Heat pumps in the form of home refrigerators, air conditioners,
etc, have been available for many years. The common kitchen fridge
is a heat pump that removes energy from the cold box and transfers
this heat to the outside air.
Your heat pump is a similar machine which removes heat from the
outside air in large volumes which it discharges to waste. It transfers
this energy to the water which in turn becomes hotter. This results
in large savings as a result of the use of the free heat energy
in the air - a heat pump will typically SAVE you 80% of the running
costs of an equivalent electric element heater.