A coolant fluid circulation subsystem for engine driven heat pump systems including a coolant fluid loop through the engine and a recuperator, and through a thermostatic control valve operable to convey and to modulate flow from a coolant reservoir during engine operation, with the coolant fluid being selectively switchable, between first or second heat exchangers in heat exchange relation with the indoor and outdoor ambient condition selectively. An auxiliary pump and coolant fluid heater are provided in the subsystem to increase the heat contained in the coolant fluid and to exchange heat to the indoor ambient condition.