used in heat pumps.
No idea if using another heat pump and lowering the temperature from say 10C (ignoring any water heating in the house) to 6C of the gray water flowing out of the house makes sense.
I thought about the exact same thing, inspired by my admittedly too long showers. Although I pondered whether too cold sewage could lead to more blockages.
Honestly? It's simply not worth the effort. Yes, we can put a grey water reservoir who receive water from washing machines, dishwasher, showers, sinks and so on, with a simple three way thermostatic valve who divert just hot water to the reservoir and than a heat pump suck heat for it to push in a room, but the quantity of heat we can recover is simply too little except maybe for some industrial activity that produce hot water as a result of some process (like an industrial laundry).
It's simply too costly creating such system than the energy we can recover.
The idea is to coil the sewage pipe with the cold water pipe before it goes into the water heater. That way the cold water will pick up some of the excess heat from the sewage pipe from the shower and it will cost less to heat overall.
This is what happens when sewage is given time to cool: https://www.youtube.com/watch?v=3i_axpk0a7Q
Reminds me of the story of what happened when traffic lights got "upgraded" to LED to save on energy. First winter all traffic lights got completely covered in snow and traffic ground to a halt. The "waste" heat as was not such a complete waste after all, it was continuously melting snow off the lights.