a method identifies at what outdoor temperature a heating or cooling system will need to operate at full capacity to maintain a desired indoor temperature . the tolerable outdoor temperature limit is identified by extrapolating data collected upon sampling the system &# 39 ; s performance at various loads . in a similar manner , the method can also predict a best achievable indoor temperature for a given outdoor temperature . the tolerable outdoor temperature limit and best achievable indoor temperature can be displayed to indicate whether the system needs servicing .