when you replace your furnance& AC units

[CaptainTruth]

2. I agree, usually not, if your existing ones are in good condition.

It's an excellent time to have them cleaned out.

As for drywall & ducts in walls (mentioned in the post above), that depends on where you live, which can influence the way your house is constructed. Here in FL most houses are slab-on-grade (built on a concrete slab) with concrete block exterior walls (for hurricane protection). There are no HVAC ducts in walls, they're all in the attic. That makes it a lot easier to install new ducts, but the low roof pitch (for less wind load, & we don't need to shed snow from the roof) means it's "fun" crawling around up there trying not to hit your head ... which I always do a few times every time I have to work in a FL attic.