I’ve always wondered when watching movies like Die Hard or Mission: Impossible or even tv shows and videogames like 24: is it possible to move through a building using the air duct system as a means of conveyance? If so, why do air ducts need to be so big? Also, knowing this, wouldn’t architects build smaller air-ducts or at least install spinning razor blades or lasers to prevent such invasions? Any architects out there know what gives?
Air in larger ducts can flow slower and with less noise.
Air ducts in large buildings generally need to be large in order to move the large amounts of air required at a reasonible velocity. The smaller the duct, the faster the same volume of air must move through it. Most people don’t want to sit under a loud jet of high speed air.
It is possible but pretty dangerous to move through the air duct system. You could get stuck in a narrow duct, the duct could break loose (which would suck if it was anchored to the roof of a 10 story atrium) or you could fall down a shaft.
Plus, in real life, it’s pretty hard to stealthily move through an aluminum air duct.
When the duct breaks loose, you’ll be happy your brother insisted on the rope.
Nitpick: air ducts aren’t made from aluminum, but from thin galvanized sheetmetal. I’m not sure what alloy is used, but it’s steel or tin based, not aluminum. Not that this has any impact on your point. 
[after dropping through the ceiling on a rope and killing nine mobsters]
Connor: Well, “Name one thing you’re gonna need this stupid fucking rope for.”
So many good rope quotes in that movie… I need to buy a copy of it.
They’re usually made of (IIRC) 16 or 18 guage galvanized steel. I used to fabricate and install duct. The return ducts are sized larger than the heated/cooled air supply ducts.
Crawling through air ducts is a ridiculous movie cliche. They’re not usually wide or strong enough to support a grown human’s weight. If the building is more than a couple years old they are usually full of dust and lint. You’d also make a lot of noise crawling around in the duct work if it held your weight.
The ducts (trunks and plenums, at least) are big enough for a human to move through, but it’s impossible to be silent as each segment’s going to announce your presence with a thunk! as you move into it and the section you just left will also let loose with a loud thunk! as it pops back into its normal shape.
Heck, even the air pressure in the ducts makes them thunk when a blower starts or stops.
Newer buildings are apt to have flexible foil/fiberglass ducts that are impossible to move through, unless it’s a really big duct and laying flat on the floor as they can’t support any weight.
Well, that would make stealth easier, not more difficult. “Klunk! Klunk! Klunk!” “Oh, dang, the air ducts are making noise again, I complained to the custodian about that just last week!”
Guess I should have said that the person in the duct would be creating a traveling *thunk…clunk! … thunk…clunk! * noise as they move from section to section, compared to a usually single thunk or short chain of thunks when the system starts up.
The Spy Museum in DC includes an air duct you can crawl through – and from which you can eavesdrop on people. It’s pretty hard to move through that quietly, and I think the duct is even carpeted.
What hasn’t been mentioned yet is that in the movies it looks all smooth inside the duct. A lot of times the insulation is on the inside, with really sharp brads to get caught on and get cut on. Also, when a large duct makes a 90 degree turn there are fins from top to bottom to make the air flow more smoothly around the corner.
I have a question: what is the risk, if any, of getting sucked, or blown, or simply falling, into a fan (or other machinery) while moving through the ductwork?
Question prompted by an old episode of “The X-Files” in which Scully was sneaking through ductwork and was tossed like a leaf and nearly julienned when the building fans went into overdrive. Of course this was BS (or in other words, it was “The X-Files”), but it did make me wonder. Ventilation ducts do have fans, after all, and it isn’t good to get caught in a moving fan, so would this be another reason to stay out of the ductwork?
See above answers re speed of air through ducts.
I’m no HVAC expert, but aren’t there often enough other fittings that would make travel a little iffy? The directional blades mentioned above? Other filters / baffles? And noise aside, as you get closer to the end of the vent, it’s likely to split off side vents (weren’t we supposed to turn left back there?) and become quite small…?
