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re: spa (hot tub) renovation - gas? solar?
19 oct 2004
chet hayes wrote:
>> >> >solar is a waste for a spa. can't provide enough heat rise...
au contraire. heating water to 105 f is easy...
>> >> evacuated tubes work well, even in cloudy climates.
focus has this equation for tube efficiency:
e = 0.717-1.52x-0.0085gx^2, where x = (tm-ta)/g. tm (c) is the mean water
temp, ta (c) is the air temp, and g is the insolation (w/m^2.) tm = 77 c
(170 f) and ta = -1 (30 f) and full sun (g = 800 w/m^2) make x = 0.0975
and e = 0.5, ie a 50% solar collection efficiency.
>> >> ...better to have a separate, well-insulated higher temp (170 f) heat
>> >> store and meter it in as needed.
>>>...with solar... you still will need some other additional heat source
>>>to get it to 105, particularly during winter.
>> i disagree.
>then nick, without an additional heat source, how are you going to get
>the spa up to 105 in a typical outdoor location in the us in winter at
>9pm when you want to use it? let's even assume it was already at 105,
>before the sun went down. as soon as you take the cover off and hop in,
>it's all down hill from there. with no other heat source, the solar
>answer is an insulated storage tank to store hot water.
good idea.
>where does that tank go, on the deck outside with the spa?
i'd say on the ground. it might be heavy.
>and how big does it have to be for the climate?
which one? how about phila in january, when 620 btu/ft^2 of sun falls
on the ground and 1000 falls on a south wall on an average 30 f day
with humidity ratio w = 0.0025 pounds of water per pound of dry air.
say your tub is a 4' cube with 8" of polyisocyanurate insulation and
an 8" cover, with a thermal conductance of 96ft^2/r57.6 = 1.7 btu/h-f
when closed. when open, pw = e^17.863-9621/(460+105) = 2.3 "hg and pa
= 29.921/(1+0.62198/w) = 0.012 "hg, so it loses 4'x4'x100(2.3-0.012)
= 3488 btu/h with the air pump disconnected, or less, if the surface
is covered with partly inflated baggies or ping pong balls. if we use
it 1 hour per day, it needs about 3488+23h(105-30)1.7 = 6362 btu/day.
with 50% efficiency, an optimal-slope flat plate solar collector would
gather 0.5(sqrt(1000^2+620^2)) = 588 btu/ft^2 on an average january day,
so we need about 6362/588 = 10.8 ft^2 of collectors, or less, in tubes,
or a bit more, to account for the heat store loss.
if the heat store is very well insulated and it's 170 f on an average
day and it can keep the tub 105 until it reaches 110, we need about
5x6362/(170-110) = 530 pounds or 64 gallons or 8 ft^3 of water for 5
cloudy 30 f days in a row. we might use another 4' cube with more
insulation inside and a folded epdm rubber liner, to bring about
a pleasing aesthetic symmetry.
>here's a excerpt from an application note from a supplier of evacuated
>solar tubes:
http://www.prime-nrg.com/water.asp
>domestic hot water; 30 tubes for family of up to 4...
>spa/hot-tub; 60 tubes for up to 1500 liters (indoor.)
heating a spa is twice as hard as heating water for a family of 4? :-)
>where do we put the 60 evacuated tubes?
...60 seems like too many.
>on the roof with pipes running around or through the house? or do we put
>them on the deck with the spa and barbecue?
how about below the deck, on the south side of the tank?
>...but wait, according to the supplier, the 60 tubes are for a 1500l
>indoor spa, which really isn't all that big, so we need even more tubes
>for the outside spa in winter.
even more tubes would make the supplier happy.
>then we have to fool around with all the plumbing issues of trying to
>insert all the solar stuff into the spa circulation system, which doesn't
>have pipe fittings sticking out that say "hook up solar collector here."
awww...
>and then perhaps another pump to move the water through the solar system...
sure. a little one. maybe 3488/5/8.33/60 = 1.4 gpm, and another small pump
for the glycol tube loop, unless that thermosyphons.
>then we need to fool around with the power pack on the spa to
>integrate this all in somehow to the std temp control system, which is
>microprocessor controlled, so when you dial in 100 or 105, it somehow
>maintains that temp.
you might disconnect the wires that run to the heater and attach them
to the new spa pump.
>who's going to take liability for doing that right?
henry kissinger?
>at the very least, you can kiss your spa warranty goodbye.
maybe not.
>and personally, i wouldn't want to sit in a spa with added pumps and
>god knows what else that somebody made up as they went along.
chacun a son bain-californie.
>and finally, how much is all this going to cost in material and labor?
you'd probably say "too much."
>the op was worried about spending money for a 240v electric run, which
>is straightforward and simple compared to all this.
maybe so. i'd further complicate things with ozone and a copper sponge
in the filter and a salinity gauge that turns on another small pump to
slowly replace spa water with fresh cold water via a heat exchanger when
the water conductance exceeds some threshold.
>don't get me wrong. solar heat can be a cost effective and workable
>solution for the right climate and the right application. but doing
>this to save the few hundred dollars a year it costs to heat a home
>spa is not one of them.
i disagree.
nick
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