Solar heating: plumbing solar mats through Intex drain port

[volcano]

Sharing this set-up here in case it's useful to others (or in case others have cautionary tales to share!)

Last year we added three solar mats to our Intex 9x18-- the effect was subtle, but definitely added a few degrees of comfort (especially since our pool gets filled with ~45˚F well water). Last year I had a branch from the pump return heading to the solar mats but I really wanted to isolate the two systems (filtration vs. solar mats).

I don't particularly trust the Intex drain adapter (the one that connects to a garden hose); I tried it for this purpose but the connection didn't feel secure for long-term use. After a few days of trial-and-error I've currently settled on this: a 3/4" Ferno coupling around the drain port with a 1.25" intex pool hose fitted over the Ferno coupling (I don't love this part since the hose clamp is holding two "soft" things together-- might replace with a barbed fitting so I can really crank on the hose clamp). This connects to a repurposed 1000 GPH pump (no filter in it), then to three solar mats in series, then out to a pool hose that returns to the pool via 1.25" PVC elbow'ed over the side.

Pics show the general set-up (pool isn't totally filled yet, we have to fill slowly via well water so it takes a few days).

 

[mas985]

Do you get enough flow rate going through the panels in series rather than parallel? The Intex pumps are not very powerful so I would expect very low flow rates with that type of setup.

 

[volcano]

Do you get enough flow rate going through the panels in series rather than parallel? The Intex pumps are not very powerful so I would expect very low flow rates with that type of setup.

pulling the water from the bottom of the pool definitely seems to help, since the water is being forced out by the overlying water pressure. Your post motivated me to make some measurements, though!

Flow rate = 3 gal/min, so ~180 GPH.
Current water temp INTO the solar heaters = 68˚F
Current water temp OUT of the solar panels = 79˚F
Current solar intensity = 748 W/m^2

 

[mas985]

Keep in mind that heat loss is proportional to the temperature difference between inlet and outlet of the panels. So the slower the water through the panels, the higher the temperature difference and the more heat is lost to the environment.

So for maximum heat transfer into the pool, you want higher flow rates and smaller temperature difference between the panel inlet and outlet. Counter intuitive but true.

The rule of thumb is that you want 0.1 GPM/sqft or more so in your case, you want at least 5 GPM.

Solar Pool Heaters - Further Reading

www.troublefreepool.com

 

[volcano]

Keep in mind that heat loss is proportional to the temperature difference between inlet and outlet of the panels. So the slower the water through the panels, the higher the temperature difference and the more heat is lost to the environment.

So for maximum heat transfer into the pool, you want higher flow rates and smaller temperature difference between the panel inlet and outlet. Counter intuitive but true.

The rule of thumb is that you want 0.1 GPM/sqft or more so in your case, you want at least 5 GPM.

Solar Pool Heaters - Further Reading

www.troublefreepool.com

True, I know I'm not maximizing the thermodynamics here! Just taking advantage of parts I have on hand (e.g., a spare Intex pump and this is the flow rate it can achieve with this set-up). The geometry of attaching the panels in parallel (rather than in series) means I'd have longer runs of hose through which to lose whatever heat was gained in the panels.

The temperature differential will likely decrease over the next week (the pool is currently being filled with 45˚F well water, so it should only get warmer from here)-- and I can imagine that the GPM will increase slightly as the pool depth increases (greater pressure of water at the drain port)? So my temp in versus temp out should decrease significantly from what I'm currently getting.

I'm looking at the TFP article for the that contains the graph you posted, and it looks like this line suggests the "sqft" is the square footage of the panels, not of the pool:

Most manufactures recommend 0.1 GPM/sq-ft. or about 5 GPM for a 4x12 panel

I've got (very approximately) 27 sqft of panels-- so wouldn't that suggest I'd want about 0.3 GPM 3 GPM (typo corrected)? (I know this ignores the temperature differential issue, though)

 

[mas985]

I'm looking at the TFP article for the that contains the graph you posted, and it looks like this line suggests the "sqft" is the square footage of the panels, not of the pool:

Yes, that is correct.

I've got (very approximately) 27 sqft of panels-- so wouldn't that suggest I'd want about 0.3 GPM? (I know this ignoring the temperature differential issue, though)

Sorry, I thought those panels were 4'x4' each. Aren't they these:

Solar Mat for Above Ground Pools - 47" x 47"

A leading brand in Above Ground Swimming Pools, Air Mattresses and PVC inflatable products. With over 50-year of history, we work hard to maintain the highest standards for safety, quality and value.

intexcorp.com

 

[volcano]

Yes, that is correct.


Sorry, I thought those panels were 4'x4' each. Aren't they these:

Solar Mat for Above Ground Pools - 47" x 47"

A leading brand in Above Ground Swimming Pools, Air Mattresses and PVC inflatable products. With over 50-year of history, we work hard to maintain the highest standards for safety, quality and value.

intexcorp.com

Yes, those are the right panels-- looks like we're both approximating around the same number! I just measured-- the operative area of each panel, once filled with water, is roughly 37"x39". I guess the actual surface area will be slightly higher since it's not flat, but definitely not the 47" that Intex suggests-- that includes the bypass paths and the margins around the mat, too.

Using these numbers for some rough calcs:
37 x 39 x 3 / 144 = 30 sq ft, for a target of about 3 GPM. At least I'm in the general ballpark of that with my kludged-together set-up!

I'll measure flow rate again once the pool is fully filled (hoping to fill it the rest of the water tomorrow-- the whole-house iron filter can handle about 1000 gallons/day and we try not to strain it excessively).