Inch :
I wasn't trying to compare overall designs, single motor vs double motor,
etc.; nor was I trying to insult anyone. My apologies if my little joke
seemed insulting, I didn't intend it so. The whole "it's cheaper to keep it
hot/cool" argument is a common fallacy, IMO promoted primarily by people
trying to sell you something.
For a given hot tub (or any object that you are trying to heat or cool, like
your house), the heat lost (or gained) is proportional to the difference in
temperature between the water in the tub, and the outside air (well, and the
ground too, but we'll ignore that). So, for example, it takes only half as
much energy input to hold the water at 10 degrees above ambient as it does
20 degrees above ambient; and no energy at all to keep the water at ambient.
Heat is heat, it takes the same amount of heat (1 kilocalorie) to raise one
liter of water by 1 degree Centigrade, whether it's from 25C to 26C or 39C
to 40C. (There are some very small effects as the water expands and
contracts, which we'll ignore.) Electricity converts directly to heat, with
nearly perfect efficiency, 1 kwh = 860 kilocalories. So, one kilowatt-hour
will raise 860 liters of water by 1 degree, or 1 liter of water by 860
degrees (except that it would boil, which is a whole different ball of worms
<g>)
If we stop heating the hot tub, at first it loses heat (energy) at exactly
the rate we would have to add heat to keep it up to temperature. But, as it
cools, it loses heat slower and slower, until at room temperature it loses
no heat at all. And the heat it has lost is exactly the amount of heat we
have to add to bring it back up to the original temperature. So, it always
takes less energy (heat) to let it cool when you're not using it.
Many years ago, the electric companies were pushing all-electric homes in
the midwest, claiming that it was just as cheap to heat with electricity as
fossil fuels, even though at the time, the most expensive fossil fuel
(propane) cost less than 1/3 as much for the same amount of energy. The
"keeping it always warm" argument was one of the arguments they used, IMO
mostly to cover up the fact that a typical electric heat installation (and
hot tubs are no exception) puts out far less heat (and hence takes much
longer to warm up) than the equivalent fossil fuel installation.
Funny thing, tho, everyone I know that owned one of those "energy efficient"
homes complained of their electric bill being 2-3 times what their former
gas+electric bill was. And that's even after the deep discounts the
electric companies offered for using electric heat. Plus many of them found
that they could not keep their homes warm in unusually cold weather, again
because the electric heat was undersized.
BTW, your hot water heater keeps the water hot all the time for a similar
reason, it's tough to build a unit that will heat the water quickly enough.
However, 'flash' water heaters are in common usage in countries where energy
is much more expensive than in the US, like Denmark and Brazil, because they
are more energy efficient.
IMO, the dual motor design follows from the "keep it hot" philosophy, which
follows from the fact most homes do not have a big enough electric service
to heat a hot tub quickly.
Randall
epetrevich@relavis.com wrote :
>
> Randall, first off, I want to say that I'm don't really totally understand
> the whole energy to heat water most efficiently idea. So you may be 100%
> correct and I may be totally hosed about it all.
>
> Here is my understanding of this...
>
> All expensive hot tubs, have good insulation and a second motor much
> smaller than the main one.
>
> It is my understanding that keeping the water at "user temp" in a well
> insulated tub is much cheaper than bringing that amount of water
> up to temp
> for occasional use (like a hot water heater in your house). Besides the
> fact that it is more convenient to have it always at temp.
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