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Using waste heat is most efficient indeed. However, waste heat (through the cooling system) doesn't come untill the engine is near full operating temp, which can take a while or forever depending on your engine or motor. Recent VW TDI have resistive heaters to provide instant heat. My older TDI could certainly use a heat pump, so do all electric cars. The problem is on convential engines, the cost/benefit ratio is way too small.
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