[Greenbuilding] A Solar Hot Tub

[dantonioli at earthlink.net]

Dan writes:
 
> 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. 
 
 " But a hot tub can't store any heat if its temperature never changes... " 
 
 
Nick, what do you mean "it's temperature never changes."  I don't follow.
The tub is a tank, it's heated, it's stored, it loses heat just like any
other storage. As such, it's temperature is always changing.
 
 
 
 >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.
 
 " Perhaps you mean the heater initially turns off after the tub reaches
104, then cycles as needed to maintain 104 F until 8 PM. "
 
No, I meant what I said, the heater turns on when the the timer turns it on
at 6:00.  It brings the temperature to 104 and turns on and off as needed
for the temperature. At 8:00, it turns off and "holds" the heat. (Again,
yes, there are temperature losses with heat storage.)

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

That would seem to make sense for people who like to jump into the tub at
any time, without waiting for it to reheat. 
 
 
All you have to do is turn the heater from auto mode to manual mode when you
get in. If the temperature is, say, 102 degrees it won't take long to bring
it up to 104. Otherwise, you have the tub running around the clock when you
don't need it. 
 
 
> 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.
 
That kind of solar heating doesn't seem to save much energy. Consider 4
scenarios:
 
1. "100% solar heat" with a secondary storage tank.
2. solar heat for 6 hours per day, with a constant 104 F tub temp
3. solar heat for 6 hours per day, with electric reheat on a sunny day
4. electric reheat on a cloudy day.
 
   Eon        Eoff          Esun          Elec          Delay
   (Btu/day)  (Btu/day)     (kWh/day)     (kWh/day)     (minutes)

1  5409.405   7496.244      3.782429      0             0
2  5409.405   7496.244      .4323834      3.350046      0
3  5390.759   7496.244      1.364025      2.41294       8.636646
4  5358.812   7496.244      0             3.767602      62.82312
 
In scenario 1 the sun provides all the heat for up to 5 cloudy days in a
row. No electricity is used for heat.  
 
If you have an oversized, expensive system and don't mind a cold tub when
you get weeks of cloudy weather. You'd have to seriously oversize the system
in order to have a solar-only tub for five days of cloudy weather.
 
In scenario 2 the sun only provides 13% of the heat, keeping the tub 104 F
with no reheat delay.
 
Solar provides most of the heat in this scenario, with electric heat coming
on minimally to keep it at 104. Granted, if you have a hot tub party from
8:00 pm till 2:00 am it will require more conventional heat.
 
 
 
In scenario 3 the sun provides 56% of the heat, like a very good direct gain
house, with a small delay. 
 
Same as #2.
 
 
In scenario 4 the sun provides no heat, with a very large inconvenient
reheat delay. 
 
Nick, have you ever lived with a hot tub? "Reheat" doesn't take long once
the tub is heated. It's not inconvenient at all with a timer, which is
standard. 
 
 
Dan Antonioli