On Jan 10, 4:56 am, wrote:

A car radiator near the top of the sunspace might store heat in a 140
F
4'x8'x3'-tall plywood tank lined with a single folded piece of EPDM
rubber,
with a $60 300'x1" PE black pressurized plastic pipe coil to make hot
water

Would a copper coil not be much more effective, high pressure PE must
have a thick wall, that does not help heat exchange.

Probably 30' of copper would work better, yes it would cost more, but
would improve collection efficiency and produce greater output!