[Greenbuilding] Hot tubs and recirculating pumps

[George J. Nesbitt]

Fun is healthy! isn't it?
    An 85W pump running 24 hrs/day 365 day/year will use 2.04 Kwh/day or 
744.6 Kwh/year. This will require an additional 0.434 Kwh AC (CEC rated) 
of PV capacity (4.7 hrs average hours sun per day Oakland CA).
    The 3.2 amp 230V (736 Watts) pump running 6.5 hrs/day will use 4.78 
Kwh/day or  1746 Kwh year. This is 1.02 Kwh AC of PV capacity.
    If you can reduce the filter run time you can reduce the energy use, 
whether you can get down to 3 hrs or 2.21 Kwh/day is hard to know. (it 
will depend some on amount of use, and how clean you keep the water).

    This is just the filter pump and not even the hot water heating! How 
much it will cost depends on the price of electricity. In some parts of 
the country people pay $0.04 Kwh for all their use. Here in California 
you pay a minimum of $0.12+/- Kwh and up to $0.55 +/- Kwh for summer 
afternoon use if you use lots of electricity.

   The 85W pump running 24 hrs looks like it uses less energy that the 
larger pump running longer hours. But I have inside information ;-)  and 
I know you are on time of use rates with a PV system. Since you get 
credit at a higher rate if your PV production exceeds your use from 
1-7pm in the summer, you might be better off using more electricity, but 
off peak.

    We had a hot tub at my parents house years ago. It was electric 
resistance heating, 110V. My parents have excess solar hot water 
production in the summer (4 to 6 25 yr old panels, 80g tank). We made a 
copper pipe heat exchanger, placed it in the hot tub (had to remove to 
use), connected with garden hose to the storage tank, ran a pump to heat 
hot tub (no controls, manual operation only, delta T controller would 
have been an improvement). In the summer we heated it exclusively this 
way. I don't remember how long we had to run the filter, or how much 
total energy we use.

    To minimize energy use buy as small a tub as possible. Pay extra for 
more insulation around the inside of the tub (have them spray it full). 
Buy as thick of a cover as you can.

   



Kevin Dalley wrote:
> Yet another hot tub question.  I'm confused about buying an energy
> efficient model.  I know, hot tubs use lots of power, no matter what.
> I may add more solar panels to offset the additional usage.  But the
> hot tub isn't for fun, it's for health reasons.  Really.
>
> Does anyone have reliable information about energy efficiency of the
> tubs?
>
> I include some of my calculations below, but my confidence isn't that
> high.
>
> The California Energy Commission established standards for hot tubs.
> These standards were supposed to take effect January 1, 2006.  The hot
> tub industry complained that the standards were not quite correct.
> The standards correct for volume and surface area of the tubs.  This
> is reasonable, but the standards don't include a minimum constant.  A
> tub needs a certain amount of energy, regardless of size.  The energy
> should increase in a way related to volume and surface area after
> that.  Volume for heating, surface area for cooling.
>
> In short, the standard still are not in effect, and I want to buy a
> tub.
>
> Many manufacturers use an 85W circulating pump which runs 24
> hours/day.  This pump filters; much of the heat produced by the pump
> is recovered and used to heat the water.
>
> The Marquis tub, by contrast, uses a higher powered pump for
> filtering, the same pump used by the jets.  It uses 3.2A at 230V. The
> official recommendation uses this pump for about 6.5 hours a day, for
> filtering, testing the heat, heating the water.
>
> Probably, I can cut this back to 3-4 hours a day or so.  This is still
> 2.2-3kWh/day.
>
> I can keep the heat off except for a short while before I use the tub.
> With insulation, the temperature will be close to correct, and just
> need a small amount of heating.  I can cut the filtering down from 2
> hours twice a day to 4 hours a day, plus one hour after use.
>
> The 85W pump *might* be better, but I don't quite have enough details
> to decide.
>
>
>
>
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