Your heating and cooling system should be inside your thermal envelope or you are wasting money. We often find the HVAC system in homes in unconditioned attic, basement, or crawl spaces. That means you are trying to cool or heat air and send it through a space that is working against that goal. You are also probably growing mold.
This not only means the HVAC mechanical equipment should be on the conditioned side of the insulation, it means the duct work should as well. While duct work has insulation, it is not as much insulation as you should have between inside and outside of your home. Keeping the mechanical system and duct work on the inside of the insulation will reduce the amount of energy required to achieve comfort.
If you mechanical system is in an unconditioned space, you are wasting money. The best solution is to add insulation to the space placing the mechanical system inside the thermal envelope. It is not a simple problem to fix if it was not done with building science in mind to start. However, it is a problem that should be solved for lower energy bills and higher comfort.