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re: elegant cheap etc spa heating
26 jul 2001
oo  wrote:
>for any region of north america there is a critical temperature (te) of
>water to be heated that obeys the following:
>"on a yearly averaged basis, when any heat from the sun is added via some
>solar thermal collector device to water at temperature te, it is
>economically neutral to choose either flat plate or evacuated tube
>technology. at temperatures above te etcs are economically better and at
>temperatures below te flat plates are economically better."

i can imagine that...

>in a spa application suppose te is less than the spa temperature of ~105.
>then you would use etcs exclusively because there is no preheat stage to the
>spa. water goes into the solar collector hot and comes out hotter...

people often want to heat water for showers, too. circulating some dhw
in a closed loop through a hose in the spa seems like a good alternative.
some spas (eg jacuzzis) already turn on a pump when the thermostat says
heat is needed. another option is slowly trickling dhw into the spa and 
preheating incoming cold water on the way to the central heater with
the outgoing spa water in an outboard heat exchanger, eg 100' of 1/4"
tubing inside a 100' reinforced garden hose. this has the advantage of
changing the spa water to reduce or eliminate the need for chemicals.
both have the advantage of keeping the spa at a constant temperature...

>so the really big question you want to ask here is "what is this te for
>where my spa is and is it less than the spa temperature of 105." it
>appears that for most of north america, with the exception of the
>noticeably warm and sunny areas (the sw and se), that this te
>is less or very close to 105f.


>*****this result assumes that the etc is four times more expensive than an
>equivalent area flat plate collector. swingsys thermalstar etcs are about
>four times more expensive per collector area than an average good quality
>flat plate.

like, how much, exactly, per square foot?

>the etcs for spas require no additional pumps or control system! this is

say it 3 times and it's true?

>simply plumb the etc collector in series after the filter and before
>the heater. water will flow though the etc and into the heater
>automatically offsetting heating costs in the most efficient manner. nice!

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.

>i hear some of you saying "what that is crazy!

"what that is crazy!" the spa water will eventually clog the collector.
minerals, chemicals, oils, particles too small for the spa filter... 


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