Lowest outside temp I can heat in older 3 unit model?

Thank you for your question. This particular single zone model (15LZAH1) can heat with outdoor temperatures as low as -15 degrees Fahrenheit. However, the minimum outdoor temperature for heating varies from model to model. For finding specifications on your older Fujitsu model or determining what model it is, we would recommend reaching out to the manufacturer. Fujitsu's customer service can be reached at 1 (888) 385-4878. For any other assistance, please give us a call.