Use a 12V BLDC pump and a thermostat set to open above 100F or so, mounted on the hot water line.
There's a version of the Laing D5 that has the thermostat built in, but that costs more than a decent Topsflo pump and a snap disc thermostat.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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