How should the air to water heat pump unit be sited to ensure the airflow and access it needs?

What is important is to ensure there is sufficient space for airflow into the unit and enough clearance at the front of the heat pump to stop cold air re-circulation.

You will also need to ensure that there is adequate space for access for service and maintenance.

You should try to ensure that the unit is sheltered from high winds as this will improve the unit is efficiency level by lowering the fan power requirement.