Solar Hot Tub

(pdf diagram)


@ the SolarStudio


summer 2006 we took an old cedar hot tub that had been salvaged from Craig's list and turned it into a solar heat exchanger.


The tub holds about 270 gallons of water, and is connected to a 32sf thermal solar panel filled with glycol that runs the hot liquid thru pex tubing underground and into a coil of copper at the bottom of the tub, just like a hot water heater would do as a heat exchanger, except we have alot more water.

Here are some stats on the tub:

Hello everyone,

The tub was up and working then a problem with the used controller, replaced it and now the hot tub is back on the blink, it seems the coolant is not moving thru the pump as it should. it seems to be lost. a leak? we are on the mend to fix it before the Build it Green Tour in the next couple of weeks.
We had some problems with the first thermostat controller but has been replaced with a new one.
Total price of materials was twice of what I expected. Originally john patterson thought the costs would be $500 for all used parts
Well it totaled up to $1100.

So as always lessons learned to expect more than you estimate.

The tub is a constant 109! well, it was 105 when we first set it but some like it hot.
There are times that the tub does not come on for days, because if the water in the tub is 109 where the sensor is set, then
There is no need to circulate the glycol, it just sits and waits for the T to drop.

The electric bill has not changed so no noticeable increase. Its only an 80 watt pump and it is hardly coming on how in the summer

The tub took 4 hours to heat up 10 degrees. That is better than expected,
And the loss each night is almost nill.

When the cooler air temp comes in with fall and winter then will check again.
But the insulation factor in this tub seems to be so good that it hardly losses any T.

Tub dimensions total 32.59cf x 7.4 gal = 241 gal approx when full


300 btu incoming solar radiation per sf here in portland
X 32sf for the solar collector (4 x 8)
Totals 9600 btus/hr x .85 efficiency = 8160 incoming btus per hour for the solar collector

8160 btus
1929.47 lbs = (241 gal x 8lbs/gal)

8160/1929.47 = 4.2 degree/hr

In 10 hours expect 42 degrees!!!

Insulation values:
R7 for polystyrene/spray foam per inch

Lid is 3” = R21
Wall 5” = R35
Bottom 12” = R84

Solar gain in portland:
Jan – 10% = 3 days
Feb – 33% = 8 days
Mar – 50% = 15
Apr – 65% - 20 days
May – 78% = 23days
June – 80% = 24 days
July – 99% = 30 days
Aug – 98% = 30days
Sept – 91%=30 days
Oct – 50% = 15 days
Nov - 23#% = 5 days
Dec- 6% = 1 day

= 55% year supplied by solar

Elec cost:
80 watt pump x 6hours day= 480watt/hours day = 1/2 kwh day
1/2 kwh = 0.04 cents per day x 204 days per year = $8.16 year

Elec hot tub pump:
Ohms law says volts x amps = watts
110v x 20 amp = 2200 watts
2200 watts x 6 hours = 13,200 watts or 13.2 kwh x 0.08 = $1.056 per day
X 160 days without sun = $168

This varies of course on the weather!!!!!!

Thanks everyone


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