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My Solar Pool Heater No Work

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. 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.
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