DIY Solar Heater - Flow Issues

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playa7seven7

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Aug 19, 2008
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Hello All!

I’ve recently finished installation of solar panels for my pool, in which I’m trying to troubleshoot my flowrate/performance. This was a DIY project that I started last fall, knowing that it will undoubtedly be less efficient than a commercial system (no going back now!).
My collector panels are made up of ½” black poly-tubing coiled on a 4’ x 4’ piece of black painted plywood. I have 12 panels, each with approximately 200 ft of poly-tubing. They are mounted on a 2 story roof (approx. 25’ high). I am running the panels in parallel, two rows of 6 panels. Panels are facing directly east in southern Michigan --unfortunately we don’t have a south facing roof, and I could not convince my wife to let me install them facing the west (towards the road).

Volume of our pool is approx. 23K gallons, with supply/return lines of 1.5” diameter. The mounting location is approximately 60’ from my equipment pad (35’ horizontal, 25’ vertical). I utilized 2” PVC from the 3-way diverter valve up to the panel headers, which reduces to a 1.5” header connecting to the ½” tubing at the panels. I am using the Pentair SolarTouch controller. I’ve attached several pics of my setup for reference.

I took suction-side and discharge-side pressure readings on my pump last year before installing the panels, which allowed me to estimate 50 feet of TDH, and 62GPM from the pump curve. My pump is a Sta-Rite Duraglas 1hp (Model: P2Ra5E-181L). My filter is a Hayward EC-65 (DE). Filter pressure is typically 8 psi when solar is off.

During normal operation, I’ve been diverting 75% of the water to the panels with the other 25% direct return to the pool. I generally see a 10 psi increase in filter pressure with solar on (reading is ~18 psi), which makes sense given the 25’ elevation.

My problem is that I’m having trouble getting adequate flow. Here’s a summary of what I’ve observed:
•Normal Operation - All 12 solar panels open; divert 75% water to solar panels. RESULT: Average combined flow rate from the 12 panels is 21 gpm (i.e. <2gpm per panel). Measured pressure at the supply header on the roof is 3 psi –does a 15 psi pressure drop from filter to roof seem excessive?
•All 12 solar panels open, divert 100% to the panels. RESULT: Average combined flow rate of 23.5 gpm from the panels.
•6 panels closed, 75% diverted to solar. RESULT: Average of 3gpm from each of the 6 open panels.
•11 panels closed, 75% diverted to solar. RESULT: 4 gpm from the 1 open panel.
•All 12 solar panels open; divert <75% water to solar panels. RESULT: Water unable to make it through the panels back to the pool.

At this point, in order to increase flow I assume I need either: A) Larger pump, or B) Booster pump. Any recommendation as to which route is better, or if there is any other trouble-shooting advice? I've seen many other posts indicating that similar sized pumps (0.75hp, 1hp) have been adequate for their solar installations on 2-story houses --not sure if the coil design is the contributing factor for my issue?

Other notes that may or may not be relevant:
•At both of the coil discharge headers, I can feel vacuum being pulled when I disconnect a coil. Gravity must be helping pull water through panels.
•When diverting at least 75% to the panels, I can tell just by feeling return jets at the pool that I’m getting less flow/circulation.

PS.
Here are the results on a very small-sample size of panel performance thus far:
We had an 80% sunny day yesterday, outdoor temp in the morning was low-70s with temps peaking at 83F in the afternoon. Pool temp was 68F at 6AM, and 73F at 4PM. Given the warm temps and large quantity of sun, I was hoping for a higher increase by the end of the day. Note: For reference, neighbor who has similar size pool without solar cover had water temp of 72F by the end of yesterday.
 

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I suspect you need a bigger pump plus more parallel flow, you could splice in another inlet and outlet on each coil to reduce back pressure (say inner 100 ft of coil then outer 100 ft.) You also may want to replumb the feed and return headers to run corner to corner to even out pressure across the array.
 
Thanks for the input!

At this point, I'm trying to decide if I'd receive much benefit in re-plumbing without first buying a larger pump or a booster pump. Re-plumbing would be more time/effort, but definitely a cheaper solution than a new pump. But, if I'm going to need a new pump regardless, I'd rather start with that change and then evaluate from there.

Is the 80% pressure loss observed by the time the water reaches the supply header on the roof (upstream of the coils) likely just due to my pump size? Or, is there potential that the pressure loss I'm observing is a result of the downstream piping/coils/gravity-pull? Not sure what others have seen on their setups.

Also, attached is a picture that better shows the layout of the supply/return configuration on the roof.

Thanks again!
Supply-Return Piping.jpg
 
A dramatic loss of pressure at the roof is completely normal and will be true with any pump. A larger pump will give you higher pressure at the roof, but it will still be dramatically lower than the pressure down by the pump. For every foot you go up you lose about 0.4 psi.

