A: The easy answer is ,"When your return on investment is at its highest." This may be difficult to pinpoint since most people aren't familiar with the average life span of HVAC components. Some equipment seems to last forever, but replacement parts for older models may take weeks to receive. Potential energy savings with newer and more efficient equipment goes unrealized as well.

Here is a short list of average life expectancies for commercial/residential equipment...

Air Conditioners (Split & Packaged) = 15 years

Heat Pumps = 15 years

Gas or Oil Furnaces = 18 years

Boilers = 30 years

Unit Heaters = 13 years 

Packaged Chillers = 20 to 23 years

Cooling Towers = 20 to 34 years

When confronted with HVAC replacement or repair decisions, this guide should help you achieve the "maximum return on your investment."