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Posts: 3
Reply with quote  #1 
I'm located in southern california and built a solar collector for my hot tub (~300 gallons).  Rather than using aluminum flashing I tried to use a corrugated steel sheet and silicone caulk the CPVC pipe to that.  I went with CPVC piping so I wouldn't have chemistry problems as the hot tub water goes straight through the collector.

My collector is ~25"x7'.  The CPVC pipes are spaced at about 2.5 inches apart.  I'm only getting a 2-3 degree temperature increase in the hot tub per a day, which isn't enough to overcome cooling.

I have it controlled by Arduino with a temperature sensor siliconed to the steel sheet in the collector and one in the water.  The collector temperature is reading up to 162F during the day while it is running.  The water going through the pipe at that point is ~97 degrees and exiting at ~100 degrees. 

Are these roughly the expected temperature differences or do I have a problem with pipes not bonded well thermally with corrugated steel sheet and I should switch to flat flashing?


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Posts: 1,845
Reply with quote  #2 
  Any idea how fast your pumping and do you have any glazing on this?  Photos are always nice.

Posts: 3
Reply with quote  #3 
Polycarbonate glazing.  I'm not sure about the pumping rate and can't measure it right now as it is solar powered.  However, the pump's nominal rate is 100 gph.

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Reply with quote  #4 
I posted these guidlines in another part the other day and just tried them with you.
Hot water panel sizing and flow.

I have asked around and searched the web and these seem like pretty good guidelines,

Gary from Build it Solar recommended .04 gallons per minute per square foot of collector.

Sizing Collector Area
A good rule of thumb for sizing collector area in northern climates 20 square feet of collector area for each of the first two family members, and 12 to 14 square feet for each additional person.

Sizing Storage Volume Hot water usage
A small (50- to 60-gallon) storage tank is usually sufficient for one to two people. A medium (80-gallon) storage tank works well for three to four people. A large tank (120-gallon) is appropriate for four to six people.

Sizing Storage Volume Hot water heating
For active solar water-heating systems, the size of the solar storage tank increases with the size of the collector—typically 1.5 gallons per square foot of collector, (in addition to the above storage if also used to heat water), this helps prevent the system from overheating when the demand for hot water is low.

So if we check the size of your panel your flow should be 27X7=175 X .04 =7 gallons per minute or 420 per hour.  You said you were pumping at 100 so it seems like your water should be getting more of an increase.

Your panels size of 175 X 1.5 = 262.5 so you should be pretty good for the 300 gallons.

It seems like your panel should do better but we have water people here that know a lot more than me on the subject, I'm just learning.  Sure seems like your panel should get hotter though.  Do you have insulation on the back?  How is it angled?


Posts: 243
Reply with quote  #5 
A good way to determine how much collector you need is to use this method:

You basically do a test to see how much the hot tub cools over a 24 hr period
when there is nothing heating it, and then look up on the graph what collector
size you need based on the cool down you measured and the size of the tub in

The method assumes a reasonably efficient collector, so if you find that it says
your 14 sqft should be doing the job, then I'd look at improvements to the
collector. But, it might tell you that you need more collector -- 14sf is
pretty small to heat a hot tub (depending on the size of the tub and how well
insulated it is).

I did this test on a CPVC collector a while back:
It preformed well -- close to the copper with grooved alum fins.
In my own mind I wonder how much good the alum flashing behind the CPVC is doing
-- I have a hunch (totally unproven) that is the close spacing of the CPVC that
makes it work and that you might not see much of a hit in performance if you
just dropped the flashing. I guess a test of this would be good.


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Posts: 1,845
Reply with quote  #6 
Thanks Gary,
  I guess I should of had my reading glasses on as you can see from my math.  I thought it was 25 feet by 7 feet not 25 inches.  That a huge error. (I've done worse)
2X7=14 X .04 = .56 X 60 = 33 gallons per hour so it seems your running to fast but....
14 X 1.5 = 21  as Gary said you need a bigger collector or smaller hot tub.
never mind the 165 degree comment, sounds like it's doing fine for the size.

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Posts: 1,067
Reply with quote  #7 
The forum software broke Gary's link for Comparing the Performance of Two DIY Solar Water Heating Collectors -- CPVC vs Copper so here it is link
Both temperature rise and airflow are integral to comparing hot air collectors

Posts: 3
Reply with quote  #8 
Is 165F normal for the internal temperature of the collector with water flowing?
Scott Davis

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Posts: 697
Reply with quote  #9 
Hi Brianinoc,

165F is well within tolerance for CPVC, and my guess is you are just referring to the air temperature inside the collector, now the fluid flowing through it, which is likely cooler than 165F and lowering the CPVC temperature below 165F.  CPVC is rated to 190F at 75 PSI.  Since we only have a few PSI in our systems, we should be fine into the low 200s. 

Take care, Scott MD
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