Air duct cleaning
I'm an HVAC tech, though I don't do duct cleaning but rather residential/commercial installs, maintenance and repairs, I do know that the equipment used for duct cleaning is often truckmount based (and if not truckmount based, it's a large standalone machine on casters) and much more powerful than any consumer or commercial vacuum. A Kirby with a hose attachment (or any vacuum for that matter) is only good for cleaning registers. If you have floor registers you can use your Kirby for cleaning the immediate area but anything past you won't do anything. If you do have floor registers, I would avoid using your Kirby because coins, legos, etc. tend to lurk and being that your Kirby is old enough to have its OE metal fan (or if it had been serviced at any time in the 80s or early 90s, possibly a Lexan fan), this wouldn't be recommended anyway as it could ruin your fan.