There is a significant plumbing issue here. You shouldn't feed the input and outputs from the same side of the roof. Instead one or the other should run the entire width of the roof and feed the array from the other end. Failing to do this means far more of the water flow is in the loops near the feed end, and very little flow occurs at the far end.

Separately, coils like yours are much less efficient than conventional pool solar panels. You want as much parallel flow as possible. You have all the coils in parallel, but a typical panel runs many many 4' sections of tube all in parallel, vastly more parallel than what you have.

While a larger pump will help you get more flow out of the array, I don't think that is the primary issue here.
 
Yes, going into this project I was completely aware that DIY coil panels are less efficient than the commercial panels. I had run across the coil concept on several solar websites, in which they appeared to have decent results –however, they didn’t express similar flow issues that I’m dealing with (and they don’t have a forum to seek advice). My end goal with these panels is not necessarily to extend my pool season here in Michigan, but rather trying to add a bit more heat throughout what can be mild summer months.

Does the optimal 4gpm flowrate often discussed pertain specifically to the commercial panels? Should I be aiming for flow higher than 4gpm for my less-efficient, non-traditional panels?

Regarding supply/return on same side of roof, I had toyed with several layouts before plumbing it. Are either of the attached layouts more along the lines of what you think would be an improvement (excuse the crudeness of my quick ‘Paint’ job sketches)? At this point, I’m fairly confident that the split-flow layout is not going to be an option, given the space available on my roof.

For what it’s worth, I have ball valves at inlet/outlet of each panel that I can use to balance flow if necessary. However, so far I’ve had pretty consistent flow across the first and last panels of each row even with all panel valves 100% open --flowrates are <5% difference between first and last panels.

My current pump is original to the pool, which has been here for 15 years. So, even though it’s running fine, I feel I could justify going larger if it will provide benefit to this solar project.

Thanks again for the input!

Opposite Supply-Returns.jpg
Split Supply-Returns.jpg
 
The upper of your two layouts is much better than what you have now.

It still has one tiny issue that I didn't mention before, though fixing this might be tricky enough to make it not worth the effort. Ideally, you want every possible path the water might take to be the same total length. You nearly have that, but not quite. On the left and right side the lower row has a slightly shorter pipe run than the upper row. Ideally you want to artificially extend the connections to the lower bank so they have the same total pipe length and same number of fittings as the pipes going to the upper bank. The challenge is to do that while also keeping the pipes traveling uniformly upwards, without any up/down/up transitions. Unfortunately, I can't see any easy way to do that, and this is much less important than switching the overall layout to the one in your upper diagram.
 
I agree with Jason on this.

One minor modification that would help to the upper drawing would be at the right, take the exit pipe of the lower row straight up and T into the exit of the upper row at the top. That would make each travel length much more uniform (like Jason was describing above).
 
Thanks for the feedback guys.

Re-plumbing the return lines to the opposite side of the roof is something that I'll add to my punchlist. Do you think this has potential to resolve the low flow through the collectors without need for a booster pump? Otherwise, my thought is that if it's going to take a combination of a booster pump and re-plumbing to obtain optimal results (given my setup), I'd probably start with the pump (less effort) and then tackle the plumbing sometime during this summer as time permits. If re-plumbing alone has a decent chance to resolve the flow issue, I'll save myself a couple hundred bucks and forgo the pump for now.

Thanks again!
 
I ended up adding a 3/4Hp Booster Pump (Hayward 6060). Without the booster pump, I was at ~21gpm return from panels, and 3 psi on the roof. With the booster pump, I'm at ~40gpm return and 14 psi on the roof. Eventually I plan to modify plumbing layout based on the above recommendations, but I'd like to collect data before I make further changes.

Here are the daily rises I've observed so far:
6/1: Pool Start Temp (PST) = 64F, Pool End Temp (PET) = 69F, Outdoor High = 68F (Partly Sunny)
6/2: PST = 66F, PET = 71F, Outdoor High = 73F (Mostly Sunny)
6/3: PST = 68F, PET = 73F, Outdoor High = 74F (Mostly Sunny)
6/4: PST = 70F, PET = 76F, Outdoor High = 80F (Partly Sunny)
6/5: PST = 74F, PET = ???

I know performance of panels varies based on geographic location/weather/etc, but is there any sort of guideline as to what would be considered typical daily temp rise when using solar?

I hope to start collecting deltaT next week between supply/return of panels, which should allow me to calculate efficiency.

Thanks!
 

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No, I have not put the solar cover on --I plan to get a reel at some point, which will make it more practical. We've had several nights this week that have dropped to 40ish, which obviously diminishes the value I'm getting during the daytime.

That being said, is there a guideline (or typical observations) as to what typical daily rise might be with solar panels? High performance vs low performance?
 
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