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Vehicle development teams find it challenging to predict what their Heating, Ventilation and Air-Conditioning (HVAC) module performance will be for cold ambient (∼ -20 deg. C) test cycles such as defrost and cabin warm-up before the car is built. This uncertainty in predictions comes from varying engine heat rejection to coolant due to cold cylinder wall temperatures, calibration changes and degraded performance of various components within the cooling system such as the coolant pump owing to higher viscosity of the coolant. Measuring engine heat rejection at cold ambient is extremely difficult as the engine warms up as soon as it is fired. Multiple measurement points require long lead time to soak to the cold target temperature. It is a common practice to adjust engine calibration parameters to warm up coolant as fast as possible for an adequate defrost and cabin warm-up performance.