There might be many approaches to carpet cleaning—from professional shampooing to lighting the fibers on fire to clear off the top layer of dirt—but by far the most popular method is steam cleaning. That name can be misleading, however. It’s true that steam can be an effective cleaning tool, but when pros talk about steam-cleaning a carpet, they're usually talking about a process technically known as hot-water extraction.

Why not steam?

Steam—which only occurs when water reaches its boiling point of 212 degrees Fahrenheit—is too powerful for many residential jobs, posing a risk of damage or shrinkage to some kinds of carpet fibers. Fortunately, water usually only needs to be heated to around 140 degrees to kill most of the microorganisms that can live in a carpet and cause odors. A skilled technician will know what range of temperatures is ideal for any given fiber.

So what’s in the van?

Whether it’s a massive van-mounted machine or a portable rolling model, a carpet cleaner has three important elements: a heating device, water jets, and vacuum suction. The heated water will sometimes be mixed with detergent, but one of the benefits of steam cleaning is that soap isn’t always necessary—pressurized jets can reach deep into fibers to get at grime. A vacuum hose then pulls most of the liquid back into the machine, though some dampness will linger. Generally, three or four hours is enough time for a carpet to be ready for you to walk on and place all your little army guys back in position.