Improving indoor air quality in homes requires fresh air; however, this is a strain on air conditioning systems. To address this issue, the use of energy-efficient fresh air units equipped with exhaust air heat recovery is recommended. These units include both passive and active types, with prominent examples being air-to-air enthalpy heat exchangers and heat pump units. Currently, the evaluation of the energy efficiency in fresh air units predominantly revolves around air-to-air enthalpy heat exchangers, rendering the commonly used heat exchange efficiency inapplicable to heat pump units. The concept of exhaust air heat recovery is perplexing and contradictory. Furthermore, the assessment of fresh air units primarily focuses on the units themselves, without considering their impact on air conditioning units and the overall system performance once combined. This study aims to establish a unified definition of exhaust air heat recovery for fresh air units, elucidating its intrinsic meaning. Additionally, it proposes a comprehensive energy efficiency evaluation method for a combined fresh air and air conditioning system. Through a case study of seven existing fresh air unit types, the necessity of exhaust air heat recovery is highlighted, and the energy efficiency levels of different unit types are compared.