When refrigerant expands, it becomes cold. When it is compressed, it becomes hot.
In the heating mode, the refrigerant is compressed in the house, giving off heat which warms the house. The refrigerant is pumped outside and expanded, needing some heat to do this, using 50[sup]0[/sup] water. It does not have to become 70[sup]o[/sup], it just needs to have enough warmth to expand, and begin the cycle again. It doesn’t collect outside heat to warm the house directly, it just needs enough heat to change state so that it can be compressed and generate more heat.