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Drainwater Heat Recovery

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Posted by Brian Graham on February 14, 2007, 4:01 pm
I can't say that I've seen anything on this topic in this group, so I thought
I'd post a quick note.

William Kemp's book 'Te Renewable Energy Handbook' is a great read. In it he
talks about Drainwater Heat Recovery and references a web site. Check out the
link below for more info.


In short, you put a heat exchanger on the shower drain. The cold supply for the
shower picks up transferred heat on its way to the shower. You'll therefore turn
down the 'hot tap' since some of the heat is coming from the cold line.

A friend built a home-made heat exchanger. Checking with a laser thermometer,
his heat recovery rings in at about 30%.  Or put another way, would you rather
buy gas at $.90 or $.60 ?

Posted by Steve O'Hara-Smith on February 14, 2007, 6:04 pm
On Wed, 14 Feb 2007 11:01:51 -0500

    That depends on how much gas purchase is saved and how much the
device costs to install and maintain.

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Posted by nicksanspam on February 14, 2007, 10:26 pm

Only 30%? If energy were more expensive, we might store a 10 gallon batch
of greywater and move it carefully through a counterflow heat exchanger
with 4 $0 100'x1/2" plastic pipes with 96% heat recovery.


Posted by Jeff on February 15, 2007, 12:31 pm
 nicksanspam@ece.villanova.edu wrote:

   Wouldn't that have a rather long time lag for heat out to show up at
cold side?

   I thought this grey water recovery was just the coolest (or rather
the hottest) thing until I started thing about:

1) The cost of the commercial product. It's many hundreds of dollars for
just one liine.
2) The length and complexity of retrofitting an install.
3) What happens when the grey water gets chunky? Like in a kitchen.

   Solar hot water makes more sense to me than this.


Posted by nicksanspam on February 15, 2007, 2:11 pm
A simple system might preheat fresh cold water with outgoing greywater
and run it into the cold inlet of a conventional tank water heater.

The cost needs to be consistent with the savings, say $00-500/year.

Rerouting plumbing to separate grey and black water could be expensive.
And if the height difference between the greywater inlet and the sewer
outlet is too small, a GFX won't work without a pump. And a GFX doesn't
store greywater, so it won't help much with a bathtub.

Backflush into the sewer line, which might be automatically indicated when
greywater starts to flow through an overflow bypass? A filter could also
help, eg a nylon stocking inside a vertical 4" PVC pipe.

There's lots of room for improvement. Very efficient heat recovery could
make solar hot water obsolete, if it only tops up 100 F water to 110.
Setting the water heater to the shower temp and fully-enclosing a shower
can also help. See


Here's a simulation with 7 $1 100'x1/2" pipes inside a $3 100'x3" 70 psi
layflat ag irrigation pipe, which might hang in an 8' flat spiral under
a basement ceiling. The inner pipes might be tied together in a hexagonal
cross section, with 6 pipes kissing a central pipe.

