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re: separate warm water and hot water systems?
29 jan 2003
david nedved wrote:
>...what i want to do is build a roughly 1500 ft^2 earth sheltered home
>off the grid. i plan on taking maximum advantage of passive solar, but
>optimizing the house for summer (here in sc hot summers are a bigger
>problem than cold winters).
columbia looks mild, with 1140 btu/ft^2 of sun on a south wall and 820 on
a horizontal surface on an average 43.8 f january day... 350 of that is
diffuse. july has an 80.8 f 24-hour average, with 70 and 91.6 daily min
and max and humidity ratio w = 0.0155. the average yearly temp is 63.1.
>i'll be aiming for a house that keeps itself mostly cool in the summer,
>therefore making a year round heating load with minimal cooling needs.
>i'll do what i can with passive, but i really don't want to have to use
>a/c in the summer because that would mean firing up the generator...
you may need dehumidification in summertime. pa = 29.921/(1+0.62198/w)
= 0.728 "hg makes the dew point td = 9621/(17.863-ln(pa))-460 = 69 f.
licl comes to mind.
>i plan on shooting for high 90% active solar for all water heating. right
>now i'm thinking i probably want two completely separate water systems.
>hot water for showers, and warm water flowing through the floors as the
>main method of distributing said heat energy.
floor heating seems overly complicated.
>the hot water system, would probably be a large high effenciancy propane
>gas heater.
you might preheat 63.1 f water with the sun, then run it into the heater.
or make a larger solar hot water system with no backup fuel.
>...i would want to add a ton of insulation and run this water
>through collectors optimized for high temp, right? something like the
>vacuum tube collectors that are good for maximizing temp, right?
sounds expensive.
>the warm water system would be what i would use to heat the house.
you might think "sunspace," and heat the house with warm water on
average days and store hotter water for cloudy days.
>i would probably want to run this water through collectors optimized for
>collecting the maximum thermal energy, such as traditional (cheap!) black
>painted brass collectors or some such, right?
wrong.
>i had been thinking that i would run the cold water going into the hot
>system through a coil in the warm water to use as much of that heat to
>preheat the water. i'd also been planning to install an "endless shower"
>type of system to maximize my use of the hot water.
good ideas.
>- is my thinking valid in that these two systems have very different needs
>and are best kept separate?
yes.
>- is anyone else doing this?
dunno.
>when sizing the warm water system for heating, i want a large enough
>storage tank to keep energy for multiple days. how large can i make my
>storage tank(s) for this system without getting silly?
depends on your silliness threshold. it makes more sense to store hot
water for multiple days.
>what sort of tank(s) do people use for when you get too big for water
>heaters?
epdm rubber-lined plywood boxes, 24' x 4' deep circular swimming pools...
>i get the feeling that after a day or two the dimenishing returns
>will make it silly to store more water than x amount...
overnight storage might make your solar house heating fraction 1/2.
another day might raise it to 3/4, then 7/8, 15/16, and 31/32...
>- am i totally off-base here and do i need to go read some more books? if
>so, recommended titles would be great!
you need to read more books. look up "ohm's law for heatflow."
nick
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