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Waste Grey Water Heat Recovery

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Posted by Mike Scaife on July 10, 2007, 1:34 am
Has anyone installed one of these heat exchanger grey water recovery units,
see http://gfxtechnology.com/
I am after something that can be retro-fitted to the main shower in the
house, to recover some of the waste heat from the water that goes down the
drain, to preheat the cold water going to the shower mixer.

Do they work effectively ??

We are mid winter here in NZ and the cold water is approx 10 C, so more hot
water is required to achieve the same shower temperature, summer time cold
is 18 C on average..

Mike Scaife (NZ)

Posted by daestrom on July 10, 2007, 9:50 pm

Yes, I have one of their large units in the main drain pipe and common water
supply feed to the bathrooms.

Because the unit I have has *two* smaller coils wrapped around the drain
pipe, and the potable water side of these coils are in parallel while the
drain pipe effectively is in series, the efficiency is lower then what it
would be for some of the smaller models that have just one large coil.

I've had it since May 2002 and taken performance measurements a few
different times.  The last round of measurements (I just looked them up) was
44.5 degF (6.9 degC) feed water in, 76.2 degF (24.6 degC) out on the potable
side, and 106 degF (41 degC) drain water in with 74.1 degF (23.4 degC) drain
water out.  That works out to about 51.7 % recovery.  With a large family
taking many showers, that works out to about $0 per year in savings.  So
I've figured it's paid for itself by now.

Keep in mind the actual savings can depend on some key things:
1) It *must* be mounted perfectly vertical.  You cannot mount it
horizontally and get anywhere near the same performance
2) Actual savings depends on how many showers you and your family take.
3) It only recovers energy when there is a steady flow of water through
*both* sides.  Things like a dishwasher, laundary washer or bathtub that
first fill with hot water, then drain later will *NOT* see any appreciable
4) If you can use one of the smaller units with a single coil and plumb it
*vertically* under the shower, you're probably better off then what I did
with a large unit.

Good luck, let us know how you make out...


Posted by jan siepelstad on July 11, 2007, 9:26 pm

that's exactly what I did with my homemade heatexchanger!


Posted by Mike Scaife on July 13, 2007, 8:51 am
Thanks for all your comments, they seem positive towards the device, they
cost NZ $30 here so seems would pay for itself after 4-5 years. I was
thinking of DIY one but don't have any tools for producing a tight spiral in
1/2" soft copper tubing.

I though the possibility of placing a spiral of copper tube inside a 2-3
foot length of waste pipe with an internal copper cone at the top to direct
the waste water to run down the sides of the copper spiral and exiting at
the bottom to the drain. This would be a single wall exchanger, but I'm sure
would be ok for many years.


Posted by daestrom on July 17, 2007, 12:27 am

The GFX design works without a central cone to direct the water.  If you
mount the device vertically (the GFX or one you build), the surface tension
will cause the grey-water to spread out in a thin 'film' around a goodly
portion of the circumference of the inside wall without special guide vanes
or 'cones'.

I'm not so sure about a spiral *inside* the tube though.  The grey-water
would have to 'stick' to the inside face of the spiral, riding in and out of
each round of tubing.  If it breaks free from the spiral tube surface and
falls free, then you may not get the same performance.  The inside of the
GFX is just a straight, smooth surface so the water stays in a film against
the wall as it slides down the tube wall.

The GFX doesn't build up too much 'scum' on the inside surface because it is
smooth-bore and the grey-water coming down constantly 'scours' the surface.
With a tube spiral inside, the spaces between the tubing might quickly fill
up with 'gunk' such as dirt and soap scum.

But I haven't done any full scale tests to verify that.  I *have* inspected
the inside of my own GFX after a couple of years and found that there was a
layer of scum on some portion of the circumference but over half of the
circumference was still pretty clean.  (of course as I mentioned, mine is
located in the main stack so it carries both grey and 'black' water).


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