16000 btu 3400 btu 10300 btu vintage unit improved comfort energy savings electricity consumed united states reduce airflow precise control efficient due reversing valve outdoor air aluminium fins aluminium coils subfreezing temperatures extended periods electrical energy properly installed refrigerant flow heat energy effective heating compressor circulating refrigerant cooling season divided provide efficient heating higher initial investment heat pumps life total heat removed total heat provided warmer months heat aid heat transfer heating season divided viable heating alternative ensuring proper installation dehumidification advanced motor partload conditions compensating technical advances electronic rejected outdoors efficiency inter defrost cycle