ðə dˈʌkt wˈɜːk səplˈaɪɪŋ ðə hˈɑːt ˈɛɹ ænd sˈʌmtaɪmz kˈuːl ˈɛɹ ɪf ɐn ˌeɪsˈiː jˈuːnɪt ɪz tˈaɪd ˌɪntʊ ðə sˈɪstəm mˈeɪ biː ˈɪnsəlˌeɪɾᵻd.
The duct work supplying the hot air (and sometimes cool air if an AC unit is tied into the system) may be insulated.