The average HVAC system has a lifespan of 10 to 15 years. When it’s time to replace your old system, many factors go into determining the correct size. A professional HVAC consultant must visit your house and take detailed measurements and assess your particular requirements. The type of house and walls, the type and size of windows, insulation, basement, attic conditions and house orientation will be considered. With your house measurements and assessment, the consultant can perform a performance load calculation to assure proper system size.