Cob Re: related box-cooker question

[HandyM2 at aol.com]

Hello Rush, Sorry for the delay, been busy with work and real life (tm).  <G>

I'm posting this to the list(s) for others comments.  Hopefully they'll post
real life experences on this subject Vs untested thoughts.  One can hope <G> 

Lets see IIRC you had questions about putting two 30 gallon tanks into a Solar
Box heater.  Your message somewhat snipped and my coments follow.   

>(Snip) But my ? is in terms of what kind of insulation, is there something
 better than polyisocyanurate? How thick should the insul be?< 

I am not aware of what polyiso...  is.  The main point of insulation is the
safe containment of heat as to keep that gained solar heat with in the water
for later use.  The reason I say that is to comment that some foam insulations
can indeed MELT in the heat of a Solar Box Heater esp if the heater is somehow
drained dry (like in a error or opps...)  I personally am fond of Fibreglas.
It's not super efficent but very reliable.  Basicly ANY insulation used in
Attic Insulation should be useful in the box heater.  Attic can get real hot
too and builders are very safety minded in this area.  

Insulation level is up to you and your percived needs.  If you are thinking
about a glass fronted box then please remember that the glass will have
virtually NO INSULATION value.  Thus going much past say R-20 for the rest of
the box is rather foolish.  You are building a daylight model only.

If you want (like I do..) Hot water for my early morning showers, some dishes
and so forth THEN you need serious insulation design.  I use my closable
insualtion panels (at least R-10) to have the reflective insulation on the
inside (when closed).  Thusly the panels can be opened during the sunny days
and the reflective insulation actually redirect MORE solar power into the
BLACK PAINTED (suggest Hi-temp Paint BTW) tank.  The rest of the box can do
well with R-20 to R-30.  The reflective cover-movable insulation panels must
be at leat 1/2 R value of the rest AND FIT WELL!! or the extra insulation
value is effectively lost.      

>What is the best package shape, I've seen ones that have a cross section of a
45 deg right angle, could the sides be shaped, curved to reflect the heat
better?< 

Honestly the interior shape is a matter of taste.  It is possable to build up
a minor advantage in interior heating with a 45 degree interior (curved are
such a pain to build/insulate for such a minor advantage) but in truth where
you live the PREVENTION OF HEAT LOSS by the closable panels far outweigh the
benifit of a fancy interior shape.  

Remember sunlight comes into the box Via the Glass front toward the direct
sun.  I remind you of that because I see so many solar heaters facing NORTH or
in shaded area....  Go Figger....  <G>  It hits interior structure.  Black
Painted Hot Water Tanks themselves or the interior walls of the insulation.
Here the sunlight turns mostly into InfraRed or heat.  This heat is adsorbed
by the hot water tanks. The biggest advantage of the 45 degree interiors is
that the air space and subsquent heating loss by convection is minimized.
This is not a reflecting boiler but a Bread Box Heater.   

Glare out of this unit indicates lost solar potental.  Think about it, why
throw away the solar gain (mostly light) via a shiney misaimed surface.  A
simple dark interior will convert the solar energy into infrared which cannot
escape easily thru the glass.  

>Plumbing wise I think that series would be better, less hookups,
 and it would deplete the hot water reserve more slowly - it would give
 the new cold water more time to heat up, I think. Is there a black paint
 that Absorbs heat better than just regular old flat black.<
 
Sounds Good.  I hope the fancy paint is not expensive and is Hi-Temp as your
box heater can get up into the 300 degree range if water is somehow lost.  I
tend to use BBQ paint.  Some folks use soot, some use tempura.  You can even
paint them Dark Green, Blue or Brown with little real effect on the solar
gain.  

A BIG TIME issue some folks forget is that WATER is HEAVY.  You are going to
build a 60 X 8 Lbs per gallon water holding structure.  Or 480 Lbs before you
add in the weight of the Tanks and structures involved.  Call is say 600 Lbs
for safety (Or Fudge <G>) factor.  The more you can spread that load on the
roof the better.  As it is hard to spread loads on the shingle side some folks
use steel reenforcement on the inside of the roof structure's typically 2 X 4
rafters.  Steel is GOOD.  Once can SCREW (not nail, please) the perforated
steel reenforcements into the  wood structure along the long axis of the
rafters and then create a spreading of load by screwed and bolted cross
members.  DONOT USE NAILS... they both tend to not hold well and tend to
weaken the wood itself due to wedge splitting effect.  

> I found a sandblaster who will clean both tanks for $25.<

Always good to clean the tanks HOWEVER I would be more interested in what the
tanks held before you came to own them...  Also pre installment is a great
time to have them pressure tested and interior painted with a epoxy based
paint.  Thus corrosion should be a minor issue in your life time.
 
> Also I was going to plumb the roof hot water into the cold before it
 enters the house gas heater tank, and also into the hot coming out of
 the house tank with a bypass. So if the roof hot is really a good system
 and supplies all my needs I could shut down the house gas heater.< 
 
Sounds good to me.  <G>  However I feel that the bypass is probablly not
needed.  If you came into a long cold spell and forgot to reengage the water
supply to the Gas Hot Water Heater as you fired it up you could create a FIRE
HAZARD.  I HATE fire hazards...  Use the Gas Water heater as a insulated extra
storage tank.  Add more insulation to it but keep the Gas Heater part clear.
Again I hate fire hazards.  

> As long as I am going to make it work, I might as well optomize the
 whole concept.<
 
<G>  Good Attitude.  

> I'm really supprised that more people here don't do this if it is so
 simple.<
 
Well it is simple.  But People hate to look WIERD.  Anything that looks
different is probably WIERD in the timid souls of mass american culture.
Solar got a poor reputation in the 70's and has never recovered.  Oh BTW in
Florida Solar Breadbox heaters were a common sight in the 1920...  But the
onslaught of Advertisements for the Gas and Electric Companys made SOlar Water
heaters old fashioned and foolish.  Sort of a why drive a buggy when you can
drive a Ford....  

Given that your Gas Water heater probably has a thermostat and will not fire
up untill the water temperature drops below say 160 degrees (or what ever it
was set at..) and the solar Breadbox heater keeps the incomming water well
above that Temperature then that Gas Heater will burn but min gas.   

> Yeah I know I'm in solar heaven. Look at Issue #66, P 66, of Home Power,
 I wrote an article about PV in an RV. I have the panels still in place
 on my 5th wheel, but there is a line running from the inverter to the
 shop at my home that I just bought. So that I can use the elec instead
 of just letting the PV system sit there...<
  
EXCELLENT!!!  Go Solar!!!  BTW How do you seperate that power from the common
power lines if the mains fail?  I have friends that work on those lines and we
HATE to get zapped by folks home generators that back flowed onto the broken
lines...  

I trust you have a Blocking Diode to keep that power at home??  

 Thanks for the help
 Rush

You are more than welcome.  Questions are welcome!

Michael