10 PI=4*ATN(1)'gwpsim
20 DIM IPT(100),OPT(100)'pipe temps
30 L0'pipe length (feet)
40 IPID=.622'inner pipe ID (inches)
50 IPOD=.742'inner pipe OD (inches)
60 IPAD=(IPID+IPOD)/2'average pipe diam (inches)
70 A=PI*IPAD/12*L'pipe area (ft^2)
80 IPVOL=L*PI*(IPID/24)^2'inner pipe volume (ft^3)
90 IPCAP=IPVOL*62.33/8.33'inner pipe capacity (gallons)
100 IPDIS=L*PI*(IPOD/24)^2*62.33/8.33'inner pipe displacement (gallons)
110 OPCAP=L*PI*(3/24)^2*62.33/8.33'3"outer pipe capacity (gallons)
120 U0'pipe conductance (Btu/h-F-ft^2)
130 CMIN*8.33'heat capacity rate (Btu/h-F)
140 N=7'number of inner pipes
160 EFF=NTU/(NTU+1)'counterflow hx effectiveness
170 IPCAP=N*IPCAP'total inner pipe cap (gallons)
180 OPCAP=OPCAP-N*IPDIS'net outer pipe capacity (gallons)
200 GRATE=3'greywater flow rate (gpm)
210 GSIZE'greywater burst size (gallons)
220 NPSP'#pipe segments
230 DL=L/NPS'pipe segment length (feet)
240 UAS=N*U*A/NPS'pipe segment conductance (Btu/h-F)
250 DIC=8.33*DL*IPCAP/L'inner pipe segment cap (Btu/F)
260 DOC=8.33*DL*OPCAP/L'outer pipe segment cap (Btu/F)
270 GTIME=GSIZE/GRATE/60'burst time (hours)
280 DT /3600'time step (h)
290 GAVG=DT*8.33*GSIZE'average gw flow rate (lb/h)
300 DGC=8.33*DT*GSIZE'greywater cap step (Btu/F)
310 IPT(NPS+1)`'freshwater inlet temp (F)
320 OPT(0)0'greywater inlet temp (F)
330 FOR BURST=0 TO 20'greywater bursts
340 FOR T=0 TO 1-DT+.000001 STEP DT'simulate 1 hour
350 IF T<GTIME GOTO 390'skip burst fill
360 FOR S=1 TO NPS'pipe segment (1<->gw in)
370 OPT(S)=(DGC*OPT(S-1)+(DOC-DGC)*OPT(S))/DOC'gw seg temps after move (F)
380 NEXT S
390 FOR S=NPS TO 1 STEP -1'pipe segments (1<->gw in)
400 IPT(S)=(GAVG*IPT(S+1)+(DIC-GAVG)*IPT(S))/DIC'fw seg temps after move (F)
410 NEXT S
420 FOR S=1 TO NPS'pipe segment (1<->gw in)
430 HEATFLOW=DT*(OPT(S)-IPT(S))*UAS'heatflow (Btu)
440 OPT(S)=OPT(S)-HEATFLOW/DOC'gw temps after heatflow (F)
450 IPT(S)=IPT(S)+HEATFLOW/DIC'fw temps after heatflow (F)
460 NEXT S
470 IF BURST  THEN EOUT=EOUT+GAVG*(IPT(1)-IPT(51))'accumulate gain (Btu)
480 NEXT T
500 FOR I=0 TO 9'freshwater temps
510 FOR J=1 TO 4
520 PRINT TAB(15*(J-1));IPT(5*I+J);
530 NEXT J
540 PRINT TAB(60);IPT(5*I+5)
550 NEXT I
560 EIN=8.33*GSIZE*(OPT(0)-IPT(NPS+1))'total water heating energy (Btu)
570 EFF=EOUT/EIN'simulated effectiveness

# inner pipes   theo eff         outer cap       inner cap

7               .9782665         21.00167        11.05245

The theoretical effectiveness is 98%, if we store 10 gallons of greywater in
one reservoir and store 10 gallons of freshwater in another and slowly feed
them into a counterflow heat exchanger and feed the output into another
reservoir. The inner and outer pipes hold 11 and 21 gallons.

fw outlet (F)

93.78576    91.04849     88.49059     86.10591     83.88806
81.83024    79.92535     78.16596     76.54438     75.05275
73.68311    72.42754     71.27815     70.2273      69.26755
68.39181    67.59336     66.86586     66.20341     65.6005
65.05206    64.5534      64.10021     63.68856     63.31481
62.97566    62.66806     62.38924     62.13663     61.9079
61.70092    61.51371     61.34447     61.19156     61.05344
60.92875    60.81619     60.71462     60.62297     60.54024
60.46555    60.39807     60.33703     60.28177     60.23164
60.18606    60.14449     60.10645     60.07149     60.03921 fw inlet (F)

simulated effectiveness: 0.8250335

The simulated effectiveness is a lot lower (it was 0.646 with 4 pipes), but
it looks like the last 50' vs the 100' pipes don't do much, with gw and fw
temps both close to 60 F. So we might not lose much if we used a 50 vs 100'
length. We'd still have a 10.5 gallon gw capacity, consistent with DOE and
SRCC test protocols.

Perhaps we can do better. I got 91% in a previous simulation. This time,
I fed 10 gallons/hour of fresh water through the greywater at a constant
rate, back into the cold side of a tank water heater. Last time, a diff
thermostat ran a pump whenever the gw inlet temp was greater than the fw
outlet temp by more than a predetermined amount. I'll try that again.
Got any more promising schemes?


PS: There's also an ICC plumbing code issue, which probably needs
a hose bibb vacuum breaker and some PE probability calcs to fix.

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