GBlist: Re: Drainwater heat recovery (DHR)

["Winston MacKelvie" <winstonworks@sympatico.ca>]

Re; DHR
Excellent analysis Glen. Thanks. One point,though, our DHR Systems also
work with batch hot water use (appliances and fixtures, ie. all the
drainwater, not just 1/2 of it as per the GFX System) so the total value of
DHR would then be about double that which you have indicated. 
Comments?

Regards,
Winston MacKelvie
WaterWatts and DrainGain DHR Systems
http://www.inventure.ca
450-243-6850

----------------Quote
For some illuminating facts on grey water heat recovery go to
http://www.vaughncorp.com/gfx.html

In certain locations grey water heat recovery CAN be economical.

For an example of favorable economics, lets assume:
  - a NE U.S. location with an average annual ground temperature of 50F
  - basement construction with hot water heater near main water drain (to
facilitate installation)
  - electric resistance heated water with electric rates of $0.10/kWh.
 - 1/2 of hot water use results in grey water draining simultaneously with
incoming cold water flow.  This would hold true for showers and unstoppered
sinks.  It would generally not hold true for "batch" water use, such as
washing
machines, dishwashers or baths, although there would be some opportunity
for
heat exchange if other hot water use were simultaneous with these
activities.
  - heat exchange efficiency between incoming cold water and warm draining
grey
water of 50%.  This means that the incoming cold water can be pre-heated to
half
the temperature difference between the two streams.  If the average
draining
grey water temperature were 90F, then the incoming cold water could be
heated
1/2 the difference between 90 and 50, or 20F.  So the incoming cold water
starts
at 50F and ends up at 70F (average grey water temperature might be 90F
(simultaneous: showers ~ 100F, unstoppered sinks ~ 85F.  batch: dishwashers
~
125F, clothes washers ~ 75F to 100F.)
  - water heating efficiency (i.e., "recovery efficiency") of water heater
is
98%.  This is the proper number to use rather than the "energy factor",
which
factors in standby thermal losses from the tank which will occur regardless
of
the presence of grey water heat recovery or not.
  - family of 5 that uses 80 gallons of hot water per day.

Then we have:
  365 days/year * 80 gallons/day * 8.34 lb/gallon * [(90F-50F) * 50% heat
exchange efficiency] * 50% simultaneous flow /(3,413 Btu/kWh) /(98%
recovery
efficiency) * $0.10/kWh ~ $73/year.  If the grey water heat recovery system
costs around $500 installed, this is a $73/$500 = 14.6% after-tax rate of
return, a bit better than a CD or saving account, and even competes with
credit
card interest paid with after-tax dollars.

It also saves over 700 kWh/year, which, on a national average basis of 1.5
lbs
of CO2 per kWh is about a half a ton.  This is also equivalent to the CO2
emissions of about 1/10 of an automobile (EPA says that an average car
emits
about 5 tons of CO2/year).

Hope you have a great day
--Glenn
EPA Energy Star Homes Program
202-564-9784, fax: 202-565-2079
email: Chinery.Glenn@epa.mail
website: www.energystar.gov/homes
info hotline: 888-star-yes



______________________________________________________________________
This greenbuilding dialogue is sponsored by CREST <www.crest.org>
BuildingGreen <www.buildinggreen.com> and Oikos <www.oikos.com>
For  instructions send  e-mail to  greenbuilding-request@crest.org.
______________________________________________________________________