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.
Now, don't forget that even if you don't use your hot tub, you still need
to run the pump (and heater) every now and they to circulate the water. (to
prevent algae and such from growing). During this time, you are trying to
heat the water also. (unless you turn your heater way down). So you are
still running your larger motor more. (It's is way cheaper to run a 1/8th
hp motor than a 5 hp motor)
I have to admit that I have never owned a single motor hot tub, so I can't
compare them. But I have friends with them that leave them empty most of
the time because they can't afford to use them. For me, I can't tell by
looking at my electrical bills which month I was using it and when I
wasn't. (I typically drain it in the summer, because I'm not home and
rarely use it then and I can't check water conditions)
Inch
Randall Young
<ryoung@NAVCOMTECH.C To:
shop-talk@autox.team.net
OM> cc:
Sent by: Subject: RE: new subject -
old spa
shop-talk-owner@auto
x.team.net
07/17/2001 07:29 PM
Please respond to
Randall Young
epetrevich@relavis.com wrote :
> This is a much cheaper way to operate a hot tub. Keeping about
> 250 gals of water at temp is much cheaper then getting it back up there
> over and over again. With a cheap unit, you need to run the jet for a
> while before you want to use it (sometimes it may be close to an hour)
Inch :
You've been listening to too many hot tub salesmen ! A little reflection
on
the laws of thermodynamics should show that it _has_ to be cheaper to heat
the tub up when you want to use it : The heat lost from the tub is
proportional to the temperature difference between it and the surrounding
air. As soon as the water cools at all, that difference is lower, so the
heat lost is lower. Less heat lost means less heat to replace to bring it
back up to temp.
The problem with heating an electric tub when you want to use it is how
long
it takes. Using your example of 250 gallons, that's roughly 950 liters, so
it takes roughly 14000 kilocalories to raise it from 25C to 40C (77F to
104F). If your heating element draws 50 amps at 220v, or 11 kilowatts, it
would take it roughly an hour and a half to heat the tub up. OTOH, my
150,000 BTU/hr heater would do it in about 20 minutes.
Randall
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