You can attach a temperature sensor to the cold water inlet (a few inches from the exchanger) and start the pumps when there's a drop in temperature. Try to arrange it so that the exchanger is right above the 40 gallon tank. Then it would tend to stay warm by convection instead of short cycling the pumps.
The ideal is to put some copper tubing in the tank but that doesn't seem to be practical for your case.
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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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