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

[Lippy225]

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. 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.

[cem-bsee]

maybe--
it is hard to see your ducts from here!

size of ducts? where are they?
size needed? -- you need a load calculation -- buy the one herein so you will know where your issues are!

[docholiday]

The location of the returns is not near as important as the duct size. If the duct system cannot move the volume of air that it needs to then it needs to be corrected. if you want performance, you'll have to know that answer first. If your installer doesnt know how to size or evaluate duct, then you probably should show him the door. We dont know what your duct system looks like and we dont know whats going to be installed. Unfortunatly, sizing duct is more difficult then sizing water lines.

[Lippy225]

The location of the returns is not near as important as the duct size. If the duct system cannot move the volume of air that it needs to then it needs to be corrected. if you want performance, you'll have to know that answer first. If your installer doesnt know how to size or evaluate duct, then you probably should show him the door. We dont know what your duct system looks like and we dont know whats going to be installed. Unfortunatly, sizing duct is more difficult then sizing water lines.

The ducts are 4" in diameter. they run all thoughout the crawlspace and walls of the house

[Lippy225]

maybe--
it is hard to see your ducts from here!

size of ducts? where are they?
size needed? -- you need a load calculation -- buy the one herein so you will know where your issues are!

The ducts are 4" in diameter. they run all thoughout the crawlspace and walls of the house

[docholiday]

The quick answer then is no.

[dash]

We had a poster that had the same thing,40 or 50 pages of posting and it worked out okay,but you ar likely better off with new ducts.

If they are capable of testing the duct static,and/or using Manual D (duct design standard) they could tell you for sure if your existing ducts will work,with or without modification.