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Using your figures a family of 2 heating with gas or propane
would save $12.47 (2.5%). But I don't say return on investment is the most
important factor, it's just that there is a budget, and you need to decide where
to put each $500 to get the most effect. Is it a greywater heat recovery device,
a better HRV, a south window and some solar mass, movable insulation, a solar
panel? Only by calculating the potential benefit from each option can you make
intelligent choices (actually quite easy to do).
On a similar topic, one builder I'm talking to who is very
proud of his energy smart houses installs a storage tank to preheat water to
room temp. before it goes into the HWT. I can't figure this one out. Aren't you
just cooling the room, increasing the heat loss from the HWT, and coming out
more or less even?
----- Original Message -----
Sent: Wednesday, June 28, 2000 8:09
AM
Subject: Re: GBlist: Using Water Heater
As Boiler
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.
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