[Greenbuilding] A Solar Hot Tub

[Frank Flynn]

You're probably sick of hearing about heating hot tubs by now but...

I have a solar hot water system in my home and it makes plenty of  
heat for the domestic hot water and a hot tub during the summer.  As  
I see it these are the options:

1 - The simplest (and cheapest by far) the hot tub is the tank, you  
have a drain back system.  When the sun shines and the tub is cooler  
than you want the water is pumped through a solar panel and heats the  
tub.  When the tub is hot enough the pump stops and the system drains  
back.  The disadvantage is you might like to go hot tubbing at night  
and the tub may have cooled a bit since - so set the tub to heat to  
106 or so it will be 104 in the evening (with insulation and a good  
cover the tub will not cool that much when covered and not in use).   
Depending where you live this may not work at all in winter or if you  
have a few cloudy days in a row.  Price is a few hundred $$

1 A - Same as above but with an auxiliary heater - most self  
contained Hot Tubs will have an electric one built in.  Use this on  
the days when the sun didn't make it hot enough.  This might not cost  
more then above except for the electricity to heat the tub when the  
sun isn't enough.

2 - Whole house domestic hot water - solar hot water panels on the  
roof heat water or antifreeze which is stored in a tank; domestic hot  
water, hot tub and possibly rooms are heated by this stored heat  
through a heat exchanger.  Since the storage tanks are big and on  
sunny day can be quite hot they can continue to heat your tub in the  
evenings and for a few days of cloudy weather - but may not generate  
enough heat in winter.  These systems can be complex with several  
pumps and controls, the technology is robust - if you like to tinker  
these can be fun.  If you have domestic hot water running through  
this system you'll probably have some kind of gas / electric back up  
heat, this can be made to heat the tub too.   Price can vary  
depending on the rest of the system - I guess $5 - 20k (but you might  
get a new heater and water heater that are solar powered too)

3 - Photo Voltaic panels make electricity - you heat your tub with  
electricity; So the max power consumed for the Sundance 880 (the  
first large spa I found on the web) was 11,520 watts - I had a  
sundance spa and I think you could keep it warm by heating it for one  
hour a day (assuming you keep it covered) plus the time you're in it  
- so 12 KWH per day.  So to run 100% solar only in winter you'd need  
maybe a 2 KW system (6 hours of full sun); cloudy days and such are  
offset by the summer when you'd have more hours of sunshine.  This  
could cost $20k  more if you want to do more than just heat your hot  
tub.

I personally have a variation of #2.  It works well 6 months of the  
year, and it works OK another 4 months, and not really that well for  
two months (I have an electric heater that came built into the tub)

Good luck -

Frank