Stop pouring hot water down the drain | Hacker News Reader

Stop pouring hot water down the drain

green.yahoo.com

125 pointschaostheory49 comments

Dup: http://news.ycombinator.com/item?id=529552

And the messages there indicate this device probably isn't practical, it has too many caveats. And if you want to do this, you attach one to the main sewer stack of the whole house - it's been available for about 20 years.

If your house is older and has copper sewer piping, it does an excellent job of radiating that heat back into your house. What you save heating water might not be too far off the heat you don't have to use your furnace to make.

Pipes generally don't go in your living spaces, they might heat your basement but that's not really where you want the heat.

Nearly all heat that starts in your basement ends up in your house. It has nowhere else to go. The copper pipes in our basement ran directly beneath the floor of the bedroom. A fine warm strip would develop whenever someone took a shower. By the time it got to the exit point in the house, the pipe was barely warm enough to notice. That is not negligible heat transfer considering its "free".

The heat in your basement does actually have somewhere else to go. Typically, uninsulated basement walls and floor are near the temperature of the surrounding earth, which ranges between 45-55 F depending on how far down you are on the wall and your latitude. Heat conduction between basement air and those cold surfaces is not negligible on the scale of home heating.

The trouble with that is that by mixing the hot drain water with cold water you spend the energy heating the cold water and lose efficiency. You want to avoid the cold toilet flush water and whatever and just get the "hot stuff".

Don't warm the entire cold water supply - just what goes to the water heater. But do it in the basement, where all the pipes already are, not under the shower, and not with a small device that is likely to get clogged.

No, he means that by putting this on the main sewer, you are running your cold drainwater through the same exchanger. If you put one under the shower only, you know you're starting with hot water.

Really, if you were starting fresh, you'd probably want a hot drain and a cold drain down in the basement.

Unless you're refrigerating your water, the water going down the drain will always be warmer or at the same temperature as the incoming supply. Since these things don't hold water, they exchange the heat as the incoming and outgoing streams move past each other, thus the effect is essentially null when you're using cold water (brushing teeth) and positive when you're using hot water (shower). When you're using water "asynchronously" (bath, toilet), there's no effect since the incoming and outgoing water move at different times.

Again, this is about efficiency, allow me to explain in more detail:

The rate of heat transfer is proportional to temperature difference. If you have a predetermined amount of time for the heat to be extracted, you will get more out of a small amount of hot water than a large amount of tepid water, even if the total energy from the temperature delta to cool incoming water is equal.

The predetermined amount of time is given by the hot water capacity of this device and the flow rate - i.e. how much time does a litre of waste water spend in the exchanger when the shower is in operation.

Plus, there are other practical advantages to using only water coming from the shower, washing machine, etc., as it will normally not contain large particles that could clog things up, particularly when the water is stagnant or slow-flowing in the exchanger.

Finally, you'd ideally feed this still relatively clean water back into the cisterns of the toilets, which don't need to run on drinking water.

Thank you for arguing my case. :-)

The one that's always confused me is having a Fridge in a cold climate.

Always wondered if there was a technology that could simply couple the fridge to the outside temperature.

This does exist. For industrial applications, there's http://www.freeaire.com/. There are also small fans designed for use with RV fridges that could do the same thing.

Unfortunately, in a cold climate, you need to heat your house anyway. Electric heat is more expensive than natural gas or oil heat, so you might save some money, but from a greenhouse gas perspective, the savings are small at best.

Yeah, good point.

"but from a greenhouse gas perspective, the savings are small at best"

Taking that argument down to an even more micro-level: most of the light in my apartment comes from overhead halogen lights (hot), and sometimes I'm inclined to switch them for energy efficient fixtures, but...

I live in a cold place anyway, so I'm not sure that switching makes a lot of sense. I know it's more efficient for the central heating system to be doing the heating (and not light bulbs, refrigerators, or laptops), but I need the heat, I need the light, and my energy bill isn't that big, so why switch?

Because natural gas is around 4x cheaper per joule than electricity.

If your energy bill is, say, $10 per month, maybe it's not worth it. Would you buy $30 in lightbulbs to save $7.50 * 12 months * 10 years = $900 over the next ten years?

If not, would you like to fund my startup?

I've heard this over and over, but in my area many people have been switching from gas to electric and saving money. Now that gas prices are supposedly starting to fall because of lack of demand maybe things will start getting rational again - even with lower distribution costs there should be no way electricity would be equivalent to gas, much less cheaper.

