This is not true. It would be true if by adding cold water you were
increasing the total flow rate out of the shower head. But the flow
rate out the shower head is basically fixed, so mixing in cold water
reduces the amount of hot water being used.
In other words, given a shower flow rate, desired shower temperature
and incoming cold water temperature, the power (energy per unit time)
that is required (flow rate) * (temperature rise required), for
suitable units. It doesn't matter whether you use that power to heat
a little bit of your shower water a lot, and then dilute it with cold,
or use it to heat all the shower water a little bit.
The one place that lowering your hot water temperature will save
energy is if you use a tank-style water heater. This reduces the
temperature differential between the hot water in the tank and the
environment, so it reduces the rate of heat loss. This savings by
reducing the hot water temperature is independent of consumption,
Everyone who has raised teenagers knows that by lowering the water
heater temperature, you reduce consumption, as they will take shorter
showers because the hot water runs out.
Around here, since we have become ecologically aware, the cost of water
is higher than the cost of heating it, so reducing consumption can save
me quite a bit.
The Graywater from just a shower alone is probably not enough to
economically justify a heat exchanger.
However, if you run a dishwasher, the wastewater from one complete
cycle will usually be hot enough and of sufficient quantity to
consider the possiblity of some sort of heat exchanger. This
combined with a shower graywater system and you might save some money.
The conventional solution is a large, insulated graywater tank that
contains a heat exchanger (copper coils) ahead of your regular hot
water heater. The insulation preservers the temperature gradient to
keep the hottest water at the top where the largest surface area of
the exchange coils are located. The copper supply line must be
isolated from the tank water so that there is no leakage (or
biological hazard from a backflow). Also, since the tank will contain
standing wastewater, some provision of cleaning or flushing the tank
when necessary should be provided.
These are the basic physical requirements. I don't think that most
US plumbing codes allow graywater systems though.
What some codes prohibit is dumping greywater into the ground separate
from the black water system.
There's nothing that prevents you from having a grey water system
that ultimately goes the same place as your black water. Ie:
running grey water into a heat exchanger, and thence into a
sanitary sewer (or septic system).
However, there _may_ be some areas that prohibit grey water in sanitary
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
Well, the shower heat exchanger is simpler, as you know there will be
hot water demand coincident with the drain water, so you don't need a
storage-type heat exhanger.
Also, I would expect that most households have 4-8 times as many
showers as dishwasher cycles. A shower also probably uses more hot
water than a dishwasher cycle (10-15 minutes @ 1 gpm hot water for a
shower, versus 5-10 gallon for a dishwasher cycle).
So I think that shower grey water heat exchange is the best place to
the heated waste water heats up the drain pipes in the basement so most
of the heat is recovered keeping your basement warm and dry...
check the temperature of the waste water that leaves your house...it's
not much to recover
Considering they were almost giving these away a while back, they were
worth it. You simply put the coil in-line with the supply line to your
water heater and it reduces the load. If you insulate it, it'll do more.
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