DIY Solar Water Heater — Copper Coil

Hobbyist build log, publication date not stated.

Key findings

A simple flat-black-painted copper coil behind glass reached sustained 165°F+ output, demonstrating that a small unglazed/glazed dark coil is a viable absorber at hobbyist scale. Uses a single continuous 50 ft roll of 3/8” copper tubing with push-to-fit connectors rather than joining shorter sections — minimizing joints/leak points, a principle that carries directly over to a small PEX coil. Flow rate achieved: roughly 20–30 gal/hr through a simple coil. Flat black paint is sufficient as an absorber coating; aluminum-backed fins add roughly 10°F over bare tube — a marginal gain that may not be worth the added complexity at a 2.5 ft³ scale.

Why it matters for this archive

A concrete, tested absorber-coil reference design (materials, coating, joint strategy, temperature/flow results) to scale down from for a doghouse-scale roof radiator, even though it uses copper rather than PEX. See Miniature thermosiphon scaling limits.