[Greenbuilding] A Solar Hot Tub

[Dan Antonioli]

Nick, a few things about hot tubs.

104 degrees is the code for hot tub temperature. You can find various ways to heat it beyond 104, but that's the legal limit in the united states due to health risks. And the differential controller I'm referring to has a high set point at 104. A standard hot tub heater has a high set point at 104.

Like any storage system, inluding the most expensive double insulated solar storage tanks, there is a temperature loss. But even with this loss a hot tub "stores hot water. A typical hot tub use pattern is to set a heater on a timer. At our house, the timer is set from 6-8 pm. At six the pump and heater turn on at it only takes about twenty minutes to bring the hot tub temperature up to 104, after which the heater turns off and the pump circulates the water through the filter. 

Some people might set their hot tubs on 24/7, but it's unusual and makes no sense.

But with solar, the energy is free so all day long when the sun shines the tub will be maintained at 104. Nothing wrong with that.

If you can afford a secondary storage tank to be the secondary heat source, great, but it's a much larger and more expensive system. If you don't have a conventional heater in a lot of climates you'll simply not have a hot tub for many cold days when soaking is at a premium, no matter how large your thermal system is and how much storage you have.


A combined solar thermal hot tub with an electric conventional heater is a good way to go, especially if you have a grid-tied pv system to offset the electricity usage. 

An additional note I'd make is that if you live in the country and have an abundance of wood for heating you can run a loop off a wood burning heater to get most if not all of your hot water for showers, radiant heating, and hot tubs. 

Dan Antonioli




-----Original Message-----
>From: Nick Pine <nick at early.com>
>Sent: Jul 11, 2007 11:58 PM
>To: Dan Antonioli <dantonioli at earthlink.net>, 'Greenbuilding Building' <GREENBUILDING at LISTSERV.REPP.ORG>
>Subject: Re: [Greenbuilding] A Solar Hot Tub
>
>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 
>