In Oslo Norway they have built an heat recovery plant attached to
their sewage treatment plant. In our houses we heat hot water
only to flush most of this hot water down the drain. A dishwasher
will heat the water it uses (6 to 10 gallons) to 140-150F. Since
there are about 8 pounds of water per gallon, this is 8*10*(150-70)
or about 6400 BTU per dishwashing cycle. This is about 1/20 th
of the energy in one gallon of heating oil or 1/16 of a therm of
natural gas. And given that your furnace may not be highly efficient,
this could mean that the it represents more saving than that. So
if during the winter months, you use your dishwasher once every
3 days, you could save a couple of gallons of heating oil or few therms
of natural gas if you could recover all the heat used by your
dishwasher. Not a whole lot-- but if all 40 million dishwashers in the
USA had heat recovery you could recover 80 million gallons of heating
oil equivalent per year! Worth more than $200 million dollars!
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Step 1: Assemble the Downspout
I am using 2 6 gallon plastic water jugs that I bought at K-Mart.
The dishwasher uses between 6 and 12 gallons depending on
the wash settings. Splitting this between two water containers
makes it easier to move them around. The plumbing connection
for this is made with 1 inch PVC fittings. One T and 2 right angle
turns are needed with short pieces (4-5 inch) of one inch wide PVC
coming out from the side of the T and connecting the right angle
segment. Water coming out of the dishwasher is fed into one
end of the tee and comes out the two sides and into the water
Step 2: Place the PVC Downspout Into the Water Cans
Assemble the PVC downspout and fit the ends of the 90 degree
fittings into the plastic water containers. If the containers are held
level the water should split well between the two containers. Leave
the air plugs on the plastic containers open so that air will not get
trapped when the dishwasher water flows into the containers.
Step 3: Run the Dishwasher
Now, run the dishwasher. Feed the dishwasher drain hose into the
downspout and into the plastic containers. After the dishwasher is
finished with the load, you can take the plastic containers full of hot
water and put them where you want to add heat. Or if you are content
to leave them where they are and use the heat there, thats OK. As an
enhancement you could add a siphon pump that runs automatically
after the water cools to drain the water away so that it is ready for the
next dishwasher cycle. Or just wait until the water has cooled and
dump it into a drain.
Another thing that could be done with the hot water would be to circulate it
around the incoming cold water to the hot water heater so that less heat is
needed there to heat up the hot water.