Can old forced hot air ducts be used for new AC?

I recently bought a 2 story home that had forced hot air system from the 60’s until the 90’s. The owners converted to baseboard heat but left all of the ductwork and registers from the original forced hot air system. The ducts are 4" in diameter. My question is…can the old ducts be used with a new AC unit? We had one person tell us that it could be done. Another told us that there are no returns up high and a completely new AC system needs to be installed which can be done through the attic. Meaning that all of the old ducts are useless. Can anyone help with a definative answer before I hire someone to do the work? I appreciate any advice and/or knowledge.


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. Ask good questions of your contractor ask does he get a permit and is his work to be inspected by Local AHJ (Authority having Jurisdiction)

There is an outside chance that you could use some existing ducts: they still may not give the comfort you desire without some changes. If the house has not been retrofitted for energy efficiency, a good retrofit may reduce the heating/cooling needs by up to 40-50%. Then the existing ducts may be able carry the reduced volumes of air required by a 2-3 ton A/C unit (400 cfm / ton). Again, get the numbers done by some one knowledgeable before you embark on the investment.

Duct sealing and insulation should be part of the retrofit.