A thermal storage heat pump system transfers heat to a passenger compartment of a vehicle from at least one of a thermal storage device and ambient air. Heat from the thermal storage device is absorbed by a first coolant flowing through it, and is transferred to a refrigerant via a first heat exchanger. The heat is then transferred from the refrigerant to a second coolant via a second heat exchanger, and then from the second coolant to air flowing into the passenger compartment via a heater core. Heat from ambient air is absorbed by the refrigerant via a third heat exchanger. The heat source is determined by at least one of the thermal storage device temperature, ambient air temperature, and ambient air humidity. At start-up of the vehicle, heat transfer to the refrigerant and to the second coolant is controlled based on low-side and high-side pressure measurements of the refrigerant.