hˈaɪɪfˈɪʃənsi kəndˈɛnsɪŋ fˈɜːnɪsᵻz kəndˈɛns ðə wˈɔːɾɚ vˈeɪpɚ wˈʌn ʌvðə baɪpɹˈɑːdʌkts ʌv ɡˈæs kəmbˈʌstʃən ænd ˈɛkstɹækt ðə lˈeɪtənt hˈiːt tə pɹˈiːhˈiːt ðɪ ˈɪŋkʌmˌɪŋ fˈɜːnɪs ˈɛɹfloʊ, jˈuːzɪŋ ɐ sˈɛkənd hˈiːt ɛkstʃˈeɪndʒɚ.
High-efficiency condensing furnaces condense the water vapor (one of the by-products of gas combustion) and extract the latent heat to pre-heat the incoming furnace airflow, using a second heat exchanger.