These people have been (most likely) switching to heat pumps, not space-heaters. The heat you get from a light bulb is purely waste from the resistance of the light bulb, so the output per unit of input will be around 1:1, comparable to that of a space-heater. Heat pumps are much more efficient, because they give you both the waste heat (from resistance) and heat from outside.

I can't claim to have done a worldwide survey of gas and electricity prices, but my 4x number is from a direct calculation off of my own utility bills. I did the calculation about 2 years ago, but both costs have remained relatively constant (within 25%, say?) where I live in Boston since then. Additionally, Massachusetts electric prices are around 2x the national average (~$0.20/kWh rather than ~$0.10/kWh).

But maybe I'm a statistical anomaly. I'd be interested to see broader evidence if you've got it.

As mentioned in another reply to your comment, it could be that people are getting some savings by using heat pumps rather than resistive heaters. For temperatures near room temperature, heat pumps have a coefficient of performance as high as 3 or 4, but as it gets colder, it approaches 1. In a warm place like California, you might beat an inefficient old gas furnace with an electric heat pump.

I remember staying in a Youth Hostel in Scotland in cold weather, and people keeping their cartons of milk on the outside windowsill 'in' the kitchen. (Presumably there was no fridge.)

The point being: if you are in a cold climate, you can just keep food outside (suitably protected, etc., etc.).

Yeah I used to live in Zurich. Great for parties. Just put the beer on the balcony :-)

Beer on the balcony. A time-honoured tradition at Hotham.

For new years we kept our keg on the patio to keep it cool. But that's just because it wouldn't fit in the freezer :P

During the winter, I kept my sodas outside my window sill, but sometimes it would get too cold and they would burst!

All this winter my wife and I have been keeping water bottles on the window sill, (I live south-west of toronto so it can get really cold at night). It often supercooled, it's really cool to see crystallization happen right in front of your eyes. (Ed: I've only ever seen supercooling once in the freezer, but I saw it almost every single night during winter, you can even drink from the bottle before it begins to crystallize. If only the same could be done with beer.)

I've avoided putting sodas on my window sill because I've had a few explode when I've stuck them in the freezer in the summer to get them ice cold. When my head is less than 2 feet away I don't want one of them exploding, if only because I don't want to strip my bed sheets at 3am. I doubt one would ever be capable of hurting someone as they always seem to split in the side.

That works well for stuff you want frozen rock solid (around here it drops to -25F in winter) and it makes it easy to run out to the grocery at lunch time and safely leave meat in the car. But for things like milk, eggs, etc. that you don't want to freeze, not so much.

In the Czech Republic people used to have two windows -- an inner and an outer. Not only is it great for insulation but you can keep things between the two windows and it stays cold without freezing.

Carlin mused about a fridge with a butter warmer.

Amory Lovins talks about this in "factor four" (great book). They go through how the refrigerator is a textbook example of loads of small design decisions ending up with something really suboptimal. In a cold climate, what you do is put the radiator coils on the outside of the house. Every kitchen would have two pipe hookups where the fridge goes, leading to the coils on the outside. Then you get the fridge and plug it in just like you do with washers. (Or you could plug it into your heat pump, which you should use to heat your house anyway.)

The purported savings are comparable to what you would save by switching to tankless water heaters: http://apps1.eere.energy.gov/consumer/your_home/water_heatin...

The tankless heater also doesn't have the energy cost of building this grey-water heat exchange device. It does have the energy cost of being built itself, but that's probably comparable to the cost of building a standard heater with a tank.

Your best bet energy-wise would probably be to go with a tankless heater as near as possible to the shower, maybe combined with a hypothetical system like the one in the article, except it would warm the water going into the tankless heater. Or you could not drain your bathtub until the water cools, if you need to heat your house anyway.

I usually wait a little while for the water temperature to stabilize, before getting in. That transient period will be much longer with this gadget.

- time it takes for the heat exchanger (gadget) to warm up due to shower drain water. - now the shower water temperature has changed (increased), so you need to adjust the valves. - another transient time for the heat exchanger to warm up, etc.

Depending on the design, you might find yourself wasting a lot more water, or jumping around in the shower as the temperature keeps changing in this slow control loop.

There are shower control systems, purely mechanical, that regulate the temperature themselves. Those systems would automatically adjust to the changing temperature of the "cold" water.

