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The most common type of shower is a head fed by a
mixer-tap that variably combines hot & cold water. The
hot water is usually supplied by the main hot water
system in the building. These can be gas, solar,
etc. but in the US as least the most common is a bloody
great (slightly) insulated tank heated by a resistive
electrical element. Heating things is energy intensive,
water especially since it has a high specific heat
This leads to big electricity bills, which are annoying.
Thermoelectric elements, or Peltier devices are small
state chip-like things that get hot on one side and cool
the other when you apply the right kind of electricity.
can use them to make horrendously inefficient mini-
fridges, they can be quite useful in temperature
small electronics etc. But they're still a sort of solution
looking for a problem. Their simple construction and the
wonders of mass production mean that these things are
now dirt cheap - ~$5 or so for a 40-50W device.
Instead of using them as coolers, when used as heaters
they get quite interesting. As an example, I considered
replacing my aquarium heater with a Peltier. The tank in
question needs about 30W to stay at 77F/25C. To
this, my 50W heater is active ~3/5 of the time,
30W. Because the temp gradient between the room and
tank is only ~5C, calculations suggest a Peltier could
heat from the room with only a net electricity cost of 2-
5W*. That's a big relative saving. For 25W, it's probably
worth it, but it illustrates the point that Peltier elements
are good at moving heat across modest gradients.
To boost a shower with Peltier element, you could heat
hot water. But, the delta of 50C or so would mean
operating <link> so inefficiently, that there's no real
However, under the bath/shower tray/behind the wall,
there is also the cold feed to the shower. Temperatures
vary, but a range of 8-18C seems reasonable. No-one in
their right mind uses a shower in pure cold mode, nor
anyone pop into the shower for a fresh glass of water, so
we're free to manipulate the temperature of the cold
where the temperature gradient is perfect.
So, we build our device. The market is the casual sunday
hardware store browser. They've been looking at heat
pump water heaters, but the $1000's more than a
conventional unit stings. Our target price is the $100-200
range. We start with an aluminium block, with a radius
relief cut/cast into it to allow good contact with
copper pipe. Then, ~10 or so 45mm Peltier devices
attached in-line, simple screw clamps heatsinks and
Pg9 of <link> suggests we can pump 55W using only 20W,
but we get to use that 20W as well.
So, our 10-element design gets us 750 extra Watts of
heating, good for ~3.0C warming at 1 gpm. This assumes
the cold is 1/2 the total flow. All for 200W input, it's
efficient, the remainder of the heat is extracted from
underbath environment, heat is reclaimed from the
draining shower water. The other components are a non-
contact flow meter for switching, Hall effect or even
will work. A controller, some heatsinks/fans and a fairly
beefy DC supply.
The result to the user will be that they simply turn the
mixer tap slightly toward cold since the average
temperature of the input water is now higher. This will
reduce flow from the water heater, reducing workload.
There will also be lower losses since less heat will be lost
through the hot pipes to the air. In summer, the pumped
heat ~550W can be subtracted from any air conditioning
load, saving electricity there.
Cleverly, the power supply can route through the
floor/wall so that only low voltage DC is in the bathroom
*online calculators exist, but power supply/control losses
Pg9 for a fairly beefy Peltier
[bs0u0155, Oct 13 2020]
Off the shelf heat exchanger for waste water [scad mientist, Oct 16 2020]
||Haven't had a hot-tank supplied shower for decades. Showers here are almost always heat-on-demand, either gas or electric. Suggest living in a developed and/or civilised country/part of the world.
||A friend of mine simply put a heat exchanger between the
incoming cold water and outgoing shower drain. Not sure
how well it works, but better than 0% (no recovery)...
||Our gas hot water heater costs maybe $400 a year
||//put a heat exchanger between the incoming cold water
and outgoing shower drain.//
||I was halfway through building one for an electric clothes
drier, until we moved and the prospect with messing
about with a gas drier made me a lot more nervous. The
idea of a heat exchanger in that application is reasonably
sound. When you think it through however, there's a lot
of downsides. The two have to be specially plumbed
together, you either build it in in the first place - no
housebuilder is adding cost and time for that kind of
thing, or you re-plumb the waste and water line. How do
you make it? Aluminium is out, too many drain cleaners
would destroy it. Stainless will be expensive.
||//water heater costs maybe $400 a year to run?//
||Ours is double that. The way this is running, the 550W
over a 15min shower saves about 135W/hrs. 4 People, 1
shower/person/day the payback time is a little over 10
years. If 2 of those people are teenage girls, it pays back
by March. Depending on the time of year, you can add 20-
50% on top of that for saved HVAC electricity. Also the
time to acceptable temperature would be less, so some
water saved also.
||The same device could also be added to the water heater
inlet, hand washing sinks, clothes washers etc.
||// How do you make it? Aluminium is out, too many drain
cleaners would destroy it. Stainless will be expensive. //
||Copper. You can order a Power Pipe heat exchanger from
Home Depot in the USA. See link.
||I have one of these under my house between the main
drain and the cold water going into my hot water tank
and the cold water taps in the tubs/showers. It's a 3 foot
long 3 inch copper pipe with smaller copper pipe wrapped
around it. It is installed vertically. The theory is that the
drain water tends to flow along the walls of the vertical
pipe, so it transfers the heat more efficiently than it
would from a pipe running mostly horizontally. Since it's
a but it's a standard 3" copper drain pipe there's no
problem with clogging or maintenance. The incoming
water spirals around this from bottom to top, making it a
counter flow heat exchanger. I forget the numbers, but I
think it was supposed to be more than 50% efficient in
good conditions. I don't see that mentioned in the
current description. It just says it will save 26% on water
heating costs. The longer pipes are more efficient, saving
up to 39% for the 72 inch model. I only had enough
elevation drop from my drains to the septic system for
the shortest pipe.
||The Power Pipe only works if you have >4 feet of drop in
the drain line. My guess is that would not apply to the
majority of houses. When I designed my house I made a
few small changes to make this work, but I think in my
case it pays for itself very quickly. Just counting energy
cost, the payback could be a while, but in my case is also
allows me to save money on the water heater.
||The building department required that I install a heat
pump water heater (or some other energy saving system
that was even more expensive). Those are efficient, but
not very powerful, so I needed a big tank. However by
recovering the waste heat I was able to buy a 50 gallon
tank rather than an 80 gallon tank. That saved almost as
much as the cost of the heat exchanger, and the heat
exchanger should last through many water heater
||The heat exchanger seems to work fine. I have been able
to feel the temperature difference of the pipe before and
after, but unfortunately I don't have instrumentation
installed to calculate how much it is actually saving me.
||And [+] for the main idea. It seems somewhat plausible
(and quieter than the compressor on our heat pump water
tank). It seems to me you might also be able to integrate
it with a heat exchanger. For my house, we don't have or
need A/C, so pumping the heat from the house doesn't
||Apparently there is also a new system called EcoDrain that is
horizontal. I'd also worry about it being more likely to clog
and possibly problematic getting a snake through it, but
their web site says it isn't a problem.
||We initially misread the title as "Clip-On Thermonuclear Shower Booster", which would be awesome.
||40 seconds of hot water ( you can stay in the shower longer but you have to turn the water off when you're not using it, so the total run time is <40 secs)