Hello, I was exploring the same option for the same size greenhouse. I was thinking about putting a small plastic pool, use a water pump to push the water in the infloor pipes. Also use an electric water heater or just a hot lamp on a metallic portion of the infloor pipe to heat the water. It is a closed circle, and the gradual walking of the pool is a bonus to help increase the temperature. I would love to hear what you think!
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