The diagram shown in the article doesn't make a whole lot of sense to me. They've got the output of the heat exchanger going directly to the supply of the shower where it mixes with the output of the water heater. This actually reduces the temperature of the shower and, as phugoid already noted, introduces undesirable temperature transients. It seems to me that it makes more sense to use the heat exchanger to preheat the feed to the water heater. This has the added advantage of making the energy exchange mechanism generally applicable to any hot water that exits via the drain which supplies the heat exchanger, not just shower water. Also, when you finish your shower and shut off the water, there is still heat in the drain and still water refilling the water heater, but the design as shown does nothing productive with that energy.

Of course, one would hope that the heat exchanger itself is made of highly corrosion-resistant material. It would be quite nasty to have a leaky drain introducing gray water into your potable water supply.

It only reduces the temperature of your shower if you would otherwise be standing under 100% hot water, which would literally be lethal with a typical hot water system (I'm guessing you don't have a typical hot water system?). It's more likely you are using a mix of, say, 70% hot, 30% cold. If you use this thingy to heat the cold water, you could get the same temperature with maybe 60%/40% mix, thus using less hot water and getting a cheaper and/or longer shower.

Perhaps most people have the thermostat on their water heater set higher than it really needs to be? On the other hand, as a bachelor, I concede that my hot water usage pattern is not the same as an average household with a family of 3 to 5 people. I typically do use 100% hot water for my showers. The only exception to this is if I run the dishwasher or the washing machine a few hours before taking a shower, which will result in 100% hot water being too hot.

Doesn't it make more sense to use the hot, gray shower water to heat the water going in to the hot water heater?

Doesn't it make more fun to wedge a peltier device between the outgoing warm and the incoming cold and use the generated power to light some LEDs in the shower?

I love peltiers, I used one as a heat exchanger on my brother's 944 Turbo on the induction system and transferred the heat to the fuel rail, creating a fuel warmer.

Result was he set the engine management system (motec) to run the fuel system richer, as the fuel that was more ready to combust (as a result of the heat) and given the air going into the turbocharger was colder (more dense) there was more air to react.

That being said - we never got the opportunity to test if there was any actual horsepower gain, but he used to suggest that there was.

Now as for LED's in the shower, that might not be as silly as it sounds. Imagine if the tiles on the floor were semi transparent (I'd suggest some kind of translucent polycarbonate) and lit up like a disco floor for those people who love to sing in the shower.

On first blush that sounds like the way to go, but for most existing homes it would probably be suboptimal.

To reduce costs, a device like this should be localized to a room rather than force you to route the water from your shower all the way back down to a water heater. The latter would necessitate a major plumbing addition.

Agree - and given the locality (i.e. there is someone using heat/hotwater right there), it would make sense to use it as soon as possible.

If you didn't have locality, the water heater approach probably make more sense -- i.e. Your washing machine, dishwasher, which tend to hold on to hot water and then dump it at the end of the cycle.

I'd actually prefer the gray water be used to heat the floor in the bathroom. If you've ever stepped out of a shower onto a tile floor you know how cold it can be sometimes. And if you've ever stepped onto a (electrically) heated floor, you'll know how awesome it is.

Running the warm water through a series of pipes below the bathroom floor would seem like a pretty good use of it to me, when compared to just dumping it into the sewer as-is.

Then of course, in a green house, the gray water would be collected and used to flush the toilets.

This would work well in my house, but not for the stated purpose.

We have a hydronic heating system (hot water circulates through pipes to toasters...kind of like old radiators but more efficient). A heat exchanger from graywater to our heating system would help our winter gas bill quite a bit.

Tying it to the dishwasher and washing machine would help out a lot too.

there is a similar idea for high insulation homes where you heat exchange the outgoing air (recirculated for freshness) with the incoming air to avoid having to cool/heat the incoming air as much

imo, heat exchangers are a good way to reduce consumption because they're usually passive and don't require much maintenance

That's a great idea. Modern gas boilers and tankless water heaters often run their exhaust and intake in concentric pipes, so that the outgoing hot air warms the incoming cold air.

Furthermore, the flow in the two pipes is usually in opposite directions because it's more efficient that way.

I think that the intake and exhaust are always in opposite directions, regardless of whether it's efficient.

My cheaper version: Stand in my own filthy gray water and then leave it overnight. Problem solved. Of course now I have to solve this jock itch on my feet...

Now this is value added. The key would be for housing developers to adopt this.

this is super cool.

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