My Solar Heater Doesn't Work

George Peteya_2

Four years ago, I installed an in deck solar pool heating system. At that time I still had a separate 400 sq ft roof-mounted SolaRoll system. I set it up so that I could run the deck, the roof, or both at the same time. Last year, the SolaRoll reached the end of its life (16 seasons, 1.5 billion btus collected, no regrets). I thought the deck heater was doing just as much as the SolaRoll, but now I don’t think so.

At a given filter pressure (Hayward EC-75, Super Pump, typically 14 psig), the total flow rate is the same no matter which path the water takes. I usually got a 4°F. rise through the SolaRoll alone. Now, the deck alone will only give me a 1°F. rise at the same flow. The rule of thumb used to be that a deck heater would provide half the btu/sq ft of an equally sized pool solar collector, so I thought I’d get at least 3°.

In July and August, I could get the pool over 90°F. with the SolaRoll if I wanted, not that I did (OK, 97°F. once, think 19,000 gallon spa on a cool evening). Now, I’m only up to 83°F. OK for now, but September is closer than you think. I’m about 50 miles south of Albany, NY. Whenever the pool isn’t being used, it has a solar cover on it.
Additional info that may help: the deck piping is 3/4” pvc, installed about 6" on center. There are 8 loops, each about 140’ long. Supply and return manifolds are 1-1/4” copper with Dahl ball valves on the return. Deck area is about 700 sq ft. Deck is in sun most of the day.

The deck is 1-1/2” bluestone over 6” of stone dust. The piping is in the stone dust, between 1/2” and 1” below the bluestone.

This year, I put a tekmar 079 sensor in the stone dust, about 2 inches deep. I did this to find out how much of a thermal mass effect there is. There’s a lot … the deck doesn’t reach maximum temperature until 2 or 3 in the afternoon. I’ve set the filter to run between 1 and 7PM. Yesterday the deck was 114°F. at 3PM. Surface temps usually reach 140°F. (by Raytek Mini-temp). I monitor pool temperature, pool return temperature, and deck (tekmar 079) temperatures with a Heliotrope General Quadra-Temp monitor. Old solar geek equipment.

I’ve seen a few solar pool heating posts on the Wall, so I know more than a few of us are thinking about it. Any ideas? No, it’s not insulated below, and I guess I won’t be pulling up 7,000 lbs. of bluestone and 40,000 lbs. of stone dust to do it now.

Attached is picture of the piping installed, before final layer of stone dust and bluestone.

hr

transfer efficiencies

The black, thin walled solar roll was directly in the sun.

The PVC is thicker walled (more insulator) and in a poor conductor (stone dust) and below the stone which also has some r value. Not ideal conduction transfer from the radiant energy of the sun to the fluid inside the PVC. These are a couple of things that cross my mind.

What kind of delt t does it run when the stone is at it's highest temperature?

Actually that Solaroll was a fairly good product for capturing solar energy, considering the simplisity, there are still some in use around my area, on roof tops! Maybe replace it with some black rubber ONIX and buy another 15 years

hot rod

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George Peteya_2

Too much R (not enough U)

HR, no doubt you’re right about the R-values … I think the R-value of the bluestone alone is about 1.2. Probably add another 1.2 for the stone dust (it’s the same material), even though it’s well compacted. Add another 0.6 for the pipe wall and you have 3.0. If the overall ∆T between the surface (say 140°F.) and the pool water (say 83°F.) is 57°F., I guess all I can expect is ∆T/R = 57/3 = 19 Btuh/ sq ft. If I really wanted some heat, I should have poured a dark pigmented concrete deck instead.

When I was laying the stone dust, somebody suggested that I would improve the heat transfer if I mixed some Portland cement in with it. Did I listen? Noooo …

Another rooftop collector is not an option. Too many water and ice dams, and roof leaks, from debris getting under the collector. The shingles are in worse shape where they were hidden under the collector than where they’re exposed. I didn’t expect that, either.

Unfortunately, there’s no place I can put a ground mounted collector without cutting down some trees which provide coolth in the summer.

Matt Clina

Did you just say coolth?

That is a great word.

George Peteya_2

Feel free ...

... to use it, I didn't invent it. I may have heard it from Larry Drake of RPA. "Coolth" is what is found in stone houses at all times of the year , and is what your ground-source heat pump is looking for around this time, too .

Larry Weingarten

coolth

coolth (kolth), n. The state of being cool; coolness. [Now chiefly provincial]... From "The New Century Dictionary" 1944. Old books is good!