
Stop pouring hot water down the drain - chaostheory
http://green.yahoo.com/blog/ecogeek/1010/stop-pouring-hot-water-down-the-drain.html
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ars
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.

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lutorm
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".

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ars
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.

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Vivtek
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.

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adoyle
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.

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pmjordan
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.

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lutorm
Thank you for arguing my case. :-)

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jwilliams
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.

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parenthesis
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.).

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

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

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rsheridan6
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...](http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820)

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.

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phugoid
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.

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eggnet
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.

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blahblahblah
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.

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extension
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.

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blahblahblah
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.

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

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_pius
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.

~~~
jwilliams
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.

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gills
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.

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matthavener
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

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pingswept
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.

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

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

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ryanvm
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...

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

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savrajsingh
this is super cool.

