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ascar1966

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Reply with quote  #1 
First let me say that I am just beginning to try to formulate a plan for a solar water heater which I plan to mount on my roof, to first use to heat my Jacuzzi. If all goes well, I can use the same plan to replace my electric hot water heater. I live in Florida, so I really think that I should be taking advantage of the SUN! So, I am wondering about circulation through pipe (most likely copper...I think?). I have seen projects that use a "Header" design, or a sort of "manifold" for the layout of the pipes rather than a continuous run with the use of "elbows" instead of "T's" . My question is: does the manifold or "header" design provide adequate circulation of the water, or would a continuous flow using elbows be better? I would appreciate any help in this area! THANK YOU!

SolarInterested

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Reply with quote  #2 
Welcome to our forum. The main disadvantage of a "continuous run with the use of elbows " (commonly called serpentine) is the increased friction head. See below for info:

Parallel Riser Pressure Drop (link)
"Why are we ignoring the pressure drop in the collector riser tubes?

The pressure drop through the collector's riser tubes themselves is small, and can probably be ignored.  We have included the collector manifolds above, and each riser only sees about 0.18 gpm of flow.  If you work through the pressure drop for a 10 ft long, half inch diameter riser, its only about 0.13 inches of water head.  Since all the risers are in parallel, this is the total pressure drop over the risers.

On the other hand, If you have a serpentine collector in which a single length of pipe is wound back and forth through through the full collector, the pressure drop will be larger and should be included.  Just as an example, if a 25 sqft serpentine collector uses a single 50 ft length of half inch copper pipe to collect heat, the the flow will be about (0.04 gpm/sqft)(25 sqft) = 1.0 gpm.   The pressure drop for 50 feet of half inch copper pipe (from the table below) is about (0.007 psi/ft)(50 ft) = 0.35 psi, or about 10 inches of water head."

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Both temperature rise and airflow are integral to comparing hot air collectors
SolarInterested

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Reply with quote  #3 
Quote:
Originally Posted by ascar1966
... My question is: does the manifold or "header" design provide adequate circulation of the water 


The short answer is yes. See the following link for some testing:

http://www.builditsolar.com/Experimental/FlowDistWideCol/FlowDistWideCol.htm

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Both temperature rise and airflow are integral to comparing hot air collectors
Virgil

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Reply with quote  #4 
We have successfully ran a serpentine all copper thermal system for years. It using 3/4" copper pipe through out the system including the two parallel plumbed copper coil heat ex changers within the plastic barrel/tank.  The vertical distance from the storage tank to the bottom of the collectors is about 20 feet... In the four parallel connected collectors there is probably 120 feet of piping... total... The single pump is a Taco 1/8 hp... The system is intentionally non-pressurized...  We use only clean fresh water that recirculates within the collectors and the storage tank...  We have successfully ran it in the frigid subzero temperatures of northern New York... We have never had an issue with this small pump pushing the water to the top of the collectors...  Of course, in this drain back type system the heated water falls by the force of gravity back to the tank... During the summer months a small fan blows cool cellar air across the motor portion of the bronze water pump.... There must be thousands of hours of operating time on this small water pump... To date we have never had a problem with it.... Key thing though, our system is completely 3/4" diameter copper pipe...  Just giving you some comparison details...  as pointed out above... Gary's build it solar web site is a wealth of information...
Virgil

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Reply with quote  #5 
One additional thought on this subject.... I would not recommend using serpentine piping in freezing climates, unless you are an excellent plumber... If the slope of the piping is not correct for fresh water drain back systems that design is prone to freeze ups...  However,,, in warm sunny Florida you could put in what is the easiest...  good luck...
ascar1966

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Reply with quote  #6 
Thank you all for your help on this question. I think that I have a little better understanding. The basic answer is: yes the header system does provide adequate circulation, but in reality, I can probably avoid most of the soldering by using a serpentine system with 3/4" copper tubing and a "small" pump. Particularly here in Florida, it really does not take much innovation to get some energy assistance from the SUN.
SolarInterested

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Reply with quote  #7 
In a frost free climate like yours it should be OK. If you're going to have multiple collectors hook them in parallel like Virgil mentioned in his post above.

