RES, the only heat source is the engine. It is cooled by coolant run through the radiator and some convection from air in the engine bay. Where I think the biggest improvement comes in is once the thermostat opens, coolant flows through the radiator and airflow to the radiator is blocked. This is my second winter with my JSW. Last year below 20F ambient the car would not warm to 190F coolant temp during an 18 mile commute with the first 75% being freeway speeds. One day near 0F I froze on my way to work and cursed the efficient diesel engine. With the winter cover from ID Parts, life is much better for me. The coolant is able to warm up quickly once the thermostat is open because airflow is restricted to the radiator. So I think that time until the thermostat opens may be improved some (minimally?), it is the reduction in cold air on the radiator that allows the engine to remain warm once the thermostat opens. I love this thing!