Sink directly into it for the summer. Heat exchanger between the source (approx 10-15C) and the water from the floor heating/cooling system. Because the heat pump is not involved it's very efficient (only the power of the well pump)

I'm actually considering if it makes sense to push a lot more heat into it in the summer (Deliberately leaving curtains open, then moving this heat into the well; or even PVT panels), but I'm not sure if the well would retain heat long enough to make a difference towards the winter.