Waste water from the bathtub, shower & clothes washer comes out at better than 90 degrees.
The planter @ 500+ sq feet by 3.5 feet deep = 1750 cubic feet of wet thermal mass.
Then think about compost. This means the planter itself is a heat source & thermal reservoir.
How to harvest – simple – bury some air tubes below the basin with small fan to blow air through the tubes.