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re: well when should i take my pool cover off??!!
8 oct 1996
andrew mckegney wrote:
>what mr. pine doesn't (want to) appreciate is that (existing) solar
>blankets -as simple as they are- are a bother to use, even with a roller,
au contraire, i appreciate that...
>and that most pool owner really don't want to look at them.
perhaps that would change if they looked like a slate patio.
>they like the look of the water swirling and sparkling in the sun.
i enjoy watching house fires, sometimes.
>a very important point about blankets has not be mentioned by others.
>that is the blankets don't work worth beans if they are wet!
we already talked about that, as regards r-value. wet insulation has little.
even 2% moisture in fiberglass insulation reduces its r-value by half, and
it's not hard to accumulate that much moisture in a house wall, if the vapour
barrier is not perfect. wind is a problem with fiberglass too. foam is better
this way, but r-value tests first developed and standardized by fiberglass
insulation manufacturers use perfectly still, dry air for testing both, so
we don't hear much about the advantages of foam over fiberglass, in the
presence of water vapour and wind.
>as i stated earlier blankets work by reducing evapouration. if there
>is water on top of them, that water evapourates,and pulls heat from
>the pool right through the blanket. it's like having holes in the blanket.
good thought. can we store heat in our slate pool cover patio all day,
and then submerge it briefly for a dip in the pool at dusk, to cool off?
here's another recipe: make 1 picture-frame mold out of 2x4s on edge with
a 4'x8' id, and put a $16 4x8x2" styrofoam panel inside and place 1.5" of
concrete on top of it, and push a 1/2" diameter hole diagonally through each
corner of the concrete, so you can tie these slabs together later. repeat
26 times. make the perimeter slabs with cedar strips and plated screws to
hold down a single $40 24'x36' sheet of 6 mil poly film, on top of the cover,
and inflate the film slightly to raise the cover and make a 768 ft^2 solar
collector, and lower it 2" every so often, letting some water leak in around
the lower film edges, using a thermostat, automatically, to remove the solar
heat. you can't walk on this, but it would keep the pool warm in december.
or build some air pockets under the cement slabs with some pvc drainage pipe
($3.88 for 4" pipe 10' long, at home depot), drilling a hole in the bottom
of the pipe to push air out of the pipe through a small piece of air tubing
stuck in the pipe, if you want to walk on water. join the air pipes end
to end to attach the slabs? use some imagination.
>during periods of bad weather the average pool owner doesn't swim, and
>unfortunately they don't watch the chemicals in their pool. if the pool
>is covered with a blanket it will actually stay reasonably warm...
suppose our pool starts out at 80 f, and only loses heat through its 24 x 36'
r1 "solar pool cover" over a cloudy week in october, in philadelphia, with
an average air temperature of 56.4 f. r = 1/768 and c is about 24x36x6x64
= 332k pounds of water, 166 tonnes. so rc = 332k/768 = 432 hours, 18 days.
nice :-) so t = 56.4 + (80-56.4) exp(-t/18), where t is in days, 78.7 f
after 1 day, 77.5 after 2, and 72.4 after 7 days.
with an r10 pool cover, we'd have an rc time constant of 180 days, so
t = 56.4 + (80-56.4) exp(-t/180), where t is in days, 79.9 after 1 day,
79.8 after 2, and 79.1 after 7 days with the cover on. hmmm.
in december, we might have t = 35.8 + (80-35.8) exp(-t/180), 79.8 after 1 day,
79.5 after 2, and 78.3 after 7 days with an r10 cover on. or t = 35.8 +
(80-35.8) exp(-t/18), 77.6 after 1 day, 75.4 after 2, and 65.8 after 7 days
under an r1 cover. not too bad, but too cold for swimming.
during a long string of average december days, with some sun, a square foot
of this pool with the perfectly insulated side walls and r1 "solar pool cover"
would absorb about 500 btu/ft^2/day of sun and lose 24(t-35.8)/r1, so
t = 35.8 + 500/24 = 56.6 f. not warm enough. adding an overhead reflector like
a 15' tall commercial plastic film greenhouse with the north poly film painted
white or ferrocement over an interesting half-dome armature made of 2x4s and
chicken wire might add 800 btu/ft^2/day so t = 35.8 + 1300/24 = 90 f. hey.
but with uninsulated sidewalls, we need a better pool cover, andy, eg an
automatically-movable one that collects sun during the day and really
holds it in at night...
>never, never swim on or over a plastic film pool cover. many people have
>drowned from trying this. mr. pine has made a very dangerous and foolish
>suggestion in an earlier post about having a plastic cover sink to the
>bottom of the pool.
i have never, never suggested swimming under a pool cover
when it is on the bottom of the pool.
especially a slate one. unless one is a mafia leader in disfavour...
("guido decided to investigate the pool cover, from underneath.")
nick
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