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re: elegant cheap etc spa heating
26 jul 2001
oo  wrote:

>> i liked your last plan better. keep some very hot water in the integrated
>> collector and release some of it into the spa as needed to maintain 105 f.
>> maybe no temperature droop over a few cloudy days in a row.

not many days... 40 gallons of 200 f water can keep a spa that needs
20k btu/day with the cover on warm for 40x8(200-104)/20k = 1.5 days...
 
>...i assure you that the etc manifold method is cheaper (marginally) and
>simplier to install and is also thermo-dynamically more efficient. the other
>method also require some kind of control scheme......manual or otherwise.

temp control seems like a fundamental problem with spas... 103 is too cold
and 105 is too hot, so we can't store much heat in all that water. hence
my suggestion to circulate some hot water from the central heater through
a heat exchanger or store some very hot water to add on cloudy days. or 
maybe keep it as close to 104 (not over) as possible with the sun and use
the electric heater as needed. or keep it at 150 and dump cold water in 
to cool it for use?

controls can get cheaper and cheaper with more volume (eg 1k to 10k units)
and the passage of time. electronics is like that, vs "hardware." we can
go out of our way to make the controls very expensive to start with if
that eliminates a few pounds of basic mechanical stuff like collectors
or plumbing or pumps or brackets or bolts. 

>> "what that is crazy!" the spa water will eventually clog the collector.
>> minerals, chemicals, oils, particles too small for the spa filter...
>
>oh come on, now you are really digging deep to find something to complain
>about :). this is a valid point but simply chemically clean the copper etc
>manifold once a year to keep things a ok.

i don't know how fast this would happen, but it seems like a potentially
serious problem. owens-illinois sky-pak etcs are so sensitive to crud that
they recommend using nothing but distilled water, with a heat exchanger.

they also caution against putting water into a hot dry tube, which can
shatter it. their drain-down system can be inefficient if pump power fails
or heavy clouds pass and the system drains and then the sun comes back.
the system can shut down early in the day, and it won't start up again
(ideally) until the next morning, to avoid shattering. 

nick




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