Radiator solar panel

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An old radiator painted black and in a glazed insulated box makes a very cheap solar collector. They work, but they have their issues.

The large water volume makes them very slow to respond to changes in insolation, so they fail to take advantage of short periods of sun, and produce next to nothing on days when the sun is in and out a lot.

Steel rusts, so you need a closed corrosion inhibited system with heat exchanger. Direct systems would flood eventually.

They're heavy, so not really safe to mount on the average roof.

The pump controller needs to suit their slowness, ie it must be thermostatic rather than a solar pv panel.

The sweetspot for these panels is on a low flat roof, powering a pre-heat tank.

They can be used with gravity circulation for an extremely cheap system. This addresses their slowness, but the lack of pump also reduces yield.