[Greenbuilding] Hot tub building

[Keith Winston]

Hi Kathy,

First, concrete is not strong in tension, but only in compression. So 
unless you have a very solid "form" that's going to remain, simply 
lining it with concrete is unlikely to work well. Also, I'd agree with 
Corwyin that you want to insulate it. Also, are you thinking you are 
going to fill it with groundwater (55F) and heat it to hot tub temp 
(105F) and then dump it out afterwards? There's a reason hot tubs have 
covers, pumps, and filters. If you have a 600 gallon tank, for example, 
that's roughly 4800 lbs of water, and 240,000 Btu to heat it up 
initially, plus whatever it takes to keep it hot, losing heat to the 
surrounding air, earth, and immersed bodies. Figure a decent solar 
collector might gather 30,000 Btu/day, you can see why hot tubs are 
energy hogs, and why you'd want to insulate it, keep it covered, and 
pump/filter the water.

Of course, the smaller the cheaper (money and energy-wise). I was 
figuring 5' diameter and 4' deep, with nothing removed for bodies or 
seats. You might be able to make it 1/2 that, for two people, if you 
shape it right... Maybe even less.

Keith