Hybrid Car – More Fun with Less Gas

just an idea - Page 2

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Posted by Winston on April 19, 2009, 5:55 am
user@domain.invalid wrote:


t there would be negligible heat loss with a 40 gallon tank of 34f

Looks like you can do it if you bury your uninsulated tank in the
basement floor.  (Thermal conductivity of air is 0.0126 or about
2000 x lower than that of your steel tank.)

Let's grab an American envelope and a pencil.

40 gallons of water weighs about 333 lbs.
It will require 333 BTU to raise the temperature of that water by
one degree F. From 34° F to 66° F is a difference of 32°.
To raise the temperature of the water in that tank to within a few
degrees of local 'earth normal' will require over 10,600 BTU/hr.
(And that is for 'no flow' conditions.)

Thermal conductivity of 1" thick steel is about 25 Btu/hr-ft-º F
If the walls of your tank are say, 0.05" thick, you will be able
to push 500 BTUs per hour through each square foot of tank surface
for every degree of difference between the inside and outside of
the tank, or 16,000 BTU/hr per square foot for a 32° difference.
Note that as the water warms up, heat flow decreases significantly.

Neglecting the top and bottom, an 18" diameter tank 40" tall
has an exchange area of over 15.5 square feet for over 7800

Leak detection and corrosion control is left as an exercise for
the student, on an hourly basis.

I *still* like copper pipe better. It has 2.5x the thermal
conductivity of your tank and lots more surface area, for
a correctly sized loop.




Don't *faff*, dear.

Posted by Lord Gow333, Dirk Benedict's n on April 20, 2009, 5:09 pm

I'd add that it depends on how he uses the water. The coil would be better
for frequent but short bursts as it would exhaust its supply quickly. The
tank would take a lot longer to heat up but would then of course have more
reserves to cover multiple showers at one time, etc.
Maybe a coil to preheat the water AND a then tank to store the 66° water
until the heater needs it? Or are we overcomplicating things at that point?

LG (figures any of it would be helpful)
"Keep it simple. If it takes a genius to understand it, it will never work."
- Clarence Leonard “Kelly” Johnson

Posted by Winston on April 20, 2009, 7:42 pm
 Lord Gow333, Dirk Benedict's newest fan! wrote:


I'm a little concerned about algae growth in a large tank held
at ca. 'room temperature'.  The flow rate in a copper tube or pipe
might prevent algae growth.

75 feet of (equiv.) 1" copper tube would continuously warm 3 gallons
of water without too much concern about algae. I think this is enough
to save the OP significant money in fuel even if the *input* temperature
to his water heater does drop significantly after the first minute
of a shower, for example.

Even the capacity of water heaters is more than just the size of the
heater tank.  'Recovery' or dynamic flow performance means that
a 30 gallon heater is good for more than 30 gallons of hot water
under reasonable flow rates.

Perfection is the enemy of "good enough".  :)



Don't *faff*, dear.

Posted by user on April 20, 2009, 9:02 pm
 Winston wrote:

I can't see a bio hazard as being a problem**, the amount of hot water
we use each morning pretty much drains the heater tank by the third
shower, meaning the tempering tank is flushed everyday with chlorinated
city tap water . I suppose even if it was an issue a filter could be put
inline between the tempering tank and the water heater. Once the water
hits the heater it's heated up to over 130f anyway.

** The water is sitting 6 inch cast iron pipes installed under my street
in 1946, I think it's bound to breed more crap there than in a tank in
the basement ;~)

Posted by daestrom on April 20, 2009, 9:50 pm

I've posted information in the past about this, but if you're using your hot
water for showers, consider reclaiming some of the heat in the drain water.
You spend money to heat that water and then let it run down the drain

GFX technologies has several models of heat recovery heat exchangers that
can be used to preheat incoming cold water for your water heater / shower
using the energy of the out-going drain water.

I've had one in my house for about eight years now and it has paid for
itself.  I've measured the performance of my unit and it has varied somewhat
but on average better than half the thermal energy is recovered by it.  If
you take a lot of showers, it's payback time can be shorter.


If you go this route, shop around. I found one at a dealer for $00 less
than the price that GFX 'suggests'.


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