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re: solar hot tub
27 sep 1996
richard lazar  writes:
>>i am interested in converting my hot tub to solar heating.

nice :-) i just got some mail from eagle pools, a local spa dealer who say
"heating elements are the #1 reason for spa failures in the spa industry."
i will call them again and suggest solar heating. they usually get defensive
and tell me spa heating only costs pennies a day.

>>i have a good south roof to use. any idea how many feet of tubeing i need,
>>or any other ideas.

is the hot tub indoors or outdoors? you might make a simple solar air heater
on the south side of an outdoor version. year-round use and the higher-than-
swimming-pool-temp argue against covering the roof with two layers of uv-poly
film and trickling water between them. plus, you need so little heat.

>the hot tube is 7x7 and holds about 400 gals. 

four feet tall, with 112 ft^2 of walls and a 49 ft^2 cover, and r20 insulation,
eg 4" of beadboard, and a thermal conductance of 161ft^2/r20 = 8 btu/f-hr?
with 104 f water inside and 36 f air outside, so it needs (104-36)8 = 544
btu/hour to stay warm, where i live, in december? that's 13k btu/day or about
4 kwh or 40 cents a day...

andrew mckegney  says:

>please remember that most hot tubs are used in the evening - usually
>after the sun has gone down.

ok. and let's assume that the tub isn't used all that often,
that the cover is mostly in place in the winter.

>if you blow air into the hot tub it is going to cool off very rapidly -

why blow air into the tub? and if it loses 544 btu/hr, and the water weighs
3200 pounds, it will lose 544/3200 = 0.17 f degrees per hour, no? how about
circulating a little sun-warmed air under the hot tub, once in a while? and
using a bigger hot tub, or one with more insulation. a 7x7x4' tall hot tub
will hold about 12k pounds of water. surround it with r40 insulation, and
it will only lose 6.5k btu/day, cooling off 0.5 f per day--4 days, 2 degrees.

>and you are going to need a "on-demand" heater -eg: natural gas or electric
>to keep the tub warm.

really? well, most people like precise hot tub temperature control: 103 f
seems very cold, and 105 seems very warm. we can't store much solar heat
for a cloudy week in that kind of temperature swing.

we could store 6.5k btu per day for 5 days in 1300 pounds of water cooling
from 130 f to 105 f. in 3 plastic 55 gallon drums full of water, sitting
inside a strawbale skirt? if the tub is built on a 4' pedestal, this could
be a completely passive system.

were we concerned that the drum surface be insufficient, 25 ft^2 each,
compared to 28 ft^2 of solar glazing, we might use 32 5 gallon plastic paint 
or drywall compound buckets with lids, or 300 2 liter soda bottles.

the tub enclosure might have a dark insulated south wall with a single layer
of polycarbonate plastic on the outside, over an air gap, and a small pv fan,
to collect about 20k btu/day of sun, or a third more, if there's a reflective
surface on the ground in front, eg a shallow frozen reflecting pool, used for
water replenishment and treatment, vs chemicals. no insulation under the tub,
and a small pv/battery-powered fan that circulates a little warm air under the
tub on a cloudy day. if the tub bottom were 6' square, and the fan moved air
at 6 mph with a 1 f delta t, the surface air film thermal conductance of the
tub bottom would be about (2+6/2)36ft^2 = 180 btu/f-hr, so 544 = 1f(180)cfm,
roughly, so we would need a 3 cfm fan, or in a more passive version with
slightly less accurate temperature control, a damper with a bimetallic spring,
like the $12 leslie-locke fv-1b automatic foundation vent from home depot.

>solar can work, but it has its' limitations, it is usually not
>installed as the only heater for a hot tub (or spa).

what we usually do is use oil to heat water, and bomb iraq when the oil price
threatens to go up. i suppose we could use gas or electricity instead, but
have you noticed the sun?


       noone is exempt from occasionally uttering nonsense.
       the misfortune is to do so solemnly.      --moliere 

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