Can old forced hot air ducts be used for new AC? Re: Can old forced hot air ducts be used for new AC? You really need to consult with a Heating and cooling contractor. A HVAC pro can do the necessary calculations and studies to properly evaluate what you have and what can be done. No definitive answer from just your description can be given. Consider the science behind heating and cooling and you will understand that this is not just a yes or no answer. I hope this helps you. Steve I have installed many many system in my life time and would state that 4 inch duct are not rated for the type of CFM that would be required for combination heat and cooling systems. Ducts that were installed in the 60's would probally have insulation deficiencies to warrant starting from scratch. I would not recommend installing a new system on old duct unless it was properly sized. Most contractors will give free estimates find a good one and go with him get more than one estimate and let the contractor know from the get go that you are going to have his work inspected by a third party weather you are or not keeps them on their toes.