I used to be in the office cleaning business so I've seen how carpets are cleaned. Usually detergents and chemicals are used. The small little machines that you can rent at the hardware store are nothing more than toys. The carpet takes a long time to dry and then much of the detergent remains on the fibers and actually acts like a dirt magnet. Your carpets end up needing to be cleaned more often because of the sticky residue that is left.
Zerorez is different. As the name implies, there is no residue left behind to attract dirt. They use some kind of new technology that activates water in some way that makes it a very effective cleaner. Their vacuum system is really awesome too. A huge, super suction vacuum is mounted on their 20 ft. truck. I don't know the exact specs but it sucks the carpet almost completely dry. It takes only about 30 minutes before you can walk on the carpet.
Before deciding to replace carpet before listing a home, you might want to give Zerorez a try. I had some really nasty carpet in my bedroom. The kids had spilled all sorts of things on it. I was certain I was going to have to rip it out and replace it. Zerorez came out and made it look like brand new.
The prices seemed very reasonable and the service was outstanding. I would consider them if you are looking for a good carpet cleaner. They have franchises all across the country.