[Greenbuilding] A Solar Hot Tub

[Nick Pine]

Dan Antonioli wrote:

>The tub is the storage tank. Thermosyphon potential aside, a regular solar 
>hot tub system uses a standard "swimming pool and spa" sensor/control that
limits the temperature to 104 degrees. When the tub temp is anywhere below 
104, and the temperature of the panel is higher than the tub water, the pump
circulates the tub water. It's one big drainback system.

So the sun heats the tub whenever the tub temp is less than 104 F?

>You still need a conventional heater, which also need to be tied to the 
>pump and filter assembly. If you take an evening soak and the temperature 
>drops
you'll want to be able to keep it at a comfortable temperature. Like most 
hot water systems, solar is the "pre-heat" and sometimes does all the 
heating.

And the conventional heater heats the tub whenever the tub temp is less than 
104 F? This seems overdetermined, like a 4-legged stool.

> A solar heated hot tub works the same way a regular hot tub works. A hot 
> tub stores hot water. If you want a "solar only" tub then the usage is 
> limited, but a hot tub heated at 104 degrees feels just as good as one 
> heated by gas or electricity.

A "solar only" tub with a separate higher-temp heat store that keeps the tub 
exactly 104 F for 5 cloudy days in a row could be ready for people all the 
time, with unlimited usage. After 5 days, the conventional heater might kick 
in, with minimal conventional energy use. If cloudy days are coin flips, 
storing heat for 1 cloudy day allows the tub to be 50% solar heated at best, 
2 days allow 75%... and 5 allow a 97% solar heating fraction.

If there's no separate heat store and the upper water temp limit is 104 F, 
the tub will often be cooler, eg at night, unless the conventional heater 
does all the heating to keep it always ready for people, in which case, why 
bother with solar heating? We might say "Heat the tub to 104 F with the sun 
whenever possible, and only use the conventional heater when the tub is in 
use," so the tub temp would droop below 104 F at night and on cloudy days, 
and there would be a waiting time (less for smaller tubs with more powerful 
heaters) and conventional energy use before usability, while the 
conventional heater warmed it back up to 104 F, and there would be 
conventional energy use while the tub is being used. Or we might say "Let 
the tub temp droop to 90 F min when it is not in use," which would limit the 
waiting time at the expense of more conventional energy use, but that's an 
unfortunate tradeoff, in my opinion, compared to a separate higher temp 
solar heat store. Why should people have to wait to use their 
frequently-fossil-fueled tub?

Nick