Yes, there are volume dampers and fire dampers also. The directional blades are called “turning vanes”. The duct does indeed get smaller as you near the diffuser (air outlet). Usually there is a “trunk” duct, which you might be able to crawl through. Then tap-offs leading to a VAV (variable air valve); which would control 1-5 diffusers. VAV’s typically range 16" dia and down. From here on the duct gets really small, typicaly 14"x6" feeding the diffuser. 99% of supply duct is insulated; most outside, some inside(15ft down stream from a VAV for noise reduction). In NYC duct is hung with 1" wide strips of galvinised steel, and concreat anchors every 8 ft. Screws throught the “band iron” into the duct; more stuff to get cought on. Plenty strong enought to hold a person, even the small duct.
As sated above crawling trough duct is a movie cliche, you would get practiclay no ware. If you ever got in, you might get a fair distance in the trunk duct, getting out would be tough. You would be filthy. Grills are tightly fastened on, no gently pushing one aside either. Kicking the crap out of one could remove it. No lights either. You couldn’t get near a blower or fan, due to dampers and filters. And only extremely high pressure ducts, might move you a bit. Crawling woud be very uncomfrotable; tossed like a rag doll, naaa.
My favorit movie parody of this is “Who is Harry Crumb?” with John Candy carwling around some quack quacks.
20 yrs Sheet Metal Worker L.U. 28 NYC Hurraa!
“Tinknockers bang better!”
In high security environments, there are often security bars (think old style jail cell steel bars, or some equivalent). I’ve only specified them once, for a room in a mixed-use facility that was to be used by a law enforcement angency for evidence storage. The ducts serving this would not be supported well enough to support an adult’s weight, and they would be barely big enough for a very small adult to fit through (about 16"x12"), but the rest of the room was made of concrete filled CMU block, with a secure door. The easiest way into the room would probably be by cutting through the metal roof.
Just about everything else here is right (except for the thickness of the galvanized sheet metal- it can be as thin as 24 or 27 gauge for smaller ducts or thinner if the installers aren’t building it to SMACNA standards). The duct hangers may or may not support the weight of a passenger, there are often spikes for fastening insulation, the metal will “oil-can” when you put weight in the middle of unsupported areas, etc.
The only realistic use of ductwork to gain access to a space is by using it as a hole through a wall. It wouldn’t be useful as a path through a building, or a good hiding space.
Scooby, P.E.
HVAC Engineer
I would add this…
Duct large enough to allow an adult to crawl around in would generally be high pressure duct, with “Ductmate” or “TDC” connections. So not only would it be pitch black, and you’d be filthy, but you’d be in a wind tunnel. (Up to the VAV, which reduces pressure to the diffuser)
Further, the only entry to this larger duct would be near the air handler—generally in the mechanical room. Even then, the only air handlers large enough for you to enter the air handler is generally in hospitals and industrial buildings. But even in most cases, there is just no way to enter the duct system. (maybe through an access door near a fire damper)
Once in the duct, reducing fittings, turning vanes, smoke and fire dampers and lots of other obstructions make travel difficult/impossible.
In the movies however, they are always entering/leaving the duct system at the diffuser. (register) At the register the most common connection is “flex”, small branch lines, round pipe 14" and smaller, and VAV boxes—none of which would accomodate a child, let alone an adult. Not to mention that in the movies the inside of the duct is always lit. 
Look cool in the movie. In real life: No way.
There is however some chance of escape or entry in some ducts. It is common in retail stores to install “burglar bars” to keep people from entering the building via the roof duct. It’s also common to see them in prisons so inmates can’t escape into the plenum as a means of escape.
Fiction aside, I think most spying is done by paying someone for information and by listening to loud-mouthed, half drunk jerks boasting in bars.
In a building which used to have a secure area with a duct, they had it set up that at night the duct would ‘swing away’ (disconnecting the secure area from the vent system) and a vault door would close, blocking the opening that the duct ran through.