Here's a possible Parallel-Serpentine layout suggested by jakedimmick in this topic (link) and shown below:



Gary from the BuildItSolar site has done some testing on drain back of a serpentine collector at the following link:
http://www.builditsolar.com/Experimental/SerpentineDrainback/SerpentineDrainbackTest.htm

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sundug

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Reply with quote  #8 
Quote:
Originally Posted by ascar1966
First let me say that I am just beginning to try to formulate a plan for a solar water heater which I plan to mount on my roof, to first use to heat my Jacuzzi. If all goes well, I can use the same plan to replace my electric hot water heater. I live in Florida, so I really think that I should be taking advantage of the SUN! So, I am wondering about circulation through pipe (most likely copper...I think?). I have seen projects that use a "Header" design, or a sort of "manifold" for the layout of the pipes rather than a continuous run with the use of "elbows" instead of "T's" . My question is: does the manifold or "header" design provide adequate circulation of the water, or would a continuous flow using elbows be better? I would appreciate any help in this area! THANK YOU!


I don't know if you use chlorine or bromine in your Jacuzzi, but know that both are incompatible with copper or aluminum. Also, do not replace the water heater, but keep it as backup. I'm sure even FL has cloudy days. Sundug

 
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stmbtwle

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Reply with quote  #9 
I'm also in Florida and have used a solar water heater to heat my hot tub/jacuzzi.  Though it's a viable approach there are a couple things you need to consider.

1: as Sundug states the chemicals in the hot tub are incompatible with copper and aluminum.   You will need a PLASTIC heat exchanger of some sort to isolate the water flowing through the collector from the water in the tub.   I think a coil of PEX or a grid of CPVC in the filter compartment would work, but I haven't tried it yet.

2: Jacuzzis operate at a fairly low temperature in relation to what your solar collector can produce.  If you run the collector any time there is sun your Jacuzzi will eventually become dangerously hot, so you need a high-limit thermostat to turn off the pump.  This works fine for the jacuzzi but WILL stagnate the collector which can easily reach boiling.  Copper or aluminum can handle this but CPVC cannot.  This creates a problem because if you keep pumping water through the collector to cool it you will overheat the tub.  Some method of dumping the excess heat is necessary or you will run into trouble.

My first attempt at a small collector for my hot tub was a CPVC collector, but it was destroyed by the high temperatures.   My second attempt was an ARETHA with an aluminum HEX and while it handled the heat well, after a year or so the passages became clogged with corrosion.  My third attempt is another ARETHA (still in building stage) but I will have the heat exchanger mentioned in (1).  However that still doesn't address the overheating problem.  I will still need some means of cooling the collector.

Solar-heating a jacuzzi is more of a challenge than meets the eye.

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Willie, Tampa Bay
sundug

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Reply with quote  #10 
I couldn't agree more with your last sentence, Willie! I have been trying on and off for a year or more to get a good year round solar heater going for my hottub. I have the added problem of dealing with freezing temperatures here in TN, and also no place for a storage tank. I am trying to flow the spa water thru the collector to eliminate a HX (heat exchanger), but this leaves me with the problem of collector stagnation in summer and freezing in winter. First I tried an evacuated tube (ET) collector- 
http://simplysolar.supporttopics.com/post/new-evacuated-tube-spa-swh-7477891?pid=1287698130
Spa water is circulated by a 20 watt PV panel, wired thru a differential thermostat plus a high temp cutout switch controlling the 15 watt pump. This worked well in summer and shoulder seasons, but the manifold and plumbing connecting the clear tubes were susceptible to freezing, leaks and algae growth. It also didn't have enuf collector area for winter.  I next tried looping 300' of 1/2" irrigation black poly plastic tubing around in a 4' x8' collector I built from metal stud channel, glazed with twinwall. This worked really well, heating the 250 gallon tub from 60*F to 100*F in two partly cloudy days. I knew it was going to overheat if stagnated, so I covered it temporarily with a weighted coroplast absorber which the wind blew off and in minutes the tubing melted down in two spots that must of had air in them. I slid the coroplast absorber back into the collector only to have it leak, which is where I am now. My plan is to use an 8' wide roll up bamboo shade to eliminate overheating. The black plastic tubing can freeze without harm. I am waiting to clear out my shop and get some help to wrestle the collector back into it so I can try the coil again, but with the shade this time. It won't be automatic, but I don't see how to deal with the temperature extremes any other way. It has to be glazed for winter, but that means overheating in summer. Sundug

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